IRON STRUTTING.
In 1862 Mr. Rziha employed old iron railway rails for strutting the Naensen tunnel, and his example was successfully followed in several tunnels built later where timber was scarce and expensive. The advantages which iron strutting is claimed to possess over the more common wooden structure are: its greater strength; the smaller amount of space which it takes up; and the fact that it does not wear out, and may, therefore, be used over and over again.
Fig. 28.—Strutting of Timber Posts and Railway Rail Caps.
Fig. 29.—Strutting made entirely of Railway Rails.
Iron Strutting in Headings.
—In strutting the headings the cross frames have a crown bar consisting of a section of old railway rail carried either by wood or iron side posts. When wooden side posts are used their upper ends have a dovetail mortise, and are bound with an iron band, as shown by [Fig. 28]. The base of the rail crown bar is set into the dovetail mortise and fastened by wedges. When iron side posts are employed they usually consist of sections of railway rails, and the crown bar is attached to them by fish-plate connections, as shown by [Fig. 29]. The iron cross frames are set up as the heading advances, and carry the plank lagging or poling-boards, exactly in the same manner as the timber cross frames previously described.
Fig. 30.—Rziha’s Combined Strutting and Centering of Cast Iron.
Fig. 31.—Cast-Iron Segment of Rziha’s Strutting and Centering.
Full Section Iron Strutting.
—The iron strutting devised by Mr. Rziha for full section work is shown by [Fig. 30]. Briefly described, it consists of voussoir-shaped cast-iron segments, which are built up in arch form. [Fig. 31] shows the construction of one of the segments, all of which are alike, with the exception of the crown segment, which has a mortise and tenon joint which is kept open by filling the mortise with sand. The segments are bolted together by means of suitable bolt-holes in the vertical flanges, and when fully connected form an arch rib of cast iron. This arch rib, A, [Fig. 30], carries a series of angle or T-iron frames bent into approximately voussoir shape, as shown at B, [Fig. 30]. Above these frames are inserted the poling-boards, running longitudinally, and spanning the distance between consecutive arch ribs. By removing the bent iron frames the cast-iron rib forms a center upon which to construct the masonry. Finally, to remove the cast-iron rib itself, the sand is drawn out of the mortise and tenon joint in the crown segment, which allows the joint to close, and loosen the segments so that they are easily unbutted.
The illustration, [Fig. 30], shows longitudinal poling-boards; more often longitudinal crown bars of railway rails span the space between connective arch ribs, and support transverse poling-boards. In building the masonry, work is begun at the bottom on each side, the bent iron frames being removed one after another to give room for the masonry. As each frame is removed, it is replaced with a sort of screw-jack to support the poling-boards until the masonry is sufficiently completed to allow their removal. The interior bracing of the arch rib shown at a a and b b consists of railway rails carried by brackets cast on to the segments. A similar bracing of rails connects the successive arch ribs. These lines of bracing serve to carry the scaffolding upon which the masons work in building the lining.
Fig. 32.—Cast-Iron Segmental Strutting for Shafts.
Iron Shaft Strutting.
—In soft-ground shaft work, the use of an iron strutting, consisting of consecutive cast-iron rings, has sometimes been employed to advantage. [Fig. 32] shows the construction of one of these rings, which, it will be seen, is composed of four segments connected to each other by means of bolted flanges. The holes shown in the circumferential web of the ring are to allow for the seepage from the earth side walls. The method of placing this cylindrical strutting is to start with a ring having a cutting-edge. By means of excavation inside the ring, and by ramming, the ring is sunk into the ground a distance equal to its height. Another ring is then fastened by special hooks on top of the first one, and the sinking continued until the second ring is down flush with the surface. A third ring is then added, and so on until the entire shaft is excavated and strutted. As in timber shaft strutting, the solid iron ring strutting is carried down only to the top of the tunnel section, and below this point there is an open timber or iron supporting framework.
CHAPTER VI.
METHODS OF HAULING IN TUNNELS.
