Railway Construction

by William Hemingway Mills

CHAPTER II.

Works of construction: Earthworks, Culverts, Bridges, Foundations, Screw piles, Cylinders, Caissons, Retaining walls, and Tunnels.

Earthworks.—Under this heading may be classified cuttings and embankments of earth, clay, gravel, and rock.

When setting out a line and adjusting the gradients, an endeavour is usually made to so balance the earthworks that the amount obtained from the cuttings may be sufficient to form the embankments. With care, this may be effected to a considerable extent; but there will be places where the material from cutting is unavoidably in excess, and others where the cuttings are too small, or contain good rock, or gravel, which can be more advantageously used for building and ballasting purposes than for ordinary embankment filling. Or there may be a large cutting which will provide enough material to form three or four of the adjoining embankments; but the distance, or lead, as it is termed, to the far embankment may be so long, and, perhaps, on a rising gradient, that it would be cheaper to run the surplus cutting to spoil, and borrow other material for the far embankment from side cutting or elsewhere. A long lead adds materially to the cost and time of forming an embankment, as it not only necessitates a considerable length of service, or temporary permanent way, but also occupies much time in the haulage of the earth waggons. For distances of half a mile and upwards, a small locomotive is more suitable than horses for conveying the waggons.

To run to spoil is the term applied to such of the material from a cutting which, not being required or utilized in the formation of the line embankments, is removed and tipped into mounds, or spoil-banks, in some one or more convenient sites near the mouth of the cutting. Sometimes the surplus material is disposed of by increasing the width of the embankments.

Material excavated in a tunnel, and hoisted through the shafts to the upper surface, has to be deposited in spoil-banks along the centre line of the tunnel.

To borrow material to form an embankment is the term used when the earthwork filling is not obtained from the cuttings on the line. This borrowing is generally done by excavating a trench on each side of the line, of such width and depth as will supply sufficient material to form the embankment. [Fig. 47] gives an example of an embankment thus made from side cutting. In some cases a piece of high ground adjacent to the embankment can be utilized for obtaining a portion, or even the whole of the filling.

Increased material is sometimes obtained by widening the cutting, or flattening the slopes, or both.

The degree of slope of a railway cutting must be regulated by the nature of the material excavated. A slope of 1½ to 1, which gives for every foot of vertical height a width of one foot 6 inches of horizontal base, as in [Fig. 48], is usually adopted for cuttings in ordinary earth, good clay, sand, or gravel. There are some descriptions of strong clay and marl which will stand at a steeper slope, even at 1 to 1; but, on the other hand, there are some kinds of clay which must ultimately be taken out to 2 to 1, and even 3 to 1.

It frequently occurs that the slopes of a clay cutting, taken out to 1½ to 1, appear to stand well for a time, but after exposure to the frost and rain of one or two seasons, the material becomes loosened, and forms into slipping masses, which slide down on to the line, stopping all traffic, and have to be cleared away before train operations can be resumed.

Cuttings through solid rock may be taken out to a slope of ¼ to 1, as shown in [Fig. 49], provided the material is compact, and there is not too great a dip in the strata or rock-beds. Where the rock-beds lie at a considerable angle, the slope on the high side will have to be made flatter than the slope required on the low side, as shown in [Fig. 50], and great care must be taken to remove from the high side all loose or disconnected pieces of rock which might come away and slide down on to the line.

Strong dry chalk will generally stand at a slope of ⅓, or ½ to 1, but when wet and mixed with flints it will be necessary to increase the slope to not less than ¾ to 1. Where the rock is loose and disintegrated, a slope of not less than ½ or ¾ to 1 will be

required, and at many points there will be detached threatening masses of rotten rock which must be cleared away to a much flatter slope for safety. In cuttings of this description it is frequently found necessary to clear out a portion of the loose pieces of the lower cavities and build in their place a facework of masonry to support the superincumbent rock. Springs of water rising in the rock, or running over any part of the rock slopes, must be properly provided for, and conducted to the nearest channel. They should be carefully watched during the winter season, when the frost, acting on the water penetrating the crevices, splits and separates large pieces which were previously firm and secure.

Instances will occur where a cutting has to be made through a thick bed of rock and several feet of soft loose strata underneath. The effect of forming a cutting through the soft strata is to induce the heavy bed of rock above to squeeze or force out the softer material below, and unless proper means were taken to avert such a disturbance, the entire cutting would have to be excavated to a very flat slope. The method adopted in such a case is to build strong face-walls of masonry, brickwork, or concrete, underneath the rock, as shown in [Fig. 51], with strong inverts placed at short distances. Suitable arrangements must be made to take away the drainage water which will collect at the back of the walls, and weeping-holes or outlets must be left in the lower part of the walls to convey the water into the water-tables on the line.

Where there is a depth of earth cutting on the top of the rock, the earth should be cut away so as to leave a bench or space of 3 or 4 feet between the edge of the rock cutting and the foot of the earth slopes, as shown on [Fig. 52].

In cases of shelving rock, with earth or clay on the top, as shown in [Fig. 53], it is frequently found necessary to remove the whole of the clay on the high side to prevent the possibility of its sliding off the rock on to the line below.

In large cuttings it is usual to push forward a gullet of sufficient width for one or two lines of waggons, as shown in [Fig. 54]. When this has advanced some distance, strong planks or half balks of timber are placed across the gullet, and the sides or wings of the cutting can be excavated, the material wheeled to the gullet, and tipped from the barrows into the waggons beneath. By this arrangement the work can be carried on very

expeditiously, as one set of men can be engaged advancing the gullet and laying the track, while others are following up and taking down the sides. A large number of waggons can thus be filled in a day, and a small locomotive kept fully employed.

Occasions will arise where the material from a large cutting, situate on a continuous gradient, as in [Fig. 55], has to be carried in both directions to embankment.

In wet weather, or if the cutting is at all wet, it would be almost, if not quite, impossible to carry on the excavation at the upper end to the proper formation level. The water would collect at the lower level, and not having any means of escape, except by pumping, would stop the work. In such a case the best way is to take out the cutting at the upper end to a slight rising gradient, as shown in the sketch, sufficient to carry away all water, and afterwards take out the lower portion in the working from the other end of the cutting.

Cases will arise where it will be necessary to make a shallow cutting through boggy peaty ground. If the boggy material be very soft, and its thickness from the formation level to the solid ground below be not great, it may be advisable to remove this extra thickness down to the hard lower bed, and fill in up to formation level with strong material. If, however, the bog or peat be too thick to justify its entire removal, it should be excavated say down to two feet below formation level, and a thick layer of branches of trees and strong brushwood closely laid and packed the full width of the road-bed. On this preparatory foundation must be placed good clean ballast to carry the permanent way. Two or three extra sleepers should be allowed to the rail length, and in some instances it will be necessary to introduce two, or even four, rows of strong longitudinal timbers—half balks—under the transverse sleepers. The object of all this extra timber is to obtain a large increase of bearing area on the soft yielding surface of the boggy material. Notwithstanding these special precautions, the trackway will sink down a little during the passage of an engine or train, but will generally return to its former level. Good side drains or water-tables should be formed at each side of the cutting to take away all rain and surface water.

In all cuttings it is desirable to have the line of formation on a slight gradient, sufficient to carry away all rain water or spring

water which may be collected in the water-tables; but more particularly so is this necessary in a rock cutting, where the material, being non-absorbent as compared with earth or gravel, requires that all drainage must be carried away to the mouth of the cutting.

In carrying out railway embankments and road approaches, it is usual to form the sides to a slope of 1½ to 1, as shown on [Fig. 56]. Occasionally the cuttings produce material which might stand at a rather steeper slope, but considering the effects which might afterwards be produced by heavy rains falling on the sides, it is more prudent to adopt the flatter slope of 1½ to 1. Some descriptions of clay will not stand at the above slope, but require a slope of 2 to 1, or even 3 to 1.

When proceeding with the earthworks, it is customary to first remove and lay aside a layer, say 9 inches in depth, of soil and earth from the seat of the embankments and top widths of the cuttings, to be used afterwards in soiling the trimmed and finished slopes of the cuttings and embankments. This soil being removed, the actual work of the excavation can be commenced. The working longitudinal section will give all the necessary particulars as to position of the mouths of the cuttings and the depths at the various chain-pegs, and the top widths of the cuttings can be ascertained by calculation, if on even ground, or from the cross-sections if on side-lying ground, according as the material may be earth, clay, or rock.

For facility of carrying on the works, reliable bench marks, or reduced level stations, must be established at convenient distances along the route of the line, and from these and the fixed chain-pegs the correct line of formation level can be checked from time to time as the work proceeds.

For ordinary earth or clay cuttings, the usual tools are picks and iron crow-bars for loosening, or getting the material, and shovels for filling into barrows, carts, or waggons. For heavy earthworks, steam excavators are now largely employed. Great improvements have been made in this class of machinery, in the way of perfecting the method of excavating lifting, and filling the material into the earth-waggons.

In nearly all rock cuttings the greater portion of the material has to be taken out, or loosened, by blasting with gunpowder, dynamite, or other explosive. The number and extent of the charges will depend upon the nature of the rock and its

stratification, and also on its position as regards proximity to buildings or residential property.

Where the rock is loose, or disintegrated, the pieces can generally be readily separated by picks and bars without having to resort to any great extent of blasting.

The first of the material excavated in the cuttings is generally conveyed in wheelbarrows to form the commencement of the adjoining embankments. When the wheeling distance becomes too far for economical barrow work, ordinary carts or three-wheeled carts, sometimes termed dobbin carts, are brought into operation where the cuttings and embankments are light; but where the earthwork is heavy, both in excavation and filling, a service or temporary road of light rails and sleepers is usually laid down to carry strong tip earth-waggons. For moderate distances these waggons are hauled by horses, but for distances over three-eighths of a mile a small locomotive is more speedy and economical. [Fig. 57] shows one form of dobbin cart; the wheels are made with good broad tyres, so as not to sink too deep into the soft ground, and the body being attached to the framework by a pivot or trunnion on each side, can be readily tilted over, and the earth tipped out, by releasing the holding-down catch. Where the ground is soft and wet, or of a very loose sandy nature, the work of hauling these dobbin carts is very heavy on the horses, and in such cases it soon becomes an advantage to lay down a service road of rails and sleepers. This service road is formed of light rails manufactured for the purpose, or old, worn rails no longer fit for main-line work, spiked down on to rough transverse wooden sleepers. The end of the embankment in course of formation, and where the earth is being tipped, is termed the tip head. Two or more roads are required at the tip head to form the embankment to its full width. [Fig. 58] gives a sketch plan of a service road near the tip head. The width is shown as for a double line. The earth-waggons are hauled along the line from the excavation, and brought to a stand at the point A. If a locomotive has drawn the waggons, it is then detached, moved forward, and shunted back into the siding BC. A horse accustomed to tipping then takes one full waggon at a time over one or other of the two turn-outs, DEF or DGH, to the tip head, sufficient impetus being given to the waggon to run the front wheels off the ends of rails on to cross-sleepers laid close, with a steep rise,

and backed up with earth. This suddenly checks the frame of the waggon, and the body containing the excavated material revolves on its trunnion, tilts up, and shoots out the material well forward, so that the man in charge of the tip head, who also knocks up the “tail-board catch,” is able to level off the filling without assistance. The empty waggon is then hauled back, and turned into the siding BC, and another full waggon taken forward and tipped, until all the waggons of the rake are emptied. Ten waggons generally form a rake when the work is pushed forward vigorously, each waggon holding about three tons. The tip head horse pulls the waggon by a trace-chain having a spring catch at the end, by which the driver releases the horse at the right moment. It is very important that this spring catch should be kept in good order, because occasionally too much impetus is given to a waggon, which, running over the tip head down the slope, would drag the horse with it if the spring catch did not act properly. Good firm foothold must be provided for the tipping horse.

The tip head should never be carried across culverts or bridges until they have been well backed up, and protected by a thick covering of earth or clay, wheeled in with barrows to an equal height on each side of the masonry, so as to prevent undue side pressure.

[Fig. 59] gives a sketch of one form of end-tipping waggon. In some cases the wheels are made of cast-iron, but as these are readily broken during the rough handling to which [earth waggons] are exposed, it is questionable whether the light wrought-iron wheels, with light steel tyres, used on some works, are not more economical in the long run. The framework and body are made of strong undressed timber, well bound and bolted together. The tail-board catch keeps the body of the waggon in its proper horizontal position while loading or running, but when released leaves the body free to tilt up, and to revolve on the front trunnion by means of the circular clip A. The same principle is also applied to side-tipping waggons which are used for the widening of embankments, or formation of platforms and loading-banks.

The permanent way of these service roads is generally made as simple as possible. A pair of movable rails are used instead of switches, as shown in [Fig. 60]. These rails are linked together by iron tie-rods, and pulled or pushed over into position for one

or other of the roads by means of the handle at A. A stout iron pin, or iron clamping-plate, serves to retain the rails in position during the passing of the waggons. In a similar manner, a short rail working on a pin, or pivot, is made to answer the purpose of an ordinary crossing. The rails are laid complete and continuous for the one road, and for the second road the outer rail is laid sufficiently high to cross over the rail of the first road. A piece of rail is then secured by a centre pin, or pivot, to the cross-sleeper, as shown on [Fig. 61]. This pivoted rail is pulled over into the position shown by the dotted lines, to allow the passage of waggons on the one road, or pulled across to the end of rail at B, for waggons to pass on or off the other road. In the latter case an iron pin or clamp serves to keep the pivoted rail in position. As these service roads are merely laid down on the soft loose material brought forward for filling, they require constant packing and lifting to prevent them working into depressions, which might cause the waggons to leave the rails.

To indicate the height of the embankment filling, strong stakes or poles must be firmly set in the ground at each chain-peg. On each of these poles two cross-bars must be fixed, the lower one placed to the correct height of the embankment, and the upper one to show the amount allowed for subsidence. The excavated material, as brought from the cuttings, is in a soft, loose condition, and an allowance must be made for its settlement, or subsidence, as the embankment becomes consolidated. This allowance will, of course, depend on the height of the embankment and the quality of the material, but for ordinary earth and clay it is customary to allow about one inch to the foot of height, which is equal to about 8 per cent.

When forming embankments over very side-lying ground, it is necessary to cut steps in the sloping surface on which the filling material has to be placed, as shown in [Fig. 62]. These steps give a hold to the new earthwork, and check the tendency to slide down the hillside.

Embankments have frequently to be carried over ground which is low, soft, and wet, but not boggy. If the culverts and drains are sufficiently large, and properly arranged, these places are not likely to cause much future trouble.

For a thoroughly soft deep bog, however, it is most difficult to make any accurate calculation as to the amount of

embankment filling which will be necessary to form a permanent foundation for the line; and the construction of a high heavy embankment across such a place is one of those undertakings which every engineer is most anxious to avoid. A large quantity of material may be tipped into the bog, and seem to stand fairly well for a time, and then suddenly disappear altogether. More material has to be brought forward, and will most likely disappear in a similar manner. The filling material being heavier than the bog on to which it is thrown, falls through, and displacing the soft semi-liquid matter, continues to sink down lower and lower until it is stopped by a harder stratum underneath. In a measure the operation somewhat resembles the tipping of earth into a lake; the material will go down until it meets with a solid bottom, and in going down it assumes its own natural slope, and forms for itself a width of base corresponding to its height. It will be readily understood what an enormous amount of filling material will be swallowed up in following out such a process. On a very soft bog, say 20 feet in depth, over which an embankment 20 feet high has to be formed, the extent of the actual earthwork filling will very probably closely approach the outline shown in [Fig. 63]. The upper portion, ABCD, representing the embankment proper, will contain about 133 cube yards to the yard forward, whereas the lower portion, CDEF, which has displaced the soft boggy matter, will contain about 266 cube yards to the yard forward, or, in other words, the filling which is out of sight will be double the filling which is in view above the section ground line.