The transportation from one point to another within the tunnel and its shafts of any material, whether it is excavated spoil or construction material, is defined as hauling. In all engineering construction, the transportation of excavated materials, and materials for construction, constitutes a very important part of the expense of the work; but hauling in tunnels where the room is very limited, and where work is constantly in progress over and at the sides of the track, is a particularly expensive process. Hauling in tunnels may be done either by way of the entrances, or by way of the shafts, or by way of both the entrances and shafts.
Fig. 33.—Platform Car for Tunnel Work.
Hauling by Way of Entrances.
—When the hauling is done by the way of the entrances, the materials to be hauled are taken directly from the point of construction to the entrances, or in the opposite direction, by means of special cars of different patterns. For general purposes, these different patterns of cars may be grouped into three classes,—platform-cars, dump-cars, and box-cars. Representative examples of these several classes of cars are shown in [Figs. 33] to [36][6] inclusive, but it will be readily understood that there are many other forms.
[6] Reproduced from catalogue of Arthur Koppel, New York.
Briefly described, platform-cars ([Fig. 33]) consist of a wooden platform mounted on tracks, and they are usually employed for the transportation of timber, ties, etc. Dump-cars are used in greater numbers in tunnel work than any other form. [Fig. 34] shows a dump-car of metal construction, and [Fig. 35] one constructed with a metal under-frame and wooden box. These cars are made to run on narrow-gauge tracks, and usually have a capacity of about one to one and one-half cubic yards. Box-cars are more extensively employed in Europe for tunnel work than in America. [Fig. 36] shows a typical European box-car for tunnel work. It is made either to run on narrow-gauge or standard-gauge tracks.
Fig. 34.—Iron Dump-Car for Tunnel Work.
Fig. 35.—Wooden Dump-Car for Tunnel Work.
Fig. 36.—Box-Car for Tunnel Work.
It is usually desirable in tunnel work to employ cars of different forms for different parts of the work. In rock tunnels it is a common practice to use narrow-gauge cars of small size in the headings, and larger, broad-gauge cars for the enlargement of the profile. Where narrow-gauge cars are employed for all purposes, it will also be found more convenient to use platform-cars for handling the construction material, and dump-cars for removing the spoil. The extent to which it is desirable to use cars of different forms will depend upon the character and conditions of the work, and particularly upon how far it is possible to install the permanent track.
As a general ride, it is considered preferable to lay the permanent tracks at once, and do all the hauling upon them, so that as soon as the tunnel is completed, trains may pass through without delay. To what extent this may be done, or whether it can be done at all or not, depends upon the method of excavation and other local conditions. In soft-ground tunnels excavated by the English or Austrian methods, it is quite possible to lay the permanent tracks at first, since the whole section is excavated at once, and the excavation is kept but a little ahead of the completed tunnel. In rock tunnels, where the heading is driven far ahead of the completed section, it is, of course, impossible to keep the permanent track close to the advance work, and narrow-gauge tracks must be laid in the heading. The same thing is true in soft-ground tunnels driven by successive headings and drifts. In these cases, therefore, where narrow-gauge tracks have to be used for some portions of the work anyway, the question comes up whether it is preferable to use temporary narrow-gauge tracks throughout, or to lay the permanent track as far ahead as possible, and then extend narrow-gauge tracks to the advance excavation. In the latter case it will, of course, be necessary to trans-ship each load from the narrow-gauge to the standard-gauge cars, or vice versa, which means extra cost and trouble. To avoid this, the method is sometimes adopted of laying a third rail between the standard-gauge rails, so that either standard- or narrow-gauge cars may be transported over the line. Whatever form the local conditions may require the system of construction tracks to assume, it may be set down as a general rule that the permanent tracks should be kept as far advanced as possible, and temporary tracks employed only where the permanent tracks are impracticable.
The motive power employed for hauling in tunnels may be furnished by animals or by mechanical motors. Animal power is generally employed in short tunnels and in the advance headings and galleries. In long tunnels, or where the excavated material has to be transported some distance away from the tunnel, mechanical power is preferable, for obvious reasons. The motors most used are small steam locomotives, special compressed-air locomotives, and electric motors. Compressed air and electric locomotives are built in various forms, and are particularly well adapted for tunnel work because of their small dimensions, and freedom from smoke and heat.
Hauling by Way of Shafts.