Apart from the large amount of filling consumed in forming this semi-artificial island, the progress of the work itself is very perplexing. A long length of the bank may have been raised again, once or twice, to the proper height, and may have carried rails and earth-waggons for some weeks, and then sink all at once several feet. The sinking, too, may not be uniform, but may produce fissures, depressions, and separation of the earthwork which will necessitate much care when bringing forward fresh filling material. The bog may not be of the same consistency throughout, there may be some layers of harder material, such as imbedded trunks of trees, and these may sustain the filling for a time, and then yield under the increasing weight of the superincumbent mass. Even when the embankment is finished throughout, and shows no sign of sinking, it should be very

carefully watched for a long time for any indication of further movement.

When the bulk of the material has been taken out of an earth or clay cutting, the work of trimming the slopes should be put in hand, so that any surplus left on the wings, or sides, may be removed, and carried away before stopping the earth-waggons. The angle of slope having been decided, a battering rule of light wooden boards is made to correspond to the slope, and in form similar to that shown in [Fig. 64]. A plumb-bob is suspended from a fixed point, A; the lower end, B, is then held against a peg or mark which indicates the correct level and width of the cutting at the place, and the upper end, C, is raised or lowered until the plumb-bob string coincides with the vertical line marked on the rule from A to D, and the plumb-bob rests steadily in the space cut for it at D. With this battering rule a length of seven or eight feet, according to the size of the rule, is first trimmed to the correct slope, and by continuing the application of the rule up the side, a correct slope line is obtained from bottom to top of slope at that place. By repeating the process at convenient distances along the cutting, a series of correct slope lines are obtained, and the intermediate space can readily be trimmed to correspond.

The same form of battering rule and method of working is applicable for trimming the slopes of the embankments.

When the slopes of the cuttings and embankments have been trimmed, vegetable soil, which has been laid aside, or reserved as previously described, should then be spread evenly over the slopes to the uniform thickness of not less than four inches, and the whole sown with good grass seeds to form a strong sward.

The trimming, soiling, and sowing of the slopes not only gives a more finished appearance to the earthworks, but the strong grass, when once well grown, binds the surface together, and helps to resist the injurious effects of heavy rains and melting snow.

There are many places abroad where a neat finish to the earthworks is considered quite a secondary matter, or where it would be difficult to obtain suitable soil to spread on the slopes. The earthworks are hurried forward to allow the iron highway to be laid down as quickly as possible, the slopes of the cuttings and embankments are only roughly trimmed, and nature is left

to supply such grass or vegetation as may spring up, or be self-sown.

The fencing in of a line of railway serves the double purpose of defining the boundary of the company’s property, and of forming a barrier for the prevention of trespass of persons and animals on to the line. For our home lines, fencing is compulsory, and the same obligation exists on many foreign railways. In our colonies, and out in the far West of the United States, and in newly opened out countries, fencing, except near towns and villages, is rather the exception than the rule; people and animals roam at will from one side of the railway to the other wherever they find a convenient crossing place, and the cowcatcher of the engine has to be depended upon for throwing aside any animal which may be standing, or resting, on the line of rails at the passing of a train.

The description of fence will be influenced by the locality, and the materials conveniently obtainable. Where stone is plentiful, perhaps brought forward out of the cuttings, and labour cheap, a masonry wall will be found a most suitable permanent fence. Any fence to be of service should not be less than four feet high. A wooden post and rail fence is much in favour in some districts, the posts being firmly set or driven into the ground, and four or five stout bars nailed on to, or set into, the upright posts. This fencing does not last very long, the pieces are small in size, and soon fail from decay. Quick or hawthorn hedges, when fully grown, make a good fence, but require careful attention to prevent gaps being made by roving cattle. They also require constant trimming and cutting. The quicks are generally planted in a mound formed by cutting a continuous ditch, or gripe, as shown in [Fig. 65]. The ditch serves as a drain to take away water running down the slopes of the embankments, small openings in the mounds, or drain pipes through them, forming leaders to conduct the water to the ditch or gripe. The outer edge of the ditch represents the boundary of the railway property, unless specially arranged otherwise.

Galvanized iron-steel wire fencing, if not made too light, is strong and durable, and very easily kept in order.

The wires may be secured to strong wooden posts, which should be creosoted, and not placed too far apart, or to iron posts or standards of angle iron or tee-iron section. The straining-posts,

whether of iron or timber, must be stronger than the intermediate posts, firmly fixed into the ground, and well stayed, to withstand the pulling and tightening of the wires. There are many places where a quick fence would not grow, and where the ground is too soft to carry a wall. In such cases a good galvanized-wire fencing will fulfil all requirements. The strand wire is better than the plain wire, as its method of manufacture necessitates the use of a superior material, and it is easier to straighten and keep in good order. An extra strong fence is often made of six, eight, or more rows of round rod-iron secured to wrought-iron uprights of bar-iron or tee-iron.

In hot countries abroad an excellent fence is obtained by planting a species of cactus or aloe in a similar manner to the quick fences at home, and as shown in [Fig. 66]. These cactus plants are readily obtained, are very hardy and quick in growth, and with their large spike-shaped leaves form such an almost impenetrable barrier that few animals will attempt to pass.

Road approaches to bridges over or under the line, or to public road level crossings, may be fenced in the same manner as the line proper. If quicks are adopted, it will be necessary to put up a light wooden fence also to protect the young plants until they are well grown. Near towns and villages it is frequently found advisable to adopt a specially strong wooden fence, or close-boarded fence, where the approach is an embankment, and too newly made to carry a wall.

Gates for farm or occupation level crossings may be made of wood or iron. As a rule, iron gates are preferred, as they can be supplied at the same cost as wood, and are very much more durable. Gates for public road level crossings have to be so placed that they will either close across the railway or across the road; their length will therefore depend upon the width and angle of the road crossing. It is better to make these gates of wood, so that, in the event of a train running through them, there may be less risk of injury to life and rolling-stock than if they were made of iron. For footpath crossings, small gates, wickets, or stiles may be adopted of such form as may be found most suitable for the requirements.

Culverts and Drains.—Before proceeding with the formation of the embankments, it is necessary to construct the culverts and drains which will be covered over by the earthworks. Any existing drains which may be of too light a description must be

reconstructed in a more substantial manner. It is a simple and comparatively inexpensive matter to rebuild a drain before the earth filling is brought forward, but it is a costly work to open out an embankment, and rebuild a culvert afterwards. Unless the seat of an embankment is well drained and kept free from the accumulation of running water, the earthwork will be exposed to washing away of the lower layers, and consequent subsidence. Each watercourse or open drain must be provided for either by a separate culvert of suitable size or, as may be done in some cases, by leading two or more watercourses into one, and thus passing all through one culvert of ample capacity. When fixing the sizes of the culverts they must not be limited to the normal flow of water, but a large margin must be allowed sufficient to meet extraordinary floods. The depth of the bed or invert of a culvert is a very important point. If laid too high, and the stream above should at any time deepen, the high invert would check the flow of the water, and would also incur the risk of being undermined and gradually carried away. If, on the other hand, the invert be laid too low, it will gradually silt up to the level of the stream-bed alongside, and there will be so much of the culvert space lost for all practical purposes. In cases when the invert of a culvert has to be laid at a special low depth to allow for future improvements in drainage, it is advisable to give extra height from the invert to the crown, or top, so as to provide ample waterway in the event of any silting up in the mean time. Particular care should be taken when building the foundation of a culvert. It has to be laid on the site of the watercourse, or on a new channel which will ultimately form the watercourse, and it should be built sufficiently deep into the ground to avert as far as possible the chance of water finding a course through below the foundation.

The invert may be of stone pitching or brick if the current is not rapid, or liable to bring down stone boulders from its gravelly bed.

With a stream-course having considerable fall, and which carries with it large stones, roots of trees, and other débris, the invert should consist of strong pitching, composed of large-sized, rough-dressed stones of hard, durable quality, capable of withstanding the pounding of the boulders brought down during floods. A soft description of stone would be quite unsuitable for the invert of such a stream; the pitching would wear away

quickly, break, and become detached, leaving the foundation and side walls exposed to the cutting inroads of the water.

Where large flat bedded stones or flags of tough quality can be obtained, they form good covers, or tops, for culverts up to two feet in width. They should have not less than nine inches bearing on the side walls, and their contact edges should be fairly dressed, so as to fit sufficiently close to prevent the embankment filling from falling through.

Where the stream, or run of water, is very small, strong earthenware pipes, 9 inches or 12 inches in diameter, well bedded, may be sufficient to carry away all the water likely to arise. For small springs in low swampy ground, dry stone drains may in many cases be used with advantage. These are made by cutting a trench, say two feet deep by twelve or eighteen inches wide, in the seat of the embankment from side to side, and filling it up with dry rubble stones, not boulders, hand-laid, the upper layer placed on the flat to keep the earthwork as much as possible from filling in between the stones.

In soft boggy ground, where the depth to a hard bottom is very considerable, wooden culverts are frequently adopted. Although these cannot be classed as permanent structures, still, when they are made of sound well-creosoted timber, and substantially put together, they last for a number of years. Sometimes they are made cylindrical in section—a species of elongated cask with strong iron hoops every few feet. Others are rectangular in section, made with two strongly trussed side frames connected and covered with cross-planking and longitudinal tie-planking on the top and bottom.

Wooden culverts are seldom made of very large size, rarely exceeding an opening of 3 feet, and it is considered preferable to use two of these culverts of moderate dimensions than one of large size. [Figs. 67] and 68 give sketches of wooden culverts of cylindrical and rectangular section, and [Fig. 69] of flag top culverts of 12-inch, 18-inch, and 2-foot openings. In masonry culverts the side walls are shown to be of rubble stonework, but brickwork can be used instead, provided the bricks are well burnt, hard, and capable of withstanding the action of the water.

In [Figs. 70 and 71] are shown types of arch-top culverts of 4 feet and 6 feet span respectively. The arch portion is shown to be of brick, which, as a rule, is cheaper than stone rings, which

must be cut and dressed to suit the small radius of the arch. The side walls may be of brick of good quality. Occasionally they are built of concrete. The wing walls may either be carried out in the direction of the stream, as in the sketch of the 6-foot culvert, or they may be built transverse, as shown on the 4-foot culvert, whichever arrangement is found to work in the best for the case in question.

For arch culverts on very steep side-lying ground it is better to build the arch-top in steps, as shown in [Fig. 72], instead of forming it parallel to the invert, or slope, of the stream-course. The level portions of the arching give a better hold for the embankment than could be obtained on a long inclined surface of brickwork or masonry.

The writer has built a large number of culverts of this type for mountain streams on steep hillsides, and has found them to prove satisfactory in every way.

In embankments alongside tidal rivers, or across the corners of estuaries of the sea, culverts have frequently to be so constructed that they will permit the passage of the drainage water from the land, or high side, without admitting the tidal water. This can be arranged by placing at the lower end of the culvert close-fitting hinged-flap valves opening outwards. When the tide has gone down the weight of the fresh, or land, water swings the flap-valve sufficiently open to allow of a free passage; and, on the other hand, when the tide rises, the pressure of the water against the face of the flap-valve keeps it tightly closed, and prevents ingress of the salt water.

Culverts are sometimes fitted with lifting-valves or doors, which can be raised or lowered to serve irrigation purposes. The door, which works in guides, is made sufficiently heavy to fall with its own weight, and the raising is effected by means of a screwed suspension-rod working in a well-secured fixed nut.

In cases of soft or treacherous ground, timber-piling or wide bed-courses of cement concrete are necessary to form firm foundations for culverts. Drains and streams which are intersected by a railway cutting have to be dealt with according to their size and their height above the finished rail level. The water from a small drain or field spring may be conducted in pipes down the slope of the cutting into the water-table, or side drain, at formation level, and will be thus carried away to the lower level at the entrance of the cutting. In many cases

streams can be diverted, and the water led away to some lower point without the necessity of actually crossing the railway. With a large stream, where it is essential that the water should be conveyed across the line and continue on its ordinary course, it may be carried over in iron pipes or iron trough if there is ample headway, or in iron syphon pipes where the height is not sufficient. The iron pipes or trough can be supported on masonry or brick piers, or cast-iron columns, the height from the rails to the underside of the conduit being not less than that adopted for the over-line bridges.

Occasionally the pipes can be carried across on an over-line bridge, either by placing them under the roadway or on small brackets outside the parapet.

With the syphon arrangement the iron pipes must be laid down the slopes of the cutting and under the road-bed of the permanent way. The pipes must be continuous, strong, and firmly connected at the joints to prevent leakage. The inlet and outlet ends of the pipes should be securely built into receiving-tanks of masonry, brickwork, or concrete, to ensure an uninterrupted flow of the stream, and also to prevent any of the water from percolating through under the pipes and on to the railway. As a precautionary measure, it is well to place iron gratings some little distance in advance of the syphon pipes to intercept and collect any brushwood, straw, or other things which might be brought down with the stream.

[Fig. 73] gives an example of the syphon arrangement as constructed with two cast-iron pipes placed side by side.

Railway works carried out in cities and large towns, whether they take the form of cuttings, embankments, arching, or tunnels, are certain to cause a very considerable disturbance of existing drains, corporation sewers, gas-pipes, water-mains and underground telegraph wires. Some of these underground works may be so peculiar and complicated as to necessitate a slight deviation from the course originally intended for the line. Suitable provision will have to be made for each of the items interfered with by the railway, and the substituted work must be carried out to the satisfaction of the constituted authorities within the municipal boundaries.

Bridges.—Amongst the many bridges and viaducts which have to be built during the making of a railway those constructed over rivers and waterways are generally the most important

The bridging across any navigable river or tidal water can only be effected in compliance with conditions imposed by the authorities controlling the navigation rights. These conditions will place restrictions as to the number and distance apart of the piers, as well as the height from high water level to the under side of the arches or girders. For rivers having a constant traffic of sea-going vessels of large tonnage and lofty masts the authorities will demand great height or headway as well as large spans; and if to this be added a deep water-way and bad foundations, the work to be constructed becomes one of considerable magnitude. The banks of the river must be carefully studied to find the most favourable point for crossing, and in some cases it may be prudent to make a detour of two or three miles. The crossing at a great height involves the construction of the approach lines at a great height also. If the river is in a deep valley with high sloping sides the natural contour of the ground facilitates the formation of the approach lines; but with a river on a low, wide, open plain, inclined approach lines add enormously to the cost of construction, as well as to the cost of permanent working.

If the number of sailing craft passing up and down the river be moderate, and, perhaps, only passing at high water, the authorities may permit a low-level viaduct with an opening bridge.

There are thus the two systems: the high-level viaduct, which allows trains to pass over and vessels to pass under at any and all times, and the low-level viaduct with opening bridge, which, if open for vessels, is closed for trains, or vice versâ.

Every crossing of a navigable river will have to be considered and dealt with according to its own individual requirements. An arrangement suitable for the one may not be admissible or prudent for the other. A frequent and important train service might be much interfered with by an opening bridge, and, in a similar manner, an opening bridge might cause much interruption and detention to the navigation of the vessels on the river.

Where a low-level viaduct with opening bridge can be adopted, there will be a very great saving of expenditure; and there are numbers of such viaducts in existence, accommodating a large railway and river traffic without inconvenience. Even with a low-level viaduct the height from water-level to the

under side of the girders of the various fixed-spans will generally be sufficient for the passage of barges and small craft, leaving the opening portion to be used by the larger vessels.

The principal openings for these large river viaducts are generally constructed for girders, partly on account of the greater facility of girder work for large spans, and also for the advantage of having one uniform height, or headway, from pier to pier.

For a high-level viaduct across a deep-water river, the cost of the lofty piers forms a very important part of the undertaking. Each pier will require its own cofferdam, caisson, or other appliance for obtaining a suitable foundation. The deeper the water, the more costly the arrangement for foundation; and the higher the pier to rail-level, the greater the amount of material in the construction of the pier. The consideration of these two points will at once show that it is very desirable not to have more of these costly piers than is actually necessary, and in studying out the design it will be a question for calculation how far the spans may be increased so as to dispense with one or more piers.