—When the excavated material and materials of construction are handled through shafts, the operation of hauling may be divided into three processes: the transportation of the materials along the floor of the tunnel, the hoisting of them through the shaft, and the surface transportation from and to the mouth of the shaft. These three operations should be arranged to work in harmony with each other, so as to avoid waste of time and unnecessary handling of the materials. An endeavor should be made to avoid, if possible, breaking or trans-shipping the load from the time it starts at the heading until it is dumped at the spoil bank. This can be accomplished in two ways. One way is to hoist the boxes of the cars from their trucks at the bottom of the shaft, and place them on similar trucks running on the surface tracks. The other way is to run the loaded cars on to the elevator platform at the bottom, hoist them, and then run them on to the surface tracks. If the first method is employed, the car box is usually made of metal, and is provided at its top edges with hooks or ears to which to attach the hoisting cables. When the second method is used, the elevator platform has tracks laid on it which connect with the tracks on the tunnel floor, and also with those on the surface.
Hoisting Machinery.
—The machines most commonly employed for hoisting purposes in tunnel shafts are steam hoisting engines, horse gins, and windlasses operated by hand. Windlasses and horse gins are rather crude machines for hoisting loads, and are used only in special circumstances, where the shaft is of small depth, when the amount of material to be hoisted is small, or where for any reason the use of hoisting engines is precluded. The steam hoisting engine is the standard machine for the rapid lifting of heavy vertical loads. Recently oil engines and electric hoists have also come to be used to some extent, and under certain conditions these machines possess notable advantages.
The construction of hand windlasses is familiar to every one. In tunnel work this device is located directly over the shaft, with its axis a little more than half a man’s height, so that the crank handle does not rise above the shoulder line. To develop its greatest efficiency the hoisting rope is passed around the windlass drum so that the two ends hang down the shaft, and as one end descends the other ascends. A skip, or bucket, is attached to each of the rope ends; and by loading the descending skip with construction materials and the ascending skip with spoil, the two skip loads tend to balance each other, thus increasing the capacity of the windlass, and decreasing the manual labor required to operate it. Skips varying from 0.3 cu. yd. to 0.5 cu. yd. are used. The horse gin consists of a vertical cylinder or drum provided with radial arms to which the horses are hitched, which revolve the cylinder by walking around it in a circle. The hoisting rope is rove around the drum so that the two ends extend down the shaft with skips attached, as described in speaking of the hand windlass. The power of the horse gin is, of course, much greater than that of a windlass operated by hand, skips of 1 cu. yd. capacity being commonly used. Horse gins are no longer economical hoisting machines, according to one prominent authority, when V(H + 20) > 5000, where V equals the volume of material to be hoisted, and H equals the height of the hoist, the weight of the excavated material being 2100 lbs. per cu. yd. As a general rule, however, it is assumed that it is not economical to employ horse gins with a depth of shaft exceeding 150 ft.
As already stated, the most efficient and most commonly used device for hoisting at tunnel shafts is the steam hoisting engine. There are numerous builders of hoisting engines, each of which manufactures several patterns and sizes of engines. In each case, however, the apparatus consists of a boiler supplying steam to a horizontal engine which operates one or more rope drums. The engines are always reversible. They may be employed to hoist the skips directly, or to operate elevators upon which the skips or cars are loaded. In either case the hoisting ropes pass from the engine drum to and around vertical sheaves situated directly over the shaft so as to secure the necessary vertical travel of the ropes down the shaft. Where the shaft is divided into two compartments, each having an elevator or hoist, double-drum engines are employed, one drum being used for the operations in one compartment, and the other for the operations in the other compartment. Where the work is to be of considerable duration, or when it is done in cold weather, more or less elaborate shelters or engine houses are built to cover and protect the machinery.