In every work of this description there is a relative proportion between span and height, which will give the most economical result from a cost point of view; the proportion varying according to the depth of the water and description of ground for foundations. An increase in the span will naturally necessitate an increase in the thickness of the pier; but where a cofferdam, or arrangement for putting in the foundations, must in any case be made, a small addition to its width may not necessarily form a large increase to its cost.

[Figs. 74, 75, 76, and 77] are sketches of high-level railway viaducts which have been constructed with great height, or headway, to allow large vessels to pass under at all times without interruption. This description of work is very costly, not only in the deep-water foundations, but also in the heavy scaffolding and appliances requisite for building piers and girders at such an elevation above the ground-level. The hoisting of the material alone forms an important item where such vast number of pieces have to be lifted to a height of 80, 90, or 100 feet.

[Figs. 78 and 79] are sketches of low-level viaducts constructed with one large opening span, or swing-bridge, for the passage of vessels. The girders and roadway of such opening span are

usually constructed as a compact framework, which revolves on a centre placed in the middle of a circular roller path or species of turn-table. The portions of the rotating opening bridge, although not always the same length on each side of the centre-pin, are generally very carefully balanced, to preserve the equilibrium of the entire mass when swinging round for the passage of vessels. To ensure stability in working, and steadiness during heavy gales, a liberal diameter should be given to the roller path of all swing-bridges having large span and great weight.

Lattice, or truss, girders are preferable to plate girders for swing-bridges of considerable opening, as they present less surface area to the action of the wind.

The opening and closing of these bridges is effected by wheel-gearing actuated by hydraulic, manual, or other motive-power. The revolving machinery should be set solid and true, well protected from the weather, and, at the same time, readily accessible for constant inspection, lubrication, or repair.

[Figs. 80 to 85] are sketches of various types of railway bridges constructed for smaller openings across narrower rivers, water-ways, or canals. [Fig. 80] is an example of what is known as a bascule bridge. This particular bridge is made in two halves, meeting in the centre of the span, the tail end of each half being provided with heavy counterweights to assist in opening or tilting up the bridge for the passage of vessels, or lowering it down for railway traffic. Each half of the bridge swings on horizontal axles, and the raising or lowering is effected by means of hand winches or other motive-power, actuating wheel-gearing working into toothed vertical segments attached to the tail end of each half. The same principle has also been applied to bridges having only one leaf to tilt up to clear the passage way.

Railway bridges of this pattern are now very rarely adopted. They have the great drawback that when raised to the vertical position, a very large area is presented to the action of the wind, and this defect might lead to very serious consequences in the case of a bridge situated in an exposed locality. An open-work floor diminishes the wind area, but a very large surface must necessarily remain.

[Fig. 81] illustrates what is known as a traversing bridge. In this case the width of the opening passage-way and the

adjoining span are made the same, and the girders for the two spans are constructed in one continuous length. By means of gearing attached to the fixed portion of the work, the continuous length of girder, with its roadway, is first slightly raised or lowered, and then drawn back on rollers sufficiently far to leave the opening span quite clear for the passage of vessels. A reverse movement of the gearing causes the movable girders and roadway to travel back and return to their original position ready for the train traffic.

Opening bridges are sometimes constructed on this system in cases where the level of the rails is only a few feet above the level of the water, and where there is only one water opening, and that not more than 20 to 30 feet wide. In such bridges the movable portion is rolled back along iron rails, or plates secured to masonry walls, or strong pile-work. This class of bridge is cumbersome, slow to move, and is now but very rarely adopted.

[Fig. 82] shows a type of simple lift bridge, of which there are but few examples remaining. In this particular bridge the girders and roadway form a solid framework, which rests on the abutments during the passage of the trains. Strong chains, secured to the corners of the framework, pass over large sheaves on the top of the iron standards, and then round drums placed below the level of the rails, and terminate by attachment to heavy counter-weights suspended in iron cylinders. The counter-weights are adjusted to approximately balance the bridge, so that a moderate power applied to the wheel-gearing on the drums is sufficient to raise the roadway to the required height. This class of opening bridge is only suitable for the passage of barges and small craft without masts; and it requires the re-adjustment of the counter-weights when the roadway varies in weight, in consequence of rain or repairs.

[Figs. 83, 84, and 85] are sketches of small swing-bridges constructed for narrow waterways. Although differing in appearance, they are all practically on the same principle, with centre pin and roller path, and are similar in general arrangement to the large-size-opening swing-bridges shown in [Figs. 78 and 79].

The swing-bridge arrangement is so simple in construction, convenient for inspection, and easy to maintain, that where possible it is now generally adopted in preference to any other

system. The weights on centre pin and roller path may be distributed as considered most expedient, and by means of suitable appliances the weight may be altogether taken off the centre and rollers when the bridge is closed for the passage of trains.

There are many wide rivers which, although not navigable in the ordinary acceptance of the term, nevertheless require bridges of large spans to provide free waterway for the floating down of rafts of timber. Away in the high ground, in the timber-growing districts, trees are felled, sawn or cut into long poles, logs, or scantlings, and hauled to the banks of the river. The timbers are then formed into large rafts of the most convenient form for floating down to the place of distribution or port for shipment. Even with old experienced floaters, using their long sweeps in the most skilful manner, it is difficult to take anything but a very irregular course down the stream. Under the most favourable circumstances one of these large rafts is an unwieldy, awkward craft to manage; but in a river full of twists and turns, with reaches varying from comparative smooth water to miniature rapids, the current carries the huge mass surging along, and only a clear, unobstructed channel will enable its navigation to be carried out with safety. The presence of a pier in the main waterway might cause destruction to the rafts and loss of life to the men. The vested interests in floating rights are tenaciously guarded, and no new bridge would be sanctioned which would in any way interfere with the waterway or endanger the passage of rafts down the river. Bridges of this description are much less costly than those over deep water—navigable rivers. Excepting the large spans, the rest of the work is comparatively simple. The water is generally shallow, and much reduced in quantity during the summer months. Good foundations can generally be obtained without going to any great depth. The headway may be kept low, or of such height as may best suit the purposes of the railway, and be sufficiently well up out of the way of the floods which may take place from time to time on the river.

[Fig. 86] is a sketch of a bridge constructed over a river much used for rafting purposes. The large span is over the main channel, and the small spans are over a wide gravelly foreshore, which is only covered with water during exceptionally high

floods in the autumn or winter. No rafting can be carried on when the river is in flood; the current would be too strong to permit of the raft being kept under control.

[Fig. 87] is a sketch of a similar bridge where the river is confined to a regular channel between two sloping banks of strong clay.

[Fig. 88] shows a bridge erected over a narrow rocky pass in the river. The channel is hemmed in by the almost perpendicular sides of mountain granite, there are no banks to overflow, the flood waters cannot spread laterally, however much they may increase in depth, and with building-stone at hand in abundance, and foundations formed in the solid rock, the situation is one of the most favourable for a strong permanent bridge. The cast-iron arch of 150-feet span has a graceful appearance, and harmonizes well with the surrounding scenery. A small masonry arch at each end of the bridge provides for communication along the banks of the river.

With rivers which are neither under the control of navigation authorities nor used for rafts of timber, there is much greater freedom for the designing and carrying out of bridges or viaducts suitable for the actual physical conditions of the locality. The headway will be guided only by the height of the railway to be carried across, and by any flood-water levels which may affect the work. The size of the spans will be regulated by the width of the river, the depth of the water, and the nature of the ground into which the piers have to be built. For broad, shallow rivers with good firm river-beds, piers may be built at moderate cost, and comparatively small spans adopted; on the other hand, with a broad deep river it will be better, as previously explained, to reduce the number of piers and increase the span. In the one case, for example, a river 150 feet wide may be crossed with three spans and two piers in the shallow water, as in [Fig. 89]; in the other it may be more prudent and economical to cross in one span, without any intermediate pier, as shown in [Fig. 90].

Next in importance to the large bridges and viaducts over rivers are the viaducts which have to be constructed for the crossing of deep inland valleys. The occurrence of one of these deep valleys between long lengths of average table-land renders necessary either a series of cuttings and falling gradients to get down to a low level, or the erection of high-level works

to continue onward the rail-level at the height already attained. A decision to adopt the latter course brings forward the consideration as to the method of carrying out the work. To form a high embankment across such a valley would entail an enormous expenditure for earthwork, and several openings, or bridges, would have to be made in the embankment for streams, rivers, and roadways. Instead, therefore, of making this part of the line entirely of embankment, it is usual to carry the earthwork forward until the height is about 25 or 30 feet, and to form the remainder of the opening of arching, as shown in [Fig. 91].

This arrangement is not only less costly than an embankment of such height, but has also the great advantage that any or all of the arches are available for the passage of streams, rivers, roads, and accommodation works.

The character of the work to be carried out in the construction of bridges or viaducts over rivers or valleys must greatly depend upon the description of materials at command. Where good building-stone is plentiful, and the price of labour moderate, works of masonry should be adopted as far as practicable. Brickwork is an excellent substitute for masonry, provided that specially selected bricks are used for all facework, or parts exposed to the weather. For water-washed piers and abutments, the lower portion should be faced with good hard stone.

Bridges and viaducts consisting of arches of masonry or brickwork form the most substantial and permanent works of construction for railway purposes; once properly built, the expenditure on future maintenance or repairs is merely nominal. For viaducts the span of the arching must be regulated by the height of the viaduct. The greater the height the larger the span. In one case 30-feet spans may be suitable, whereas in another it may be more economical to introduce spans of 60 feet or more, and so reduce the number of lofty piers. From a cost point of view there is, however, a limit to the span of arching, and, except for special cases, where expenditure is of secondary importance, large spans are very rarely adopted. Arches of large spans, no doubt, have been built both in masonry and brickwork, and have been a complete success in every way except expense. Unfortunately, the quantity and weight of materials in arching, and the corresponding cost, increase

very rapidly as the span increases, and for openings of more than 60 or 70 feet girder-work becomes much cheaper than arching.

[Figs. 92] and [93] are examples of viaducts having piers of masonry, with girders to carry the roadway. In the one case the roadway is carried on the bottom flange of the girders, and in the other on the top. The latter arrangement affords greater facility for securely bracing the girders together, while for the former it is claimed that the girders form a massive parapet, which would serve as a protection in the event of an engine or vehicles leaving the rails.

In the early days of railways, many large viaducts were constructed having masonry piers, and timber trusses to carry the roadway. Much ingenuity was displayed in designing the trusses, and in the introduction of cast-iron joint-shoes and wrought-iron bracings. Many of these wooden superstructures served well for several years, but they were always exposed to the imminent risk of destruction from fire, and however carefully the logs may have been selected, the decay of the timber was only a question of time. The deterioration of one piece was equivalent to the weakening of the entire truss, and the renewal of any part was both difficult and costly. The shrinkage of the timber, and the working at the joints, caused the trusses to deflect considerably under a passing load, and although the actual strength of the structure may not have been much impaired, the creaking and depression had anything but a reassuring effect. Timber superstructures for anything but small spans are rarely adopted now, except for temporary works, or on lines abroad, where the transport on girder-work would be very costly, and where good timber is very cheap and abundant. Even in the latter case the wooden superstructure is generally looked upon as a temporary expedient, to be replaced at no very remote date with iron or steel girders, when the materials can be conveyed over the entire completed line.

[Figs. 94, 95, and 96] are sketches of three types of timber trusses as constructed in viaducts of several spans.

There are many localities, especially abroad, where suitable stone is most difficult to obtain, and very expensive to work and convey. In such cases it is compulsory to use as little of it as possible, and to resort to iron or steel both for the girders and a large portion of the piers. The piers may be made of cast-iron,

wrought-iron, or steel, of suitable form and arrangement to ensure strength and stability. Not only must the piers be strong enough to carry the weight that may be brought upon them vertically, but they must have sufficient width of base to ensure lateral steadiness. The design should admit of facility of erection, with a minimum of scaffolding, and the pieces should be of convenient length and weight for transport. The lower length of river piers, or portion liable to be in contact with flood-water, should be of solid masonry, to resist the action of the water, or of any débris brought down by the current. More than one fine viaduct has been swept away for want of due attention to the latter precaution.

[Fig. 97] illustrates a type of pier composed of cast-iron columns, well braced and stayed with wrought-iron. The ends of the columns and all contact surfaces should be properly turned and faced by machinery to ensure true and perfect joints, and the socketed ends should be turned and bored to fit closely. The latter is important, and if not carefully carried out, a slight sliding movement of the flanges may take place, and throw undue strain on the bolts.

[Fig. 98] shows a very similar pier, constructed entirely of wrought-iron or steel.

Each of the above-described piers has a liberal amount of taper or batter, both in the front and transverse elevation.

The size and number of the columns, and the dimensions of the braces or stays, will depend upon the height of the pier and the weights and strains to be sustained.

Many important and lofty viaducts have been erected on this principle of iron piers springing from masonry foundations, more particularly across deep rugged ravines abroad, where iron piers offered the only practical, substantial means of dealing with what appeared otherwise an impossibility.

[Fig. 99] is a sketch of the Kinsua Viaduct on the Erie Railway, one of the highest railway viaducts in the United States. In the transverse elevation the piers have a large amount of taper; but in the front elevation they are vertical, and of width to correspond to one of the small spans of the main girder. This arrangement of long and wide base gives great stability to the pier. The spans of the girders, which are of the ordinary lattice type, are not large, being 61 feet for the clear spans, and 38 feet 6 inches for those over the piers. The principal interest is in

the great height and simplicity of the piers. The rail-level over the top of the pier is 301 feet above the level of the water in the Kinsua stream. The width of this pier on the top is 10 feet (for single line), and the width at the bottom 103 feet.

[Fig. 100] is a sketch of the Loa Viaduct on the Antofagasta Railway, Bolivia, stated to be the highest railway viaduct in the world. The arrangement of spans and piers is very similar to the Kinsua Viaduct. The main spans are 80 feet, and the pier spans 32 feet. The width of the pier on the top is 10 feet 6 inches (for single line), and the width at the bottom of the highest pier is 106 feet 8 inches.

In contrasting these light iron piers with what would have been required if constructed of masonry, an idea may be formed of the enormous amount of material, labour, and time, which would have been expended to erect the work in stone.

Before the principle of lofty iron piers had been thoroughly developed, many high piers had been built of timber both at home and abroad. More particularly was this the case in the United States of America, where the presence of magnificent timber close to hand offered special inducements for the use of wood. Like a mammoth scaffolding, each pier was constructed with a most liberal supply of material, judiciously selected and carefully put together, but the danger of destruction by fire was ever present from the beginning. Probably more timber piers and bridges have been destroyed by fire than have been removed on account of natural decay.

One of the most notable of these timber-pier constructions was that of the Old Portage Viaduct, on the Erie Railway, U.S.A. [Fig. 101] is a sketch of one or two of the piers. This viaduct was more than 800 feet long, and 234 feet high from the bed of the river to the rail-level. The spans were 50 feet each. Masonry piers were carried up to about 25 feet above the ordinary water-level of the river, and upon these the timber superstructure was erected. Each timber pier consisted of three complete sets of framework, securely connected together, and also well stayed and braced to the adjoining piers. This viaduct was destroyed by fire in 1875, and was reconstructed with piers and girders of iron.

Railway bridges over or under public roads of primary or secondary importance must be constructed to the widths and heights prescribed for such works in the fixed regulations of the

country in which they have to be built. As a rule, these road-bridges are simple and inexpensive in character, except in towns, or in cases where the line crosses the roads very obliquely, or where the road is situated at the top of a deep cutting, or bottom of a high embankment. Away from towns and out in the open country, permission is generally obtained to divert the roads to a moderate extent, so as to obtain a more favourable angle and height for the bridge; but in towns, where the roads become streets, sometimes of great width, with houses and shops on each side, little or no diversion can be allowed.