Choice between the method of hoisting the skips directly, and the method of using elevators, depends upon the extent and character of the work. Where large quantities of material are to be hoisted rapidly, it is generally considered preferable to employ elevators instead of hoisting the skips directly. In direct hoisting at high speed, oscillations are likely to be produced which may dash the skips against the sides of the shaft and cause accidents. The loads which can be carried in single skips are also smaller than those possible where elevators are used; and this, combined with the slower hoisting speed required, reduces the capacity of this method, as compared with the use of elevators. Where elevators are employed, however, the plant required is much more extensive and costly; it comprising not only the elevator cars with their safety devices, etc., but the construction of a guiding framework for these cars in the tunnel shaft. For these various reasons the elevator becomes the preferable hoisting device where the quantity of material to be handled is large, where the shafts are deep, and where the work will extend over a long period of time; but when the contrary conditions are the case, direct hoisting of the skips is generally the cheaper. The engineer has to integrate the various factors in each individual case, and determine which method will best fulfill his purpose, which is to handle the material at the least cost within the given time and conditions.
Fig. 37.—Elevator Car for Tunnel Shafts.
The construction of elevators for tunnel work is simple. The elevator car consists usually of an open framework box of timber and iron, having a plank floor on which car tracks are laid, and its roof arranged for connecting the hoisting cable ([Fig. 37][7]). Rigid construction is necessary to resist the hoisting strains. The sides of the car are usually designed to slide against timber guides on the shaft walls. Some form of safety device, of which there are several kinds, should be employed to prevent the fall of the elevator, in case the hoisting rope breaks, or some mishap occurs to the hoisting machinery, which endangers the fall of the car. Speaking tubes and electric-bell signals should also be provided to secure communication between the top and bottom of the shaft.
[7] Reproduced from the catalogue of the Ledgerwood Manufacturing Company, New York.
CHAPTER VII.
TYPES OF CENTERS AND MOLDS EMPLOYED IN CONSTRUCTING TUNNEL LININGS OF MASONRY.
The masonry lining of a tunnel may be described as consisting of two or more segments of circular arches combined so as to form a continuous solid ring of masonry. To direct the operations of the masons in constructing this masonry ring, templates or patterns are provided which show the exact dimensions and form of the sectional profile which it is desired to secure. These patterns or templates will vary in number and construction with the form of lining and the method of excavation adopted. Where the excavation is fully lined on all four sides, the masonry work is usually divided into three parts,—the invert or floor masonry, the side-wall masonry, and the roof-arch masonry. At least one separate pattern has to be employed in constructing each of these parts of the lining; and they are known respectively as ground molds, leading frames, and arch centers, or simply centers. In the following paragraphs the form and construction usually employed for each of these three kinds of patterns is described.
Ground Molds.
—Ground molds are employed in building the tunnel invert. They are generally constructed of 3-inch plank cut exactly to the form and dimensions of the invert masonry as shown in [Fig. 38]. To permit of convenience of handling in a restricted space, they are generally made in two parts, which are joined at the middle by means of iron fish-plates and bolts. Either one or two ground molds may be employed. Where two molds are used they are set up a short distance apart, and cords are stretched from one to the other parallel to the axis of the tunnel, by which the masons are guided in their work. Extreme care has to be taken in setting the molds to ensure that they are fixed at the proper grade, and are in a plane normal to the axis of the tunnel. Where only one ground mold is employed, the finished masonry is depended upon to supply the place of the second mold, cords being stretched from it to the single mold placed the requisite distance ahead. The leveling and centering of the molds is accomplished by means of transit and level.
Fig. 38.—Ground Mold for Constructing Tunnel Invert Masonry.
Fig. 39.—Combined Ground Mold and Leading Frame for Invert and Side Wall Masonry.
Two modifications of the form of ground mold shown by [Fig. 39] are employed. The first modification is peculiar to the English method of excavation, and consists in combining the ground mold with the leading frame for the lower part of the side walls, as shown by [Fig. 39]. The second modification is employed where the two halves or sides of the invert are built separately, and it consists simply in using one-half of the mold shown by [Fig. 38]. When the last method of constructing the invert masonry is resorted to, extreme care has to be observed in setting the half-mold in order to avoid error.
Fig. 40.—Leading Frame for Constructing Side Wall Masonry.
Leading Frames.
—As already stated, leading frames are the patterns, or molds, used in building the side walls of the lining. Like the ground mold they are usually built of plank; one side being cut to the curve of the profile, and the other being made parallel to the vertical axis of the tunnel section. The vertical side usually has some arrangement by which a plumb bob can be attached, as shown by [Fig. 40], to guide the workmen in erecting the frame. The combined leading frame and ground mold shown in [Fig. 39] has already been described. The use of this frame is possible only where the masonry is begun at the invert and carried up on each side at the same time. This mode of construction is peculiar to the English method of tunneling; in all other methods the form of separate ground frame shown by [Fig. 40] is employed. The ground frames are lined in and leveled up by transit and level; and, as in setting the ground frames, care must be taken to secure accuracy in both direction and elevation.