A railway passing through a portion of a densely populated town must deal with the streets as they exist, as any great alteration in their course or continuity would involve a large destruction of property. With careful laying out it is possible to obtain favourable crossings for many of the streets, but a number of others must be crossed obliquely, and these oblique crossings very frequently result in a span twice the width, or even more, of what would be necessary to cross the street on the square. Bridge-work in towns is more costly than in the country, as a higher class of work is demanded, more finish or dressed work in the masonry or brickwork, and more ornamentation in the screens and parapets in connection with the iron girder-work. The work itself has to be carried on in a confined locality, with limited space for materials and appliances, and where the thoroughfare must be kept open.

Where the height is sufficient, and suitable materials readily obtained, it is preferable to adopt an arch bridge, as being of a much more permanent character than girders.

[Fig. 102] is an example of an ordinary over-line arch bridge to carry a public road over a double line of railway in a cutting of moderate depth.

[Fig. 103] shows a somewhat similar over-line arch bridge, but its height from rail to road-level being greater, side arches are introduced in preference to long heavy wing walls.

[Fig. 104] shows an over-line arch bridge in a rock cutting. In this case, by increasing the span and forming the springing bed in the solid rock, the masonry of abutments and wing walls may be reduced to a minimum.

[Fig. 105] is a sketch of an ordinary under-line arch bridge to carry a railway over a public road in an embankment of moderate height.

[Fig. 106] shows a similar under-line bridge, but with curved instead of straight wing walls.

[Fig. 107] is an example of an under-line arch bridge in a rather high embankment, and where side arches have been adopted instead of long wing walls.

The above six types are equally applicable for private roads crossing the railway, but, as previously mentioned, a lesser width and headway will be accepted for under-line bridges for private or occupation roads, than for public roads. For the over-line bridges, however, the width and headway will be regulated by the number of lines and standard height of the railway.

When these arch bridges have to be built on the skew to suit an oblique crossing of the road, extra care will be necessary in setting out the work, and marking on the centering the spiral courses of the arching.

Arch bridges may be built of masonwork or brickwork, or a combination of the two. If the available quarries do not yield good flat bedded stones readily worked, it is better, where possible, to use strong hard bricks for the arching, and utilize the stone for the remainder of the work.

Although arching undoubtedly forms the most durable type of bridgework, numbers of cases occur where the available height or space between rail-level and road-level is too small, or the cost of masonry and brickwork too great, to admit of anything but girder-work. Detailed sketches of some of the many forms of girder bridges are given in [Figs. 132] to [153], illustrating various systems of roadways and parapets. In some instances the main girders are made sufficiently deep to serve as parapets, while in others a shallower girder has been adopted, on top of which has been placed a light cast-iron parapet composed either of close plate-work or of ornamental open railings. The open ironwork parapet has a good appearance, but as a screen is not so efficient as the close cast-iron plates.

In addition to the bridges required for the regular public roads, it is usually necessary to construct a certain number of occupation or private road bridges over and under the line to accommodate portions of estates and large properties intersected or severed by the railway, and which would be inadequately provided for by ordinary gate crossings on the level. The position and description of these occupation bridges is generally matter of private arrangement. The bridges will be somewhat

similar in character to the public road bridges, but of much less width for the roadway. Those over the railway must have the standard span and height adopted as a minimum for the other over-line bridges, and those under the railway must have the full width on the top for the lines of rails, but will have less width between the abutments for the roadway.

Foundations.—So much depends upon the soundness and security of the foundations of any bridge, viaduct, or large building, that it would be almost impossible to devote too much care to the selection and treatment. Unless the foundation be firm, the entire structure will be exposed to the risk of failure, either in subsidence of masonry, giving way of arches, or depression of girders. A small matter overlooked during the construction of this part of the work will be most difficult to correct or adjust afterwards.

The insistent weight of all structures built of masonry or brickwork will cause the mass to settle to a certain extent, according as the joints of mortar or cement become compressed by the number of superincumbent courses. In a similar manner the gravel and clay of a foundation will compress more or less according to its compactness and the weight of the structure. No inconvenience will, however, arise if the settlement or compression be uniform throughout the entire area.

In ordinary average, dry, solid ground, a good foundation can usually be obtained at a moderate depth. The removal of a few feet of the surface layers will generally lead to a good hard stratum of natural material sufficiently firm to carry the abutments and piers of railway bridges and viaducts. Two or more footings are usually adopted so as to distribute the weight over an increased area, as shown in [Fig. 108].

Where the weight to be carried is considerable, it is better to increase the number of the footings, and give them a smaller projection, as in [Fig. 109], rather than have a lesser number and greater projection, as in [Fig. 108]. There is greater liability of fracture of the material in the latter than in the former.

Care must be taken to distinguish between made ground and natural ground. Hollows which have been filled in must not be relied upon to sustain heavy weights; the material may have been consolidating for years, but it is safer to cut through it and found upon the natural stratum beneath.

Soils of a clayey nature must be dealt with very cautiously.

If the ground be fairly level, and the material firm, a solid foundation may be obtained, but the excavated portion should be covered up as quickly as possible to prevent any decomposing action taking place upon exposure to the open air. The expansive nature of some clays must be carefully kept in view, so as to guard against any disturbance in the finished foundation. There are some descriptions of shale which when first opened out appear to have the solidity of hard rock, and yet, after a few days’ exposure to the atmosphere, are changed to the consistency of soft mud.

Sand, being composed of such small particles, is almost incompressible, and makes an excellent foundation so long as it can be retained in its position. Little or no settlement will take place if the sand remains undisturbed, but so soon as it comes under the influence of running springs, or underground drainage, the fine particles of the sand will be gradually but surely carried away with the water, and the entire foundation be undermined. The opening out of a neighbouring excavation, or the carrying out of some low-level drainage, would endanger a construction which otherwise would be solid and permanent.

In many cases of soft ground, more particularly abroad, sand piles have been adopted and have given very good results. The system is carried out by first driving a large wooden pile down through the soft material into the more solid stratum below. The timber pile is then carefully withdrawn and the cavity filled with clean sand. The number and distance apart of these sand piles will depend upon the nature of the ground and description and weight of structure to be carried.

Clean, compact gravel is one of the best materials to build upon, being almost incompressible and quite unaffected by exposure to the atmosphere. It is easily excavated and levelled off to the surface required.

A foundation of rock may be considered in the abstract as the most solid base to be obtained, but it must be treated judiciously, and a proper surface secured. The outer portion of many descriptions of rock consists of blocks or layers of stone partially or entirely separated from the main bed, and these, lying in a loose condition, are deceptive and treacherous as a foundation base. The exposed rock should be carefully examined, and all detached or outlying pieces or layers removed before placing any foundation course. Special care must be paid to all

shelving rock, and a level seating cut into it for the entire width of the foundation, as shown in [Fig. 110].

A thick bed of concrete, as in [Fig. 109], makes an excellent foundation course. When firmly set it becomes one solid massive base from end to end, and prevents the yielding or dropping of masonry at any intermediate points.

There are many places in soft, wet ground where instead of attempting to excavate all the soft material down to a harder stratum, it is better to adopt timber pile foundations, as shown in [Fig. 111]. The size of the piles and their distance from centre to centre must be regulated by the description of material into which they have to be driven and the weight they have to sustain. Double waling pieces should be properly checked and bolted on to the heads of the piles, and trimmed or levelled off to receive a double floor of thick planks. The spaces round the heads of piles and walings should be filled in and levelled up to under side of flooring, with cement concrete.

For bridges of moderate span, over soft ground or over shallow fresh water, strong cast-iron screw piles can be adopted with great advantage. [Fig. 112] shows a very usual form of screw pile, made with an external screw at the lower end and with a sharp cutting edge to facilitate penetration into the ground. The upper portions are made in suitable lengths, and all to one pattern and template, for convenience in carrying out the work. The screwing into the ground is generally effected by means of a capstan or cross-head fixed to the top of the first working length of pile, and which is pulled or turned round by ropes worked from stationary windlasses. In some cases long bars or levers are attached in radiating positions to the capstan-head, and a number of men are employed to walk round and round, pushing the levers, and in this way screwing the pile into the ground. As the pile goes down the capstan-head has to be removed, and additional lengths bolted on, until the pile enters a solid stratum, or is considered deep enough for the duty it has to perform. The last or top length has generally to be cast to a special length to bring the work up to the exact height to receive the girders. The core of excavated material passes up into the interior of the pile, and in some cases becomes so compressed or tight as to require the use of an internal augur to remove a portion of it to enable the screwing to proceed. The pile shown in [Fig. 112] is one of a number which were successfully screwed

into the ground to depths varying from 42 to 48 feet. A toothed or serrated edge, as in [Fig. 113], is sometimes given to the lower edge for screw piles which have to cut their way through a hard stratum.

All bolting flanges should be accurately turned and fitted to ensure close, parallel surfaces when bolted together.

The joint shown at A, [Fig. 112], is one the writer has used to a large extent for the bolting flanges of cast-iron screw piles and cylinders. It is very simple in form, readily coated with white lead to ensure a water-tight joint, and as the upper length is practically recessed, or let into the lower length, the exact continuity of the different castings is secured.

Solid screw piles of wrought-iron or steel, similar to [Fig. 114], are used for some descriptions of work. These are generally made in long lengths, in sizes varying from 4 to 8 inches in diameter, and with screw blades of wrought-iron or cast-iron fixed in the most secure manner to resist the strain produced when screwing into the ground. The couplings for these solid piles must be very carefully made, all contact surfaces truly faced and fitted, bolts turned, and bolt-holes drilled.

[Fig. 115] is a sketch of a hollow cylindrical water-jet pile, which has been used successfully in cases of light sand. The lower end of the pile is made externally in the form of a solid disc, terminating in a conical point, having an aperture in the centre to correspond to the water-jet. To the top of the pile is secured a tight-fitting cover through which a tube passes from a force pump. Water at high pressure is pumped into the tube, and as it forces its way out through the conical point the sand is stirred up and loosened, and thus allows the pile to descend. When the pile has been lowered to a sufficient depth the pumps and tube are removed, and the sand settles down into its former compact condition.

Great care must be used with the first two or three lengths of any screw pile to ensure the pile taking a correct or true vertical position. Each series of screw piles should be properly braced together to obtain stability under moving loads.

Hollow cylinders of cast-iron, wrought-iron, or steel form most efficient foundations or piers for large bridges over soft ground or fresh water of considerable depth. Made open at the bottom, and constructed of complete rings, or, if of large diameter, of rings built up in segments and securely attached together

with water-tight joints, the cylinder is placed in its proper position on the ground or lowered into the water preparatory to sinking. The lower length is made with a sharp cutting edge to facilitate penetration. By excavating and removing the material round the cutting edge and base inside the lower length, the cylinder descends gradually either from its own weight or by assisted weights, and length after length is added until it is sunk to the depth required. The excavated material is filled into buckets and hoisted to the surface by a winch fixed on the top length. When sinking in water the working top of the cylinder is always kept at a suitable height above the water for convenience in removal of the earth or clay from the interior to barges or gangways alongside.

Some strata are more favourable for cylinder sinking than others. Material of a strong clayey nature admits but a small amount of water into the excavation, and a moderate-sized pump will keep the working fairly dry until considerable depth has been reached. Some other materials are so open that the water cannot be kept down with ordinary pumps, and the cylinders can then only be lowered by the pneumatic process. This process has been carried out in two methods, one of them on the vacuum principle, and the other by air pressure, or, as it is termed, the plenum system. With the former method the cylinder is placed in position, and an air-tight cap, through which a pipe passes, is secured on the top. Powerful air-pumps are then set to work, and the partial vacuum thus created in the interior causes the material round the cutting edge and base to be loosened and drawn into the cylinder, the cylinder at the same time going down or sinking by its own weight, or assisted, if necessary, by added weights. The cap is then taken off, and the material removed from the interior, the operation of exhausting and emptying the interior being repeated until the cylinder is sunk to its proper depth. This method has been found to work well in strata which contained a large proportion of clay to assist in excluding the air and water, but was not nearly so successful when applied to material containing stones and large boulders.

The plenum process is based on the principle of the diving-bell, the water being prevented from entering at the bottom by keeping the cylinder full of compressed air. An air-chamber, or air-lock, with perfectly air-tight joints, is securely fixed to the

top or upper working length of the cylinder, and no access can be obtained to the interior of the cylinder without passing through this air-lock, which has one lower door or valve opening into the cylinder, and an upper door opening out into the open air. Temporary inside staging is formed by putting planks across from flange to flange, and placing short ladders on these landings for the use of workmen descending or ascending. The excavated material is hoisted by a winch, generally placed on the landing just under the air-lock. The air-pump is placed in some convenient position outside, near at hand, the pressure-pipe passing through the air-lock into the interior of the cylinder. Air is forced into the cylinder to a pressure sufficient to drive out and keep out the water from the interior, and allow the workmen free access for excavating the material round the cutting edge and base of cylinder. The amount of pressure required will depend upon the depth of the working below the level of the water alongside. Men accustomed to the process can work without much inconvenience under a pressure of 20 to 22 pounds per square inch, equal to a depth of 45 to 50 feet; but when the pressure exceeds 25 pounds, the duty becomes very trying, and is attended with considerable risk. Instances are recorded of men working at depths of 105 and 110 feet, necessitating a pressure of over 45 pounds per square inch; but it is very questionable whether the men exposed to such a severe ordeal were not permanently affected, if some of them did not actually succumb.

It will sometimes occur that, after sinking through soft porous strata to a considerable depth, a layer of clayey material is penetrated sufficiently retentive to keep out the water and permit of the removal of the air-lock and the completion of the sinking as an open-top cylinder.

When working on the plenum system everything must pass through the air-lock, both materials and men. The excavated material is hoisted up to the level of the air-lock, the upper and lower doors of which must be closed, and the pressure inside the air-lock brought to the same as that inside the cylinder by means of a regulating valve. The lower door is then opened to admit the excavated material, and then closed again to cut off all communication with the interior of the cylinder. The upper door is then opened, and the material hoisted out into the open air. The same process has to be adopted for the egress of the workmen,

and the reverse arrangement for the ingress of men and materials. The shape and dimensions of the air-lock may be varied according to circumstances, but the principle will remain the same.

When the cylinder has been lowered to what is considered a sufficient depth, it is usually loaded with a certain amount of dead weight in the shape of old iron or other convenient material, and allowed to remain loaded for some days to ascertain if it will sink any further. Should this test be found satisfactory, the dead weight is removed, and the interior of the cylinder pumped dry and carefully filled with good cement concrete.

Cylinders for foundations are generally made circular in section, that form being the most convenient for turning and facing the flange-joints. They can, however, be made oval in section, or of any section that may be found most suitable for the work required.[ Figs. 116] and [117] give the particulars of a double-line railway bridge carried on cylinder piers across a river. The detail sketches explain the form of cutting edge, flange joint, and method of bracing. This bridge is one that was reconstructed and widened from a single-line to a double-line bridge. Traffic was carried over on one line while the second line was being erected, hence the reason why one strong central girder was not adopted.

Cylinders of 7 feet diameter and upwards are sometimes filled with concrete in the lower portion, on which is built either a circular lining or a solid mass of masonry or brickwork up to the level of the girder-blocks. In some cases the cylinders proper, together with their concrete filling, terminate a little above the water-level, and upon these foundations are erected strong cast-iron columns, plain or ornamented in design, to carry the girders and roadway. The cylinder itself is generally considered merely as a casing or medium for obtaining a foundation, the weight of the superstructure being carried on the internal filling or lining.

Caissons constructed of plates of wrought-iron or steel are much used for the foundations of large piers in deep water. Practically they may be considered as cylinders on a large scale, with the difference that whereas cylinders are generally continued up to the under side of the girders of the superstructure, caissons are only carried up to a short distance above the water-level. A caisson forms a strong water-tight iron cofferdam, from

which the water can be excluded, and a masonry or brickwork pier constructed inside. It may be made all in one piece to correspond to the form of the pier, or in separate pieces to form one whole, each being sunk independent of the other, and connected together afterwards. Being built up of plates cut to the proper size and shape, it is a very simple matter to rivet on additional tiers of plates as the caisson is lowered deeper and deeper into the bed of the river. The lower length is made with a cutting edge to penetrate the ground; the exterior is made without any projection larger than the rivet heads, and the interior is strengthened with T-irons or double L-irons at the joints, and strong cross-bracing to resist the pressure of the water. About 7 or 8 feet above the cutting edge a strongly framed iron floor is riveted to the vertical sides, and strengthened by plate-iron under-brackets placed at short distances. The excavators work in the space below the floor, and the excavated material is passed up through openings formed in the floor at convenient points to suit the working. The methods of lowering a caisson are the same as for lowering a cylinder. If the pneumatic system has to be adopted, then two or more air-tight tubes of liberal dimensions (say 5 to 8 feet diameter), according to the size of the caisson, must be attached to the floor, and on the top of each of these tubes air-locks must be secured for the removal of men and materials. The masonry or brickwork of the pier is built upon the iron floor, and a portion of this building work is usually carried on during the sinking of the caisson to obtain weight to assist in the lowering. When down to the proper depth, the space below the floor is properly cleared of débris and water, and then carefully filled in with cement concrete.

Some caissons are made with vertical sides throughout their entire height; others have an outward taper for 15 or 20 feet on the lower end. The former are not only simpler in construction, but are more easily kept in a vertical position during the sinking. Caissons are usually put together in some convenient place near the edge of the water, and then conveyed on pontoons to the sites of the piers. Great care is required in lowering them into position in the bed of the river, and guide-piles, guy-chains, and other appliances are frequently necessary to keep them vertical during the sinking.

The form, dimensions, thickness of plates, cross-bracing, and general arrangement will depend upon the size and depth of the

pier to be constructed. Caissons for heavy work on difficult or treacherous ground require great care, not only in their construction, but also in placing them in exact position, and in sinking them correctly to their proper depth. A tilted caisson is a most difficult subject to handle, and entails heavy expenditure to restore it to a true vertical position. By making careful borings, the engineer can ascertain very closely the depth to which the caisson will have to be lowered to obtain a good firm foundation. With this information the caisson can be so constructed that the upper portion, termed the temporary caisson, commencing a few feet above the bed of the river, can be detached, and removed at the completion of the work from the lower or permanent portion sunk below the ground line.

[Fig. 118] gives sketches of a wrought-iron plate-caisson applied to a deep-water river pier, and lowered to its full depth by the pneumatic process; dotted lines show the air-tubes through which the excavated material is hoisted and emptied into barges alongside.

Many large and important pier foundations have been constructed on the system of brick cylinders or wells, particularly in India, where the foundations for large river viaducts have to be carried down to great depths through thick deposits of soft material. These wells are built upon V-shaped curbs to facilitate the penetration when sinking. [Fig. 119] is a section of a well with a wrought-iron curb, and [Fig. 120] is a similar well with a wooden curb. The wrought-iron curb is made in segments for convenience of transport, the pieces forming the complete ring being bolted or riveted together at the site of the foundations. The wooden curb is composed of several thick layers of hard wood planking cut to the proper shape, and laid with broken joints, the whole being bound together with suitable bolts and spikes. In some cases the lower or cutting edge of the wooden curb is strengthened or protected by a sheathing of wrought-iron plates.

Well foundations are usually put in when the rivers are at their lowest, and reduced to a few small channels in the great width of dried-up river bed. This condition enables the greater portion of the curbs to be conveniently and accurately placed in position on dry ground, or on ground which, although soft and muddy, is not covered with water. Should the site of one of the wells occur in one of the small channels, the stream can be

diverted to one side, and a small artificial island made to receive the curb above water-level. When a curb is fairly fixed in position, the work of building the brick well can be commenced. With the wrought-iron curb the triangular cavity between the vertical plate and sloping plate must be filled with concrete to form a level base for the first course of brickwork. The wooden curb being composed of horizontal layers of timber, is ready to receive the brickwork without further preparation. To strengthen and keep the brickwork firmly tied together, strong wrought-iron vertical tie-rods, 1¼ or 1½ inch in diameter, are generally built into the work—as shown in the sketches—at distances about four feet apart. The lower end of the bottom tier of tie-rods is secured to the curb, and the upper end passed through a strong wrought-iron plate-ring, which is continuous all round the brickwork. A long deep nut is screwed down over the top or screwed end of tie-rod until the plate-ring is down tight on the brickwork. The tightening nuts are made sufficiently deep to receive the lower ends of a second series of vertical tie-rods, which in like manner pass through another wrought-iron plate-ring on the next section of brick well, and the same arrangement is continued for the full height of the well. The lengths of the tie-rods will depend upon the lengths of the section of brickwork to be built at a time, and may vary from 10 to 15 feet.

As the work of building proceeds the curb and brick well will sink gradually into the ground, and down to a certain depth, varying according to the material of the river bed, the weight of the brick well itself will effect the penetration and lowering. Beyond this depth the lowering must be done by scooping or dredging the material from the inside of the well, and placing heavy weights of old railway iron or other convenient masses on the top. When one section or length of well has been sunk down, then another set of tie-rods are inserted into the deep nuts, and another section of brickwork commenced. The operation of lowering is rather tedious, as all the weights have to be hoisted up on to the top of the length in hand, and piled so as to leave space for lifting out the material dredged from the interior; and then, when the length has been lowered, all the weights must be removed before the brickwork can be resumed on another length. Where the river bed consists of soft material, the excavation inside the well can generally be

effected by suitable dredges or scoops worked from the surface or top of brickwork. Should trees or other obstructive masses be met with embedded in the strata, it will be necessary to employ divers to remove them piecemeal out of the way of the curb.

When the brick well has been lowered down to the full depth, and is thoroughly bedded in a stratum of strong material, the test weights should be left on for some time to ascertain if there is any further sinking. After all the weights have been removed the bottom of the well can be dredged out clean, and the interior filled in with concrete to such height as may be considered necessary.

Brick wells must be watched carefully to ensure that they sink down in a perfectly vertical position. Any inclination away from the perpendicular must be corrected at once by means of guys and struts, the same as in sinking iron cylinders. The principal difficulty will be with the first 20 or 25 feet.

The diameter of the well will depend upon the weight it has to carry, and its height from river bed to under side of girders. The wells may be either circular or polygonal in section, and built singly or in pairs, as shown in sketches ([Fig. 121]).

Many piers and abutments of bridges in shallow or moderately deep water are built by means of coffer-dams of timber and clay puddle. The coffer-dam forms a water-tight wall round the site of the foundation, from which the water is pumped out, and the excavation carried down to the depth required. In very shallow water it is sometimes sufficient to drive only a single row of piles, and form a bank of good clay puddle on the outside, as shown in [Fig. 122]. In deep water it is necessary to drive a double row of piles, 3 or 4 or more feet apart, and fill in the space between with clay puddle, as shown in [Fig. 123]. The piles for coffer-dam work should be carefully selected, of good timber straight, and correctly sawn on the contact faces. Guide-piles are first driven in proper line and position round the intended foundation. To these strong horizontal double waling pieces are securely bolted, one on each side of the guide-pile, one pair near the top, and the other pair as low down as can be placed. The sheeting piles, which are lowered down between the horizontal waling or guiding pieces, are driven as close to one another as possible, being assisted in doing so by the sheet-pile shoe, shown on [Fig. 124], which is made not with a point like

an ordinary pile shoe ([Fig. 125]), but with a cutting edge slightly inclined, so that in driving the tendency of the pile is to drift towards the pile previously driven. Sometimes the outer row of piles consists of whole balks, and the inner row of half balks; the size of the piles must, however, be regulated by the depth and current of the water. When both rows of piles have been completed, the space between should be dredged out, and then filled with carefully prepared clay puddle. To enable the puddle to adapt itself thoroughly to the wooden sides, it is desirable to remove the inside walings after all the piles are driven, as any internal projections interfere with the proper punning and settling of the puddle. The swelling of the puddled clay has a tendency to force apart the two rows of piles, and to counteract this as much as possible, iron tie-rods should be passed through from side to side every few feet, and screwed up against large washers placed on the outside of the outer walings. Strong struts or cross-bracing of timber must be placed from side to side inside the coffer-dam to resist the pressure of the water in the river. This cross-bracing can be removed gradually as the work of building progresses upwards, and be replaced with short struts wedged in against the sides of the finished courses.

In cases where the ground is soft, and when it is not considered prudent to excavate the foundations deeper for fear of disturbing the stability of the coffer-dam piles, rows of large, square bearing-piles may be driven in the floor of the foundation, as shown in [Fig. 111]. The tops of these bearing-piles must all be sawn off to the same level, and a platform of strong double planking securely fixed to the piles to receive the foundation course of concrete, masonry, or brickwork. The spaces around the tops of the piles and the under side of the timber platform should be filled in with good cement concrete.

The interior of the coffer-dam is kept dry by constant pumping, either by hand pumps or steam pumps, according to the volume of water finding its way into the foundations. When the finished pier or abutment has been carried up above the river water-level, the coffer-dam is no longer required, and may be removed. Sometimes, to save the timber, the piles are drawn by means of strong tackle fitted up for the purpose; but in doing this there is considerable risk of disturbance to the foundations, and it is better to leave the piles in the ground

and employ divers to cut off the tops a little above the bed of the river.

In preparing the design for a large foundation it is absolutely necessary to first ascertain by careful borings the description of material upon which that foundation must be placed, so as to proportion the area of bearing surface to the weight to be sustained. Some materials will naturally carry more weight than others, and although the engineer cannot always select the material he would prefer, he can, however, control the superficial area of the foundations. Much valuable information has been obtained both from experiments and from comparisons of actual practice, and the following memoranda may be useful for reference, as indicating the pressures per superficial foot which may be safely put on various materials:—

Moderately stiff clay	2½ tons.
Chalk	4 ”
Solid blue clay	5 ”
Compact gravel and close sand	6 ”
Solid rock	12 ”

Doubtless the above weights have been exceeded in many cases, but it is better to be on the safe side, and leave a good margin for stability.

Large subaqueous foundations for heavy piers and abutments are costly and tedious, and especially so when the pneumatic process has to be adopted. Special appliances and well-trained, experienced workmen are requisite, and if all the men and materials have to pass through the air-locks, the progress of the work must necessarily be slow. When the foundations have been completed up to the level of the water, the construction can be pushed on more rapidly, as the work of scaffolding, hoisting, and building, can all be carried on in the open air.

Amongst the very many types of arch-work and girder-work adopted for railway purposes, the following examples from actual practice may be useful for reference:—

[Fig. 126] represents small 24-foot span, low viaduct arching suitable for a line passing through towns or villages, where ground is valuable and the area to be covered must be kept as small as possible. The arches may be utilized for stables, stores, or roads of communication between the lands and properties intersected by the railway. The segmental form gives a better headway underneath than the semicircular, besides containing less material in the arching proper, and requiring a smaller

amount of centering. Every precaution should be taken to prevent water percolating through any portion of the arching, or haunching, and a thick layer of good asphalte should be placed over the entire upper surface, and carried well up the lower portion of the parapet walls, as shown on the sketch. The cast-iron pipes with rose heads form a very efficient means of taking away the rain-water which filters through the ballast and filling. The pipes should be carried down in chases, or recesses, built in the fronts of the piers, to protect them as much as possible from injury in the yards below. Rose heads, pierced with holes, and surrounded with small stones hand-laid, serve well to conduct the water into the pipes. Where the arching is of considerable length, recesses or refuges for the platelayers may be obtained by substituting a short length of cast-iron-plate parapet, instead of the stone or brick parapet, over some of the piers, as indicated in the sketch.

[Fig. 127] shows a similar description of arching for spans of 30 feet. The above two examples represent plain substantial work, but if circumstances warrant more external finish, this can readily be added without interfering with the general arrangement. In a similar manner, if considered preferable, the arches may be made semicircular or elliptical.

In the sketches shown of the arched over-line and under-line bridges, the arching and coping of parapets are in brick, and the remainder of the work in stone. In very many cases brick will be found cheaper and more expeditious for arching than stone, unless the quarries turn out stone in blocks which can be conveniently trimmed for arching. All bricks used for arch-work should be hard and well burnt, and special care should be taken in the selection of those to form the under-side course, which will be exposed to the atmosphere. For moderate spans arches have been successfully constructed of concrete. For this description of work the materials should be carefully gauged and mixed together, and the finished work should be allowed to stand some time on the centres to allow the concrete to become thoroughly set.

In [Fig. 102], the cutting being deep, almost up to the level of the public road, the foundations of the wing walls are built in steps, resulting in a minimum of masonry below the finished ground line. Where the cutting is shallow, and the public road has to be brought up to the bridge on an embanked

approach, the greater portion of the wing walls will have to be built up from the solid or original ground, and there will be a large amount of masonry below the finished ground line, as indicated in [Fig. 128].

In some cases of over-line bridges it is necessary to curve the wing walls to correspond to the road which turns off to the right or left after crossing the railway, as shown in [Fig. 129]; or the wing walls may have to form two separate curves where the road branches off in two directions after leaving the bridge, as shown in [Fig. 130].

[Fig. 131] shows plan, elevation, and cross-section of an under-line arch bridge, considerably on the skew, carrying a railway over a river. The wing walls are curved, and very similar in type to some of those in preceding examples. The river bed and ground alongside being of solid rock, good foundations were obtained at a very moderate cost.

On many railways constructed in the beginning as single lines only, the over-line bridges have been built for double line. The additional cost in the outset has been small, compared with the great expenditure which would be incurred afterwards in reconstructing the bridges to suit a double line.

The general arrangement of abutments and wing walls shown in the foregoing examples will apply to similar classes of bridges where girder-work is adopted instead of arching.

There are many ways of forming the floor or deck of a girder bridge intended to carry a railway over a road or stream. In some cases it will be imperative to have a thoroughly water-tight floor to prevent rain-water percolating through to the roadway below; while in others, such as bridges over streams, and secondary roads, this special provision will not be necessary, and a lighter and more economical floorway can be adopted. A strong wrought-iron or steel-plate flooring, with its corresponding filling and ballasting, means not only so much additional cost in the flooring proper, but also so much additional dead weight to be carried by the main girders.

[Fig. 132] is a sketch of rolled joist-iron I-girders and timber floor frequently adopted for small farm roads and cattle creeps of 10 or 12 feet span. A beam of timber is fitted in between the two rolled joist-irons, and the three pieces securely fastened together with strong iron bolts placed about 3 feet apart. These small compound girders rest on bearing-plates of wrought or cast

iron, and are held together and to gauge by tie-rods, as shown. The rails are spiked or bolted down on to the timber beams, and the flooring formed of strong planking.

[Fig. 133] shows an arrangement of plate girders for a 16-foot opening over a stream. The girders are placed immediately under the rails, and are tied together by plate-iron cross-bracing the same depth as the main girders. The flooring consists of 4-inch planking laid with ¾-inch spaces, on which are laid longitudinal rail-bearers 14 inches wide by 7 inches thick.

[Fig. 134] is a sketch of a somewhat similar arrangement for a lattice-girder bridge, 45 feet span, carrying a single line of railway over a river. The main girders are tied together by lattice-work cross-bracing. The floorway consists of 5-inch planking, laid with ¾-inch spaces, on which is placed the 14 feet by 7 feet longitudinal rail-bearers. Plate-iron outside brackets are riveted to the main girders to carry the ends of the planking and light tube-iron parapet.

[Fig. 135] illustrates an example of trough girders, constructed to carry a double-line railway over a country road 25 feet wide, where the space from under side of girder to rail-level is small. The girders are constructed in pairs, with short, shallow cross-girders at 3 feet 6 inch centres, riveted in between them to carry longitudinal timbers on which the rails are laid. Bottom plates, 5/8 inch thick, unite the two girders for the length of their bearing on the abutments, and a similar plate, 9 inches wide, unites them at the centre; the remainder of the span is left open to prevent the lodgment of rain-water. Three strong tie-rods are placed to keep the girders to gauge. Curved wrought-iron ballast-plates are used between the running-rails, and plank flooring forms the rest of the covering.