Arch Centers.
—The template or form upon which the roof arch is built is called a center. Unlike the ground molds and leading frames, the arch centers have to support the weight of the masonry and the roof pressures during the construction of the lining, and they, therefore, require to be made strong. Owing to the fact that the pressures are indeterminate, it is impossible to design a rational center, and resort is had to those constructions which past experience has shown to work satisfactorily under similar conditions. In a general way it can always be assumed that the construction should be as simple as possible, that the center should be so designed that it can be set up and removed with the least possible labor, and that the different pieces of the framework and lagging should be as short as possible, for convenience in handling.
Tunnel centers are usually composed of two parts,—a mold or curved surface upon which the masonry rests, and a framework which supports the mold. The curved surface or mold consists of a lagging of narrow boards running parallel to the tunnel axis, which rests upon the arched top members of two or more consecutive supporting frames. The supporting frame is built in the form of a truss, and must be made strong enough to withstand the heavy superimposed loads, consisting of the arch masonry during construction, and of the roof pressures which are transferred to the center when the strutting is removed to allow the masonry to be placed. The framework of the center is supported either by posts resting upon the floor of the excavation, or upon the invert masonry when this is built first, as in the English and Austrian methods, or it may be supported directly upon the ground where the arch masonry is built first, as in the Belgian method of tunneling.
In describing the various methods of tunneling in succeeding chapters, the center construction and method of supporting the center peculiar to each will be fully explained, and only a few general remarks are necessary here. Centers may be classified according to their construction and composition into plank centers, truss centers, and iron centers.
Fig. 41.—Plank Center for Constructing the Roof Arch.
One of the most common forms of plank centers is shown by [Fig. 41]. It consists of two half-polygons whose sides consist of 15 in. × 4 ft. planks having radial end-joints. These two half-polygons are laid one upon the other so that they break joints, as shown by the figure, and the extrados of the frame is cut to the true curve of the roof arch. The planks commonly used for making these centers are 4 ins. thick, making the total thickness of the center 8 ins. Plank centers of the construction described are suitable only for work where the pressures to be resisted are small, as in tunnels through a fairly firm rock, although there have been instances of their successful use in soft-ground tunnels.
Where heavy loads have to be carried, trussed centers are generally employed, the trusses being composed of heavy square beams with scarfed and tenoned joints, reinforced by iron plates. Different forms of trusses are employed in each of the different methods of tunneling, and each of these is described in succeeding chapters; but they are generally either of the king-post or queen-post type, or some modification of them. The king-post truss may be used alone, with or without the tie-beam, as shown by [Fig. 42]; but generally a queen-post truss is made to form the base of support for a smaller king-post truss mounted on its top. This arrangement gives a trapezoidal form to the center, which approaches closely to the arch profile. Owing to the character of the pressures transmitted to the center, the usual tension members can be made very light.
Fig. 42.—Trussed Center for Constructing the Roof Arch.
The combined center and strutting system devised by Mr. Rziha has already been [described] in a previous chapter. In recent European tunnel work quite extensive use has also been made of iron centers consisting of several segments of curved I-beams, connected by fish-plate joints so as to form a semi-circular arch rib. The ends or feet of these I-beam ribs have bearing-plates or shoes made by riveting angles to the I-beams. Centers constructed in a similar manner, but made of sections of old railway rail, were used in carrying out the tunnel work on the Rhine River Railroad in Germany. The advantages claimed for iron centers are that they take up less room, and that they can be used over and over again.
Setting Up Centers.
—According to the method of excavation followed in building the tunnel, the centers for building the roof arch may have to be supported by posts resting on the tunnel floor; or where the arch is built first, as in the Belgian and Italian methods, they may be carried on blocking resting on the unexcavated earth below. Whichever method is employed, an unyielding support is essential, and care must be taken that the centers are erected and maintained in a plane normal to the tunnel axis. To prevent deflection and twisting, the consecutive centers are usually braced together by longitudinal struts or by braces running to the adjacent strutting. Only skilled and experienced workmen should be employed in erecting the centers; and they should work under the immediate direction of the engineer, who must establish the axis and level of each center by transit and level.