[Fig. 136] is a sketch of a plate-girder bridge over a country road 28 feet wide, with the load carried on the lower flange of girder. Three main girders carry the double line of railway, the centre one having double the strength of each of the outside girders. On the top of the cross-girders, strong angle irons are riveted to serve as guides and supports for the longitudinal timbers which carry the rails. Every third cross-girder has raised ends to give increased lateral stability to the main girders. A close cast-iron plate parapet forms a screen to the roadway. Wrought-iron ballast-plates are used between the running-rails, and the remainder of the flooring is of timber.

[Fig. 137] gives the particulars of one 60-foot span of a viaduct carrying a double line of railway over tidal water. The main girders are placed one under each line of rails, and all the four are strongly tied together by lattice-work bracing the full depth of the girders. The outside footpaths for the platelayers are carried on strong brackets, riveted to the main girders. Longitudinal timbers, coped with angle iron, are placed as outside guards, alongside each rail, for the full length of the viaduct. Wrought-iron ballast-plates are placed between the running-rails. The remainder of the footways consist of timber planking, laid with half-inch spaces, and covered with a layer of small pebbles as a protection against fire.

[Fig. 138] shows a very similar arrangement in a viaduct carrying a single line of railway across a river. The two main lattice girders—66 feet span—are placed at 9-foot centres, to obtain greater stability. The cross-girders are extended to carry the outside footpaths and handrailing. Outside guards are placed alongside each rail as in the preceding example. Wrought-iron ballast-plates are fixed all along between the running-rails, and timber planking used for the rest of the floorway.

[Fig. 139] gives cross-section of a lattice-girder bridge, 82 feet span, carrying a single line of railway over a river, with the load carried on the lower flange. The cross-girders are placed at 4 feet 3 inch centres. Wrought-iron ballast-plates compose the floorway between the rails, and timber planking covers the rest of the bridge. Plate diaphragms, or stiffeners, of the form shown at A, A, A, A, are riveted to the main girders at five places in their length.

[Fig. 140] shows cross-section of a lattice-girder bridge of 200 feet span, carrying a single line of railway over a river, the load being placed on the lower flange. The floorway consists of plate-iron cross-girders, spaced at 4-foot centres, on which are placed the longitudinal rail-bearers and planking, the latter being covered with a layer of clean pebbles for the width between the running-rails. As the depth of the main girders was sufficient to admit of overhead bracing, strong plate-iron diaphragms, of the form shown on the sketch, were riveted to the main girders at every 50 feet. These diaphragms thoroughly brace the two girders together, and effectually prevent any tendency to side-canting, at the same time imparting an effective appearance to the bridge.

[Fig. 141] shows cross-section of a plate-girder bridge, of 36 feet span, carrying six lines of way across a street. Strong plated cross-girder bracing, at 4 feet 8¼ inch centres, is riveted to the main girders, and the top is covered with old Barlow rails, 12 inches wide, and weighing 90 lbs. per lineal yard. A layer of asphalte, about 1½ inches thick, is carefully laid all over the upper surface of these rails to make a thoroughly water-tight floor. Clean gravel is placed on the top, on which are laid the sleepers and rails of the permanent way. Rain-water passes through the gravel into the hollows of the Barlow rails, and finds its way into suitable drains provided at each abutment. This arrangement not only prevents the falling of drip-water into the street below, but permits of the alterations of the lines of way, or putting in of cross-over roads on the surface above. The outside main girders are made deeper, and are surmounted by close cast-iron parapets.

[Fig. 142] gives the particulars of a three-span plate-girder bridge, constructed to carry a double line of railway over two other railways and a canal, the load being placed on the lower flange. Two main girders are used for each line of way. Strong plated cross-girders are placed at 5 feet 3 inch centres, and on the top of these is laid a flooring of old Barlow rails, terminating at the sides with sloping wing-plates riveted to the cross-girders and main girders, the entire surface being covered with an inch and a half layer of asphalte. Good gravel ballast is placed on the top, on which are laid the sleepers and rails. One central main girder of sufficient strength would have been as efficient as the two central girders, but there was a practical difficulty which prevented its adoption. The new girder-work was built to replace an old structure of peculiar arrangement, and to keep the traffic going on one line there was no alternative but to make each line of way complete in itself.

[Fig. 143] illustrates an example of jack arches in concrete built between strong plate-girders. The span of the girders was only 16 feet, but the opening or roadway was of considerable length, and passed under a portion of a busy station yard. The girders are placed at 6-foot centres, and tied together in pairs by 1¼-inch tie-rods, three to the span, spaces of 6 inches in plan being allowed between each set of the rods. The concrete was curved up to the top plate of the girder, as shown, and the entire surface covered with a thick layer of asphalte, on which were

placed the ballast and permanent way. Brickwork might have been used for the jack-arching, but concrete was considered more convenient.

[Fig. 144] shows the cross-section of a truss-girder bridge of 123 feet span, carrying a double line of railway over a wide thoroughfare, the load being placed on the lower flange. There are two main girders, each 12 feet 6 inches deep in the centre, and 8 feet deep at the ends. Plate cross-girders are placed at 4 feet 6 inch centres, on which is riveted longitudinal plate-iron troughing, extending across the bridge and terminating at the sides with wing-plates, as shown. The entire floor is covered with a thick layer of asphalte previous to filling in with ballast to receive the permanent way. Plate stiffeners are adopted in this bridge very similar to those in [Fig. 139].

[Fig. 145] gives plan, elevation, and cross-section of a plate-girder bridge of 95 feet span, carrying a double line of railway over a very busy street. There are two curved-top main girders, each 10 feet 9 inches deep in the centre, and 6 feet 7½ inches deep at the ends. The arrangement of cross-girders, longitudinal plate-iron troughing, and permanent way, is very similar to that in the preceding example, but the side wing-plates are carried up higher, and are riveted up to the web-plate of main girder, forming continuous stiffeners from end to end of the main girders. A light, ornamental, close cast-iron parapet is bolted on to the top of the curved, or upper, boom of the main girder, the top line of the parapet being carried out parallel to the bottom boom of girder. This bridge crosses the street very obliquely, and, although cast-iron columns were allowed at the edge of the footpaths, the main spans are unavoidably large. When designing the above bridge, the writer had to adopt a girder that would form a screen, to provide a deck, or floor-way, which would be not only water-tight, but also deaden as much as possible the sound or vibration of passing trains, and at the same time give some ornamental appearance to the girders and parapets. This bridge carries a constant service of heavy trains; it is perfectly dry underneath, and is remarkably free from noise or vibration.

[Fig. 146] shows cross-section of a plate-girder bridge of 40 feet span, carrying a double-line railway over a street, in a situation where the depth from top of rails to under side of girders had to be made as small as possible. Three main girders

were used, the centre one being double the strength of each of the outside girders. Instead of ordinary cross-girders, transverse plate-iron troughing was adopted, very similar in section to the longitudinal iron troughing in [Fig. 145], but stronger. The troughing rested on the angle iron of bottom flange of main girder, and was riveted to the vertical web-plates of main girders, shallow additional vertical plates being inserted alongside web-plates to prevent any drip-water or moisture coming in contact with the main web-plates. The entire surface of the troughing was well covered with asphalte before filling the hollows with gravel ballast. An ordinary transverse wooden sleeper was placed in each hollow, and on these sleepers the rails were secured as shown. In this case—as in others of transverse troughing—the rain-water had to be conveyed away from the hollow of each trough by a separate outlet into longitudinal gutters shown at A, B, and continued on to the abutments.

Transverse troughing is always more troublesome than longitudinal troughing, as both ends of each trough must be effectually closed to prevent the drainage water leaking out on to the web-plates, or angles of the main girders. With longitudinal troughing the water is readily carried away from each hollow, to cross drains constructed at the piers, or abutments.

[Fig. 147] shows cross-section of a truss-girder bridge, 120 feet span, carrying a single line of railway over a river. The cross-girders are placed at 10-foot centres to correspond to the vertical members of the main truss-girder. Longitudinal plate-iron rail-girders are riveted in between the cross-girders, and the entire floor is covered with curved wrought-iron ballast plates, as shown. The rails are carried on longitudinal timbers, which are bolted on to the rail-girders. Angle iron brackets, riveted on the top of the cross-girders, keep the rail timbers in position and gauge.

In each of the above examples, where longitudinal rail timbers are adopted, flange rails are shown, as many engineers prefer to have a continuous bearing for the rails on bridges, in case of rail fracture. There is nothing, however, to prevent the chair road being laid on longitudinal timbers, and for this purpose the writer has used chairs of the ordinary pattern, specially cast with side lugs to grip the timber, as shown in

[Fig. 148]. Chairs of this form have a very firm hold on the longitudinal timber, and the side lugs check any tendency of the splitting or opening of the wood when putting in the spikes or screw bolts.

[Fig. 149] shows cross-section of a plate-girder over-line bridge, 32 feet span, carrying a private road, 12 feet wide, over a double-line railway. The road traffic being small, the floorway was constructed of creosoted planking carried on rolled I-iron cross-girders placed at 3 feet 8 inch centres, and riveted to the main girders. The horse-tread track was provided with a second layer of planking, laid transversely, to take up the wear, cross battens, 4 inches by 2 inches, being placed at 12-inch centres, and sand spread between to give good foothold. A light lattice-work parapet was bolted on to the top of the main girders.

[Fig. 150] gives cross-section of a plate-girder over-line bridge, 30 feet span, carrying a private road, 20 feet wide, over a double-line railway. The main girders are tied together by lattice-work bracing, spaced at 7-foot centres. Curved wrought-iron plates are laid across from girder to girder, and butt against a narrow horizontal plate, which forms part of the upper boom. The curved plates are riveted on to the top of girder, and form a continuous iron floor, or deck, from side to side of the bridge. Upon this iron floor is laid an ordinary asphalte roadway. The outside girders are made deeper, and carry an ornamental cast-iron parapet. In some bridges of a similar construction, the roadway is formed of creosoted wooden block paving, on a foundation of asphalte.

BRIDGE CARRYING THE D. W. AND W. RAILWAY (LOOP LINE) OVER AMIENS STREET, DUBLIN.   [To face p. 144.

[Fig. 151] shows cross-section of a plate-girder over-line bridge, 28 feet span, carrying a public road, 35 feet wide, over a double-line railway. The main girders, 2 feet 4 inches deep, are placed at 5 feet 2 inch centres, and are tied together by plate-iron cross-bracing 2 feet deep. Jack-arches of brickwork, 9 inches thick, are built in between the main girders, the haunching being filled in with concrete. The entire surface is covered over and made watertight with asphalte, on which is laid the metalling of the roadway. The outside girders are made considerably deeper, and have strong cast-iron-plate parapets bolted on to the top booms. There is no doubt that jack-arching of brickwork or concrete makes a very strong and permanent floorway, but its dead weight is very great, and its adoption is

not to be recommended where iron or steel plate troughing can be obtained at a moderate price.

[Fig. 152] gives cross-section of plate-girder over-line bridge, 41 feet 6 inch span, carrying a public road, 25 feet wide, over three lines of way. Two main girders are used, of sufficient depth to form parapets or screens for the finished roadway. Plate cross-girders, placed at 6 feet 6 inch centres, are riveted to the web-plate and lower angle irons of main girders; and on these is placed a flooring of plate-iron longitudinal troughing to carry the metalled roadway.

[Fig. 153] gives the particulars of a plate-girder over-line bridge, carrying an important public road, 35 feet wide, over several main lines and sidings. The carriage-way is carried by two girders placed at 25-foot centres, and on the lower boom of these are riveted lattice-work cross-girders to receive the plate-iron longitudinal troughing and roadway. The footpath girders are set at a higher level, and the load placed on the lower flange. The curved side brackets merely act as bracing between the carriage-way girders and footpath girders. A cast-iron-plate parapet is bolted on to the top of each of the footpath girders, making a close screen, 6 feet high, above the footpath. Lattice-work cross-girders were adopted for the convenience of supporting small water mains and gas mains below the road-level. The roadway is formed of ordinary metalling, and the footpaths of asphalte pavement; the kerbing is of granite, and the side water-tables of crushed granite concrete.

[Fig. 154] is a cross-section of a small uncovered lattice-girder footbridge 41 feet span, and 5 feet wide, suitable for small roadside stations. The top and bottom flange consist each of two angle irons, those in the bottom flange being placed table side upwards, so as to bring the entire section of both angle irons fairly into play, and also to provide a better bearing for the channel-iron cross-girders which carry the planking of the footway. When planking is carried on the inside of light angle iron, as in [Fig. 155], a severe strain is produced at the point A; this is entirely obviated by placing the bottom angle irons table side upwards, as in [Fig. 156]. Three of the channel-iron cross-girders are extended outwards, and to the ends of these are riveted tee-iron stiffeners to steady the main girders. In some cases stamped, or ribbed, wrought-iron plates are used for a footway, but, although more durable, they do not give such a

secure or agreeable foothold as timber. The ascent or descent of the bridge may consist either of steps and landings, or of ramps, according to circumstances or expediency. Sometimes these bridges are made with curved tops, terminating in steps when nearing the steps, or ramps. It is very questionable whether such an arrangement is a good one or a safe one. There is always a feeling of insecurity when walking over a sloping surface broken up by steps, and experience points out that it is better to continue the footway level right across to the place where the passenger must change his direction to go down the stairs or ramp.

[Fig. 157] gives cross-section of a covered lattice-girder footbridge, 62 feet 6 inches span, and 10 feet wide, suitable for an important station. The upper boom of girder consists of two angle irons and top plate, and the bottom boom of two channel irons. The cross-girders are rolled joist-irons resting on the top tables of the channel irons. Four of the cross-girders are extended outwards, and carry plate-iron outside vertical brackets to stiffen the main girders. Three-inch longitudinal planking is laid down from end to end of the bridge, and on this is laid 1¼-inch transverse flooring, in narrow widths, to form the walking deck. The footbridge is lighted from the sides by continuous glazed sashes fixed in strong wooden framework, as shown. The roof is covered with canvas bedded in white lead, and painted in the same way as an ordinary carriage roof.

The above examples of under-line and over-line bridges are given more with a view of illustrating some of the many different descriptions of flooring, rather than to point out or suggest the type of main girder to carry the load. The description and size of the main girders can be varied to suit the span of the bridge, the requirements of the traffic, and the opinion of the designer. For spans up to 50 feet it will generally be found that web-plate girders are both simpler and cheaper than lattice or truss girders; at the same time, there are occasions where plate girders can be advantageously adopted for very much larger spans, as, for instance, in the example given in [Fig. 145], where the deep plate girders form a most efficient screen.

[Figs. 160] to 194 give diagram sketches of a few out of the many forms of open, or truss, girders which have been adopted for large spans. There are many types from which to make a selection, each one possessing its own special features and

advocates. In working out the details of any, or all of them, there are some points which should always be kept in mind when deciding the distribution of material in the main booms. Rain-water, or moisture of any kind, is the great enemy of wrought-iron or steel work, and therefore the plates, angles, tees, or channel sections, should be so arranged as to afford the least possible facility for the collection or lodgment of water. With open, level booms, as in [Figs. 137], [139], [140, 144], and [145], the rain-water cannot collect, but runs off at the sides, and the plates are quickly dried by the sun and wind. With trough booms, as in [Fig. 158], the collected rain-water can only get away through holes drilled for the purpose in the bottom plates. These holes are liable to become choked up, but even when open they rarely carry off all the accumulated water; some of it remains to corrode the plates, and is only dried up by evaporation. The inside of trough booms should be constantly inspected, and the exposed plates more frequently painted than the rest of the girder. In a similar manner, in small double-web lattice girders, with the lattice-bars inserted between two angle irons, as in [Fig. 159], the rain-water finds its way into the spaces at A, A, in spite of the most careful packing or filling with cement or asphalte. Numbers of small girders of this latter type have had to be taken out after a comparative short life, in consequence of the great corrosion and wearing away of the lower ends of the lattice-bars and angle irons into which they were inserted.

It is most essential, also, that all portions of the girder-work should be conveniently accessible for inspection and painting. Complicated connections, and parts which are difficult to examine, are liable to be overlooked, or, at the best, only painted in a very imperfect manner. Neglected corners soon create deterioration, the paint scales off, corrosion commences, and the working section is gradually reduced. A discovered weakness in some of the important parts points to an early condemnation of the entire structure. The difficulty of access to the interior of box or tubular girders, especially those of small or moderate dimensions, is a great objection to that type of girder. Experience has pointed out that open girders, free and exposed to the light and air, can be so much more effectually inspected and painted.

Perhaps one of the most anxious tasks which falls to the lot of an engineer is the renewal of under-line bridges and viaducts on a working line. On a new line in course of construction the

entire site of the work is at the disposal of the erectors, and the building of a bridge or viaduct can be carried on with a freedom which cannot be obtained on an open line. On a working railway, the train service must be kept going, irrespective of renewals, and very often the best that can be done is to reduce the double line to single line working at the site of the operations. It is not always expedient or possible to make a temporary bridge and diverted line for traffic purposes, as the expenditure to be incurred might be too great to warrant the outlay, or there may be local difficulties to effectually prevent the introduction of a provisional structure. The taking down of one half of the old structure may necessitate the removal of stays and bracing affecting the stability of the half remaining to carry the traffic, and thus render temporary shoring and bracing necessary. The erection of the new work in such a limited space has to be watched with great care; all cranes, lifting appliances, and scaffolding must be kept clear of vehicles moving over the running-line, and very frequently it is found prudent to cease erecting operations during the passage of a train.

In very many cases of renewals, the description and arrangement of the old structure will materially influence or control the design for the new one, and the details of the latter must be schemed out so as to disturb as little as possible the stability of the old work remaining as the working road.

The following list gives the lengths of the main spans of some railway bridges, and may be found useful for reference:—

Lengths of Main Spans of some Large Railway Bridges.

Name.	Span.	Description.
	feet.
Forth Bridge	1,710	Cantilever.
Niagara	821	Suspension.
Sukkur	820	Cantilever.
Poughkeepsie, U.S.A.	548	Cantilever.
Douro	525	Arch.
St. Louis	520	Arch.
Cincinnati	515	Linville truss.
Haarlem	510	Arch.
Kuilemburg	492	Lattice bow.
St. John’s River	477	Cantilever.
Niagara	470	Cantilever.
Britannia	460	Tube.
Ohio River, Pennsylvania	442	Pratt through truss.
Saltash	434	Tube and girder.
Hawkesberry Viaduct	410	Compound truss.
Conway	400	Tube.
Vistula	397	Lattice.
Spey River, Garmouth, N.B.	350	Bowstring.
St. Laurence	330	Tube.
Hamburg	316	Double bow.
Cologne	313	Lattice.
Runcorn	305	Lattice.
Sunderland	300	Bowstring.
Rondout Bridge, Buffalo	264	Pratt through truss.
Newark Dyke (New)	259	Lattice bow.
Tay Bridge (New)	245	Lattice bow.
Ohio River, Louisville	245	Fink truss.
Beaver Bridge, Pennsylvania	230	Pratt deck truss.
Craigellachie Bridge	200	Lattice.
Rohrbach Bridge, St. Gothard River	197	Wrought-iron arch.
Windsor Bridge	187	Bowstring.
Victoria Bridge over Thames	175	Wrought-iron arch.
Shannon River Bridge	165	Bowstring.
Carron Bridge over Spey	150	Cast-iron arch.
Preston Viaduct	102	Cast-iron arch.
Trent River Bridge	100	Cast-iron arch.

Retaining Walls.—Instances frequently occur during the construction of a railway where it is advisable, if not absolutely necessary, to substitute retaining walls in preference to forming the slopes of cuttings and embankments.

The excavation of a cutting may be greatly reduced in quantity by introducing low retaining walls, as in [Fig. 195], and the saving in the material to be removed will be all the more important in those cases where cutting is in excess of embankment.

The amount of filling for an embankment and the land on which it has to be formed may both be considerably diminished by building a low retaining wall, say 6 or 7 feet high, at the foot of the slope, as shown in [Fig. 196]. Such a retaining wall makes a most efficient fence and well defined boundary of property.

The policy of adopting low retaining walls in cases like the

above will depend mainly upon the cost of building materials as compared with the cost of earthwork and land.

Where land is very valuable, and where residential property, streets, or roads must be interfered with as little as possible, the retaining walls may have to be carried up to the level of the original surface of the ground, as in [Fig. 197], which is shown as for a cutting 25 feet deep. The walls may be built of masonry, brickwork, or concrete, or a combination of them, and the dimensions or thickness will depend upon the description of material to be supported. Weeping holes, or small pipe drains, should be formed in the walls, a little above formation level, to take away any water which may collect at the back.

Where the cutting is through soft, wet, treacherous clay, liable to slip or expand, it may be necessary to insert arched thrust girders extending from side to side, as in [Fig. 198], so that the outward pressure against the one wall may counteract against the outward pressure of the other. The thrust girders should be placed at from 10 to 15 feet centres, and be well braced together in plan to enable them the better to resist any tendency of bulging out of the walls.

A similar arrangement of high retaining wall may be introduced in embankment to lessen the encroachment on streets or public roads, as shown in [Fig. 199].

In making a railway through thickly populated towns, it is generally preferable to construct the line on arches rather than on earthwork filling between two high retaining walls. The numerous openings are available for future streets, or means of communication from one side to the other, and the arches themselves can be profitably utilized for stables, stores, offices, and workshops.

[Fig. 200] shows a narrow rocky pass with deep rapid river on the one hand and high cliffs on the other, the only available ledge being already occupied by a public highway. By building a retaining wall, as indicated on the sketch, and excavating a little out of the cliff, space may be obtained for a line of railway; or the arrangement may have to be reversed, and the retaining wall for the railway built along the margin of the river, as in [Fig. 201].

In both the cases, [Figs. 200 and 201], not only must there be a number of weeping holes left in the lower part of the wall, but there must be sufficient well-built drains and culverts under the

filling and through the wall to carry away all ordinary or flood water coming down from the cliffs and hills above. Where a retaining wall is built along the margin of a river, the lower portion, which will be in contact with the water when the river is full, should be constructed of selected large heavy stones to withstand the scouring action of the water, and any brushwood or floating timber which may be brought down by flood water.

Where retaining walls are built to support wet clay, or in embanked places on wet side-lying ground, the efficiency of the work will be much increased by constructing a layer, two or three feet in thickness, of dry, flat, bedded stones carefully hand-laid, from the foundation to the top of the wall, as shown in [Fig. 199].

These dry stones form a continuous vertical drain to take away water from any part of the earthwork down to the outlets left in the lower portion of the wall.

The building of retaining walls entirely of dry stone is very questionable economy, and entails a constant expenditure in maintenance and renewal. The working out of one stone loosens the surrounding portion of the wall, and if not at once repaired, a length of the wall will fall down, bringing with it a large quantity of the earthwork.

If readily obtained, large heavy stones should be selected for the coping of retaining walls, so as to minimize as much as possible the chance of their disturbance or displacement. Where lighter stones have to be used, or bricks laid on edge, they should be bedded and pointed in cement.

In many places it is necessary to form wide and massive foundations of concrete on which to build the retaining wall; and in some cases of soft, treacherous ground, timber piling may be necessary.

Tunnels.—It would be difficult to assign a date to the first examples of subterranean works constructed for utilitarian purposes. Nature had furnished so many grand specimens of caves, grottoes, and underground passages formed in the solid rock, that man soon grasped the principle, and essayed to carry out similar works on his own account. The early attempts would probably be limited to forming places of shelter, storage or security. Advantage would be taken of those rocks which from their locality, accessibility, and compactness of material, promised favourable results. The appliances being few and

primitive, the work of construction would be laborious and slow. So long, however, as the workers restricted their operations to the solid rock, they had merely to contend against the hardness of the material, as the opening or passage-way, once made, required no further support or attention; but as the wave of progress swept onward, man was compelled to deviate from the lines originally followed by nature, and had to form his subterranean pathway through softer material, where the workings required substantial support. The search for minerals of various kinds led to the driving of long headings or galleries underground, and as these had frequently to penetrate through strata of a soft and yielding character, strong timber framework had to be introduced to afford stability to the works, and safety to the workers. For ordinary mining operations, strong rough timber supports may meet all requirements, and may last until the heading is worked out and abandoned; but for subterranean passages or tunnels which are intended to form permanent means of communication, the strongest and most durable materials must be used to protect the interior as far as possible from deterioration or decay. Heavy timbering might be sufficient for mere temporary purposes, but substantial masonry or brickwork side walls and arching became necessary for permanent work in those portions where the tunnel required artificial support.

The first tunnels of any importance were most probably those constructed for canal purposes. Many of them were of considerable magnitude, and in some instances were from two to three miles in length. They were substantially lined with masonry or brickwork at all places where the tunnel passed through soft material or loose rock, and from the solid nature of the work, and the many years they have been in existence, they thoroughly testify to the ability of the constructors.

The introduction of railways involved the making of a large number of tunnels, perhaps more so in the beginning, when it was thought that the use of the locomotive would be confined to very moderate gradients, and when engineers hesitated to adopt the steeper inclines and sharper curves which form the practice of modern times. Another element of consideration also consisted in the fact that the first railways were designed to connect the most populous and busiest districts, where the prospects of heavy traffic would appear to warrant a large outlay for works of construction. As the system spread and

railways extended further away from the important centres, the probabilities of traffic would become less promising, and efforts would be made to keep down cost of construction, and avoid tunnel work as much as possible.

It is not easy to define where cutting should end and tunnelling begin. There is no practical difficulty in making a cutting 50, 60, or 70 feet deep, with slopes to suit the material excavated, and the estimated cost per yard forward may even compare favourably with the cost of average tunnel-work. But there are other questions which must be kept in view—the time required to form the cutting, the space to be obtained on which to deposit the enormous quantity of excavated material, and the probable difficulty in obtaining the large area of land necessary for the cutting.

Before deciding the actual position of a tunnel, both as to line and level, it is necessary to obtain the most reliable information possible regarding the strata through which it has to pass. In addition to the geological indications on the surface and in the locality, borings should be made, and trial holes or shafts sunk along the proposed centre line of the work, and from these an approximately accurate longitudinal section can be laid down on paper, showing the respective layers of material to be cut through, and the angle at which they lie. With these particulars before him, the engineer may, in some cases, consider it more prudent to change the position of the tunnel in preference to incurring specially difficult or tedious work in dealing with some recognized unfavourable material. Occasionally the route may be slightly varied and better material obtained, but very frequently there is little to be gained except by a wide deviation from the original line.

Solid rock, except for the slow progress, is perhaps the most favourable material for tunnelling, as the timbering, side walling, and arching can be almost, if not entirely, dispensed with.

Loose rock, although more readily removed, necessitates strong timbering to prevent large masses breaking away and falling into the tunnel.

Some clays are very compact and tenacious, and will stand well with moderate timbering, but even these should not be left long before following up with the side walls and arching.

Many clays give much trouble by expanding, or swelling out, when the excavation penetrates the layer, and although extra

strong timbering may be used, and be placed closer together, the logs and planks are frequently bulged out and broken by the action of the clay. Specially strong supports are required for this description of clay, and extra thickness of material in the permanent work of side walls and arching.

Solid unbroken beds of chalk are not difficult to cut through: the material is easy to work, and the excavation will stand with ordinary timbering; but where the chalk is broken and intersected with deep pockets of gravel and sand, the operations are very much impeded. The loose material, once set free by cutting through the confining barrier of chalk, will quickly fall into and fill up the excavation if not held back by strong timbering. Side walls and arching are generally necessary for tunnels through chalk.

Soft wet clay, quicksands, or other strata having springs of water percolating through them, are serious obstacles in the way of expeditious tunnelling. No sooner is one cube yard of this soft material removed than another slides down, or is washed down, to take its place. When once the excavation taps the water-bearing strata, large volumes of water will find their way into the workings, and must be conveyed away to the mouth of the tunnel, or pumped up through the nearest shaft. The timbering of the sides and roof through this description of working is very tedious, and attended also with a considerable amount of risk. The absence of really solid ground on which to place or shore up the supports, taxes the skill of the excavators, and very often, when a short length has been made apparently secure, it will come down with a run, compelling all hands to beat a hasty retreat. The permanent lining through such treacherous material should follow the excavation very closely, and special care should be exercised in building the walls, arching and invert.

In the excavation through stratified rocks it is necessary to note carefully the lie of the strata, whether horizontal, vertical, or shelving, as with each one the excavators are exposed to risks, against which every precaution should be taken. A large horizontal slab of solid-looking rock will suddenly break and fall down without any warning. A heavy mass from a vertical layer, perhaps unkeyed, or loosened, by an adjacent blasting operation, drops down when least expected; and pieces from the high side of the shelving layers detach themselves and slide into the working in a most unaccountable manner.

No attempt should be made to carry a tunnel through material which has been disturbed or at all affected by any natural slip or cleavage, as although the strata may be hard and compact in themselves, they have really no solid or fixed foundation. The sliding away, once initiated, is certain to continue, and, accelerated by the tunnelling operations, will most likely, sooner or later, crush in the tunnel and sweep away every vestige of the work. Amongst the great mountain ranges these natural disturbances are by no means rare, and it will be wiser to keep away from their locality, even at the expense of a longer tunnel. Unfortunately, instances are on record of tunnels made, or in course of construction, through hillsides which had already commenced to slide away from the more solid rock, and the ultimate results were a further sliding away and total destruction of the work.

The lower slopes and outlying portions of high mountains are the most exposed to these natural slips, and they should be most carefully studied before commencing any tunnelling operations through them.

To facilitate drainage, it is essential that a railway tunnel should be laid down with a gradient or gradients falling in the direction of one or both ends of the tunnel. In nearly all tunnels a considerable amount of water finds its way in through the weeping-holes left for that purpose in the side walls, and must be carried away in suitable drains. If the quantity of water be small, ordinary water-tables, one on each side, may be sufficient; but for large volumes of water it will be necessary to build substantial side-drains, or an ample culvert below the level of the rails.

The gradients in a tunnel should be moderate, and not by any means excessive, or likely to tax the hauling powers of the locomotives. When an engine is working nearly to the utmost of its power on a steep tunnel incline, and the speed has become very slow, the exhaust vapours or gases from the funnel strike the arching with great force, and are deflected down on to the footplate in such dense volumes as to almost suffocate the driver and fireman. The writer will never forget two or three trying experiences in foreign tunnels, when he and the engine-staff were compelled to leave the footplate and climb forward to the front of the funnel, leaving the engine to work its way alone. Except for very short tunnels it is wiser to have easy inclines, and to restrict the steep gradients to the open line, where

the very slow travelling, or even the coming to a stand from “slipping,” may not produce unpleasant or alarming consequences.

In tunnels of any length it is usual, where possible, to construct shafts extending from the surface of the ground overhead down to the tunnel below. These shafts serve the double purpose of enabling the excavation to be carried on at an increased number of faces, and act as permanent ventilators after completion. In some cases the shafts are sunk exactly over the centre line of the tunnel, in others a few yards away from the centre line. The latter arrangement, if not quite so convenient for hoisting material while carrying on the excavations, has certainly the great after advantage that anything falling or maliciously thrown down the shaft cannot strike a passing train. The short side-gallery, or space between the tunnel and the shaft, provides a good refuge for workmen employed in repairs, and a convenient site for storing a few materials advisable to keep on hand.

Occasionally favourable opportunities present themselves for making horizontal shafts. For a portion of its length the tunnel may be located at no very great distance from the precipitous sides of some deep mountain ravine, or run near to the cliffs on the sea-coast, and advantage can be taken to drive a lateral heading or gallery through which the material from the tunnel excavation may be conveyed and thrown out into the gorge or seashore below.

In many cases the surface of the ground rises so abruptly from the faces of the tunnel and ascends to so great a height, that shafts of any kind are entirely out of the question, and the whole of the work must be carried on from the two ends. The rate of progress is consequently much slower, and the ventilation more difficult. In a shaftless tunnel of considerable length, and with a frequent train service, the question of providing suitable appliances for promoting artificial ventilation is of the utmost importance.