Lagging.
—By the lagging is meant the covering of narrow longitudinal boards resting upon the upper curved chords of the centers, and spanning the opening between consecutive centers. This lagging forms the curved surface or mold upon which the arch masonry is laid. When the roof arch is of ashlar masonry the strips of lagging are seldom placed nearer together than the joints of the consecutive ring stones, but in brick arches they are laid close together. Besides the weight of the arch masonry, the lagging timbers support the short props which keep the poling-boards in place after the strutting is removed and until the arch masonry is completed.
Striking the Centers.
—The centers are usually brought to the proper elevation by means of wooden wedges inserted between the sill of the center and its support, or between the bottom of the posts carrying the center and the tunnel floor. These wedges are usually made of hard wood, and are about 6 ins. wide by 4 ins. thick by 18 ins. long. To strike the center after the arch masonry is completed, these wedges are withdrawn, thus allowing the center to fall clear of the masonry. Usually the center is not removed immediately after striking, so that if the arch masonry fails the ruins will remain upon the center. The method of striking the iron center devised by Mr. Rziha has been [described] in the previous chapter on strutting.
CHAPTER VIII.
METHODS OF LINING TUNNELS.
Tunnels in soft soils and in loose rock, and rock liable to disintegration, are always provided with a lining to hold the walls and roof in place. This lining may cover the entire sectional profile of the tunnel, or only a part of it, and it may be constructed of timber, iron, iron and masonry, or, more commonly, of masonry alone.
Timber Lining.
—Timber is seldom employed in lining tunnels except as a temporary expedient, and is replaced by masonry as soon as circumstances will permit. In the first construction of many American railways, the necessity for extreme economy in construction, and of getting the line open for traffic as soon as possible, caused the engineers to line many tunnels with timber, which was plentiful and cheap. Except for their small cost and the ease and rapidity with which they can be constructed, however, these timber linings possess few advantages. It is only the matter of a few years when the decay of the timber makes it necessary to rebuild them, and there is always the serious danger of fire. In several instances timber-lined tunnels in America have been burned out, causing serious delays in traffic, and necessitating complete reconstruction. Usually this reconstruction has consisted in substituting masonry in place of the original timber lining. In a succeeding [chapter] a description will be given of some of the methods employed in replacing timber tunnel linings with masonry. Various forms of timber lining are employed, of which [Fig. 44] and the illustrations in the [chapter] discussing the methods of relining timber-lined tunnels with masonry are typical examples.
Cross Section.
Longitudinal Section.
Figs. 43 and 44.—A Typical Form of Timber Lining for Tunnels.
Iron Lining.
—The use of iron lining for tunnels was introduced first on a large scale by Mr. Peter William Barlow in 1869, for the second tunnel under the River Thames at London, England, and it has greatly extended since that time. The lining of the second Thames tunnel consisted of cylindrical cast-iron rings 8 ft. in diameter, the abutting edges of the successive rings being flanged and provided with holes for bolt fastenings. Each ring was made up of four segments, three of which were longer than quadrants, and one much smaller forming the “key-stone” or closing piece. These segments were connected to each other by flanges and bolts. To make the joints tight, strips of pine or cement and hemp yarn were inserted between the flanges. Since the construction of the second Thames tunnel, iron lining has been employed for a great many submarine tunnels in England, Continental Europe, and America, some of them having a section over 28 ft. in diameter. Where circular iron lining is employed, the bottom part of the section is leveled up with concrete or brick masonry to carry the tracks, and the whole interior of the ring is covered with a cement plaster lining deep enough thoroughly to embed the interior joint flanges. In the succeeding [chapter] describing the methods of driving tunnels by shields several forms of iron tunnel lining are fully described.
Iron and Masonry Lining.
—During recent years a form of combined masonry and iron lining has been extensively employed in constructing city underground railways in both Europe and America. Generally this form of lining is built with a rectangular section. Two types of construction are employed. In the first, masonry side walls carry a flat roof of girders and beams, which carry a trough flooring filled with concrete, or between which are sprung concrete or brick arches. Sometimes the roof framing consists of a series of parallel I-beams laid transversely across the tunnel, and in other cases transverse plate girders carry longitudinal I-beams. In the second type of construction the roof girders are supported by columns embedded in the side walls. Where the tunnel provides for two or four tracks, intermediate column supports are in some cases introduced between the side columns. In this construction the roofing consists of concrete filled troughs or of concrete or brick arches, as in the construction first described. Examples of combined masonry and iron tunnel lining are illustrated in the succeeding [chapter] on tunneling under city streets.
Masonry Lining.
—The form of tunnel lining most commonly employed is brick or stone masonry. Concrete and reinforced concrete masonry lining has been employed in several tunnels built in recent years. The masonry lining may inclose the whole section or only a part of it. The floor or invert is the part most commonly omitted; but sometimes also the side walls and invert are both omitted, and the lining is confined simply to an arch supporting the roof. The roof arch, the side walls, and the invert compose the tunnel lining; and all three may consist of stone or brick alone, or stone side walls may be employed with brick invert and roof arch. Rubble-stone masonry is usually employed, except at the entrances, where the masonry is exposed to view. Here ashlar masonry is usually used. The stone selected for tunnel lining should be of a durable quality which weathers well. Where bricks are used they should be of good quality. Owing to the comparative ease with which brick arches can be built, they are generally used to form the roof arch, even where the side walls are of stone masonry. Masonry lining may be built in the form of a series of separate rings, or in the form of a continuous structure extending from one end of the tunnel to the other. The latter method of construction produces a stronger structure; but in case of failure by crushing, the damage done is likely to be more widespread than where separate rings are employed, one or two of which may fail without injury to the others adjacent to them. The construction is also somewhat simpler where separate rings are employed, since no provision has to be made for bonding the whole lining into a continuous structure. Where a series of separate rings is employed, the length of each ring runs from 5 ft. up to 20 ft., it depending upon the character of the material penetrated, and the method of construction employed. For the purpose of detailed discussion the construction of masonry lining may be divided into four parts,—the side-wall foundations, the side walls themselves, the roof arch, and the invert.
Concrete and reinforced concrete linings are now extensively used on account of cheapness and facility of handling, but they have the great disadvantage of resisting pressure after they become hard, which is some time after being placed. The strutting should, therefore, be left to support the roof so as to prevent direct pressure on the fresh material. The roof, as a rule, is supported by longitudinal planks held in position by five or seven segments of arched frames placed across the tunnel. A large quantity of timber and carpenter work is thus entirely wasted and these costly items, in many cases, make the concrete lining of a tunnel more expensive than the one built of brick and stone. To avoid these inconveniences tunnels have been successfully lined with concrete on the side walls and concrete blocks in the arches. These blocks have been built by hand and molded in the shape of the arch voussoirs.
Foundations.
—In tunnels through rock of a hard and durable character the foundations for the side walls are usually laid directly on the rock. In loose rock, or rock liable to disintegration, this method of construction is not generally a safe one, and the foundation excavation should be sunk to a depth at which the atmospheric influences cannot affect the foundation bed. In either case the foundation masonry is made thicker than that of the side walls proper, so as to distribute the pressure over a greater area, and to afford more room for adjusting the side-wall masonry to the proper profile. In yielding soils a special foundation bed has to be prepared for the foundation masonry. In some instances it is found sufficient to lay a course of planks upon which the masonry is constructed, but a more solid construction is usually preferred.
This is obtained by placing a concrete footing from 1 ft. to 2 ft. deep all along the bottom of the foundation trench, or in some cases by sinking wells at intervals along the trench and filling them with concrete, so as to form a series of supporting pillars.
Fig. 45.—Diagram Showing Forms Adopted for Side-Wall Foundations.
The form given to the foundation courses and lower portions of the side walls varies. Where a large bearing area is required, the back of the wall is carried up vertically as shown by the line AB, [Fig. 45], otherwise the rear face of the wall follows the line of excavation AC. For similar reasons the front face of the wall may be made vertical, as at FG, or inclined, as at FH. The line FE indicates the shelf construction designed to support the feet of the posts used to carry the arch centers during the construction of the roof arch.