When the centre line of the tunnel has been accurately set out on the ground, and the levels of the different parts of the work decided, the construction of the shafts and the driving of the headings can be commenced. Working shafts intended to serve for permanent ventilation are generally made nine or ten feet or more in diameter, and are usually lined with substantial

brickwork or masonry. When the well-like excavation has been carried down a few yards, or as far as it can be taken without the risk of the earth falling in upon the sinkers, a strong curb of hard wood or iron of the same diameter as the finished shaft is laid down perfectly level and to exact position, and on this curb the ring or lining of brickwork or masonry is built up to the level of the ground. The first length finished, the excavation downwards is resumed, and the interior lining continued, either by allowing the first length to slide down as the material below is gradually removed, and building further lining on the top, or by excavating and propping up the curbing with strong timbers below. When working to the latter method, stout wooden props of convenient length, stepped on to sole-pieces, are adjusted to the under side of the wooden curb above, the material is then removed for the thickness of the brickwork or masonry, and another curb accurately set to level and position; on this is built a length of lining up to the first curb.

This work of under-building or under-pinning must be carried out with great care and in segments; no props must be removed until the curb immediately above is well supported by the new lining, and the inside of the lining must be watched and tested to prevent any tilting. All spaces at the back of the work must be filled in and well packed with hard dry material. As the shaft is continued downwards the mode of working may have to be varied; different descriptions of material may be encountered, and blasting, shoring, and pumping may each in turn be necessary.

When down to the full depth, the lower length of the shaft will have to be securely supported by strong timbers, until it can be properly built into and incorporated with the arching of the tunnel or side gallery.

The completion of the shaft enables the workings to be commenced on each side, the excavated material can be hoisted to the surface, and building material lowered down. When the tunnel works are finally finished, the lining of the shaft should be carried up until it is 15 or 20 feet above the level of the surface of the ground, and a dome-shaped iron grating placed on the top as a protection against stones or other articles which malicious persons might attempt to throw down the shaft.

Some shafts are only intended for the temporary purpose of lifting the excavations from below, or lowering building materials

down, and when the work is completed they are filled in again and closed. These service shafts are generally made square in section, and are merely lined with wood. Strong vertical timbers are placed at the four corners, to which horizontal double cross-pieces are bolted, thick planking being placed vertically at the back of these cross-pieces to support the sides of the excavation.

The heading of a tunnel is a narrow passage or gallery cut through from end to end of the works in the direction of the centre line. Where there are shafts, the cutting of the heading can be pushed on from several points, and be completed much more rapidly than when the working is restricted to the two ends. Headings are usually made just sufficiently large for the miners to work, say about 5 feet 6 inches high by about 3 feet wide, the object being rather to expedite the driving of the driftway than to remove large masses of material. They must be set out with great accuracy, and be constantly checked as the driving is in progress. When completed from end to end, the centre line can be checked throughout, and the course actually taken compared with the course intended. If there has been much variation in the narrow pioneer pathway, either in line or level, the amount of the divergence must be rectified when ranging the final centre line for the full-size excavation.

Tunnels cannot always be delayed until the heading is cut through for the entire length. In many cases the heading, the full-size excavation, and the permanent lining have all to be carried on at the same time, but as the work of the heading is smaller in extent, that portion of the operations can usually be kept well in advance of the others. The critical moment arrives when the headings from opposite directions meet, as any deviation or want of coincidence must be adjusted in the portion of the tunnel still remaining to be opened out to full size. Some tunnels of moderate length have been constructed without any heading at all, the excavation being taken out to the full dimensions from the commencement.

The heading of a tunnel assists not only in the correct alignment of the work, but furnishes at the same time an accurate knowledge of the strata passed through. It is also of service for ventilation, communication, and drainage.

In some cases the heading is driven at the bottom of the tunnel section, as in [Fig. 211], and in others at the top, as in

[Figs. 202] and [204]. Many of the earlier tunnels were constructed on the former system, while of late years the latter method has been very largely adopted. The bottom heading may perhaps in some instances be more efficacious for drainage, but it is very liable to be frequently choked up when taking out the excavation to the full size, and the lower surface is much cut up by the movement and conveyance of materials. Another disadvantage arises from the necessity of removing such a large amount of the cutting approaching the tunnel entrance before a beginning can be made to the bottom heading. The top heading has the advantage that it requires less removal of open cutting previous to its commencement, and, being high up in position, there is less chance of its being stopped up by falling material, the finished excavations being carried out on the sides and below the heading.

Where the headings are cut through solid rock, stiff shale, or compact chalk, little or no supports are necessary, but where they pass through clay or loose material, timbering will be required for sides, roof, and floor. Rough round poles, about 6 inches in diameter, are generally used for verticals, and are firmly secured to transverse sole-pieces, and on the top of these verticals strong transverse top-sills are fastened by means of rough tenons or checks. Strong boards are inserted at the back of this framework to keep the earth from falling into the working. The distance apart of the verticals will depend upon the description of material excavated; in very soft places they will have to be placed very close together, but where fairly sound and tenacious they may be placed at about 3-foot centres. The excavated material must be conveyed away to the entrance of the heading in small hand-trucks running on planks or light rails.

The widening out of the excavation to the full size will be a repetition on a large scale of the work carried out in the heading, with the difference that, the exposed surfaces being of so much greater extent, extra care and precautions must be taken with the framework and shoring of the timbering.

The form and arrangement of the timbering, as well as the number, sizes, and positions of the pieces, must be determined by the material of the excavation and the contour line of the finished arching or lining. The framework, which would be sufficient to support ordinary soft material, must be largely augmented both

in quantity and scantling to meet the requirements for wet treacherous clay.

[Figs. 202 and 203] give end view and longitudinal section of timber framework frequently adopted for average tunnel work. The positions of the different pieces will explain themselves and the duty they have to perform. The main struts, or raking pieces, which have to sustain great pressure, may be shored against the finished lengths of masonry or brickwork. The timbering of the sides can be removed as the lining proceeds, but in many cases the round logs and boards near the crown cannot be withdrawn, and have to be left in the work, the space between the top of the arching and under side of the boards being firmly packed with brickwork, masonry, or dry rubble stonework.

As the tunnel lining is generally carried forward in short lengths, following up the main excavations, the centering for the arching should be of such description that it can be readily transferred or moved forward as the work proceeds. The form of the centering, and the spacing of its upright supports, must admit of sufficient width for one or more lines of rails for the waggons required to remove the excavated débris and convey the building materials used in the lining.

Picks, bars, and shovels are the tools used in the excavation of the softer material and loose disintegrated rock, but for the hard rock, blasting will be necessary. The tunnel opening being comparatively small, only moderate blasting charges can be used with safety, and these must be placed so as to break up the rock-bed in a suitable manner for working, and without shaking or damaging the already completed excavation. Ordinary hand-drills, or jumpers, may be used for forming the charge holes, a number of them being at work at the same time, and the charges fired very closely one after the other. As the blasting operations necessitate the retiring of the miners to a considerable distance, out of the way of flying fragments, and the remaining away until the foul air has been dispelled, it is advisable to fire off several charges about the same time, and thus minimize as much as possible the stoppage to the drilling and clearing away the loosened material.

Mechanical drills, worked by compressed air or other motive-power, are now very extensively used where the rock is solid and continuous. They are much more expeditious than the hand

drills, but they are costly in their installation, and also in their working and maintenance.

In some tunnels, where the material has been firm and dry, the upper portion of the excavation has been first removed, and the masonry and brickwork lining built in position down to about the springing of the arch, the remainder of the excavation being afterwards taken out, and the side walls built by means of shoring and underpinning.

In other tunnels the complete section has been excavated and timbered, and the work of building commenced from the foundation of the side walls. A strong continuous invert from side wall to side wall is necessary where passing through soft swelling clay or loose strata intersected with small streams of water. Where the material is very solid and dry, it is not necessary to introduce inverts, but the foundations of the side walls should be laid at such a depth below rail-level as not to be affected by drain-water running through the tunnel.

The side walls and arching may be either of masonry or brickwork, but should be of the best description, especially for the facework. For brick arching only the best hard-burnt bricks should be used, and the inner or exposed ring should consist of selected hard fire-bricks to withstand the heat and gases escaping from the funnels of the locomotives. The thickness of the side walls and arching will depend upon the description of material to be supported. In some places a comparative thin lining may be sufficient, while in others extra thickness must be given to resist the great pressure exerted by expanding clay and loose wet strata.

Weeping-holes, or small drain-pipes, placed low down must be left in the side walls every three or four yards, or closer in very wet places, to allow the water collected at the back of the walls to escape into the side drains of tunnel. In building the arch portion every effort should be made to have close solid work without any open joints or spaces through which the water may run, and the crown of the arch and a few feet down on each side should be coated with cement or asphalte to lead all water away from the top to the sides. Water dripping from the under side of the arch on to the line is a great destructor of the permanent way materials, especially the fastenings; and bolts, nuts, fish-plates, and spikes placed in a wet dripping tunnel will not last half the time they would out in the open line, where they would have the sun and wind to dry them.

Small arched recesses or niches should be formed in the side walls at convenient distances to serve as refuges for platelayers or others working in the tunnels.

It is most essential that the space between the masonry and brickwork lining and the facework of the excavation should be carefully filled in and hard packed, so as to prevent the possibility of pieces of rock or other material falling on to the top of the arch. The neglect of this precaution may lead to a casualty years after the tunnel has been completed.

It would be impossible to over-rate the importance of a constant faithful supervision of the building of the lining, especially the arching. The work has to be carried on by workmen in cramped positions, with imperfect light, and surrounded by all kinds of obstacles and inconveniences, and unless a detailed inspection be rigidly maintained, a carelessness in the selection of the materials, and a laxity in the workmanship, will be the inevitable result.

[Figs. 204] to [219] are sections of tunnels which have been constructed for double and single line railways. The sections give the normal form and dimensions adopted in each case, although there may have been many portions of the work where unfavourable or treacherous material necessitated an increase in the thickness of the side walls, or of the arching, or of both. The types vary in accordance with the opinions of the designers as to the most suitable section for the purpose, and range from the comparatively thin lining and vertical side walls shown on [Fig. 207], to the almost circular form and very thick lining shown on [Fig. 216]. The latter is the section which experience has proved to be the best to sustain the enormous all-round pressure exerted by certain descriptions of swelling clay.

Careful judgment will be required to decide which parts of a rock tunnel may be left unlined. The apparently solid-looking portions are oftentimes deceptive, and numbers of instances are on record of large pieces of rock falling down in tunnels which for many years had been considered as thoroughly secure. Where there is any doubt it is better and safer to put in a lining, even if only to the extent of an arching springing from side walls of solid rock, as shown on [Fig. 206]. A moderate additional expenditure at the time of construction may prevent a serious catastrophe afterwards.

The faces or entrances to tunnels may be constructed with

curved wing walls, as in [Fig. 220], or with straight wing walls, as in [Figs. 221, 222, and 223]. Where the approach cutting is in rock, the latter form is generally adopted.

It would be misleading to put down any average price for tunnel-work. So much depends upon the locality, the description of material to be excavated, the cost of masonry or brickwork, and the cost of labour. Added to these come the unforeseen troubles of slips and water-laden strata, creating difficulties which baffle the miners for a time, and add enormously to the expenditure. Some tunnels for double line have been constructed in good ground, and under favourable circumstances as to building materials and labour, for as low as £32 per lineal yard; while others, carried out under adverse conditions, have cost as much as £150 per lineal yard. A medium somewhere between the two should represent the cost of tunnel-work through ground which does not present any special difficulty. At the same time it must be borne in mind that simple tunnelling which can be done in one locality for £50 or £60 per lineal yard, would be increased 20, 30, or 40 per cent. in another, where building material for the lining is scarce and expensive.

Tunnel-work abroad will generally cost more than the same work at home. The native labourers may perhaps be procured at low rates, but the skilled workmen must be brought from a distance, and will obtain high wages.

Another form of tunnel-work, generally termed the covered-way system, is frequently adopted in towns and places where land and space are very valuable. This method consists in the excavating and removing of earthwork to admit of the building of the side walls and arching of a suitable tunnel-way, and then filling in over the top to a depth of three or four feet, or up to the level of the original surface of the ground. This work may be carried out by either removing the entire width of the earthwork before the commencement of any building operations, or by first forming two deep, well-shored trenches, in which to build the side walls up to about arch-springing. In bad ground the latter arrangement has the advantage, as the shoring and strutting to hold up the sliding material is limited to the widths of the two narrow trenches, and the centre block of earthwork is left untouched as a support to the strutting. When the side walls have been built sufficiently high the upper portion of the centre block of earthwork can be removed to allow of the

erection of centering and building of the arching, and afterwards the remaining portion of earthwork can be removed at convenience. In this manner a tunnel-way may be constructed under streets, gardens, and even under buildings. Being nearly all done in the open, the work is more under control than in an ordinary tunnel, but it is usually very costly. Temporary or diverted roads must be arranged; the excavated material must generally all be removed by carts, sometimes to long distances; and provision must be made for diverting the network of sewers, gas, and water pipes which are intercepted along the route.

[Fig. 224] is a sketch of covered-way with brick arching. [Fig. 225] illustrates another type where cast-iron girders and jack-arches of brickwork were introduced on account of the small headway. In soft yielding clay it is necessary to construct strong inverts, as indicated in the sketches. Recesses for the platelayers should be provided every ten or fifteen yards.

The above systems of covered-way were largely adopted in the construction of the underground portions of the Metropolitan Railway and District Railways in and around London.

In addition to the ordinary type of tunnel formed by first excavating the material and then lining the opening with brickwork or masonry, tunnels of moderate size have been constructed of cast-iron tubes, similar in section to [Fig. 226]. The tubes were cast in short segments, bolted together inside, the outer circumference, or surface in contact with the earth or clay, being left free from projections of any kind. By making the segments with bolt-holes exact to template, they were readily fitted together in the work, and a thin layer of suitable packing material placed between the bolting-flanges sufficed to render the tubes water-tight. The tunnelling was carried on by means of a short length of slightly larger tube, or cap, made of plate-iron or steel, which fitted over the leading end of the main tube. The front end of this cap was made very strong, and provided with doors through which the miners could work. A series of hydraulic presses attached to the cap were brought to bear on the bolting-flange of the last completed ring, and as the excavated matter was removed by the miners from the front the cap was forced forward by the hydraulic presses, and another ring of cast-iron segments inserted. On the City and South London Railway, constructed on the above system, the small annular space formed round the cast-iron tube by the operation

of the sliding cap was filled in with cement grouting by means of an ingenious machine designed for the purpose.

Large tunnels under rivers or tidal estuaries must each be dealt with according to the particular circumstances of depth below stream-bed, material to be cut through, length of tunnel, and gradient. The chief obstacle to be contended against in so much of the river tunnel-work is the large volume of water which pours into the workings through fissures in rock or seams of gravel and sand, necessitating the constant use of most powerful pumps. In ordinary land tunnels the gradients are generally laid out to fall towards one or both entrances, and any water finding its way into the excavations may be led away to the entrances by drains or pipes. On the other hand, in a river tunnel the gradients generally fall away from the entrances down towards the centre of the river, and all water coming in must be pumped out and raised up to at least the level of the river. In places where the water comes streaming in from many points, any failure or stoppage of the pumps would place the lives of the miners, and the security of the work itself, in great jeopardy. Iron shields, or protection chambers for the miners advancing the excavation, have been used with great success in carrying on work through loose wet strata which appeared to defy all other means of progress. Solid rock, chalk, or compact clay, may present no difficulty so far as they go, but a continued dip in the gradient, or a line of fault, may suddenly change the entire course of operations, and require the immediate use of the most powerful pumping machinery and protective appliances. The special features of each case will demand special precautions, and the judgment and inventive powers of the engineer will be severely tested in coping with the difficulties with which he is surrounded.

[Contents]