Side Walls.
—The construction of the side walls above the foundation courses is carried out as any similar piece of masonry elsewhere would be built. To direct the work and insure that the inner faces of the walls follow accurately the curve of the chosen profile, leading frames previously described are employed.
Roof Arch.
—For the construction of the roof arch, the centers previously described are employed. Beginning at the edges of the center on each side, the masonry is carried up a course at a time, care being taken to have it progress at the same rate on both sides, so that the load brought onto the centering is symmetrical. As soon as the centers are erected, the roof strutting is removed, and replaced by short props which rest on the lagging of the centers and support the poling-boards. These props are removed in succession as the arch masonry rises along the curve of the center, and the space between the top of the arch masonry and the ceiling of the excavation is filled with small stones packed closely. The keystone section of the arch is built last, by inserting the stones or bricks from the front edge of the arch ring, there being no room to set them in from the top, as is the practice in ordinary open-arch construction. The keying of the arch is an especially difficult operation, and only experienced men skilled in the work should be employed to perform it. The task becomes one of unusual difficulty when it becomes necessary to join the arches coming from opposite directions.
Invert.
—In all but one or two methods of tunneling, the invert is the last portion of the lining to be built. In the English method of tunneling, the invert is the first portion of the lining to be built, and the same practice is sometimes necessary in soft soils where there is danger of the bottoms of the side walls being squeezed together by the lateral pressures unless the invert masonry is in place to hold them apart. The ground molds previously described are employed to direct the construction of the invert masonry.
General Observations.
—In describing the construction of the roof arch, mention was made of the stone filling employed between the back of the masonry ring and the ceiling of the excavation. The spaces behind the side walls are filled in a similar manner. The object of this stone filling, which should be closely packed, is to distribute the vertical and lateral pressures in the walls of the excavation uniformly over the lining masonry. As the masonry work progresses, the strutting employed previously to support the walls of the excavation has to be removed. This work requires care to prevent accident, and should be placed in charge of experienced mechanics who are familiar with its construction, and can remove it with the least damage to the timbers, so that they may be used again, and without causing the fall of the roof or the caving of the sides by removing too great a portion of the timbers at one time.
Thickness of Lining Masonry.
—It is obvious, of course, that the masonry lining must be thick enough to support the pressure of the earth which it sustains; but, as it is impossible to estimate these pressures at all accurately, it is difficult to say definitely just what thickness is required in any individual case. Rankine gives the following formulas for determining the depths of keystone required in different soils:
For firm soils
d = √(0.12 r2s),
and for soft soils,
d = √(0.48 r2s),
where d = the depth of the crown in feet, r = the rise of the arch in feet, and s = the span of the arch in feet. Other writers, among them Professor Curioni, attempt to give rational methods for calculating the thickness of tunnel lining; but they are all open to objection because of the amount of hypothesis required concerning pressures which are of necessity indeterminate. Therefore, to avoid tedious and uncertain calculations, the engineer adopts dimensions which experience has proven to be ample under similar conditions in the past. Thus we have all gradations in thickness, from hard-rock tunnels requiring no lining, and tunnels through rocks which simply require a thin shell to protect them from the atmosphere, to soft-ground tunnels where a masonry lining 3 ft. or more in thickness is employed. [Table II]. shows the thickness of masonry lining used in tunnels through soft soils of various kinds.
The thickness of the masonry lining is seldom uniform at all points, as is indicated by [Table II]. [Figs. 46 and 47] show common methods of varying the thickness of lining at different points, and are self-explanatory.
Figs. 46 and 47.—Transverse Sections of Tunnels Showing Methods of Increasing the Thickness of the Lining at Different Points.
Side Tunnels.
—When tunnels are excavated by shafts located at one side of the center line, short side tunnels or galleries are built to connect the bottoms of the shafts with the tunnel proper. These side tunnels are usually from 30 ft. to 40 ft. long, and are generally made from 12 ft. to 14 ft. high, and about 10 ft. wide. The excavation, strutting, and lining of these side tunnels are carried on exactly as they are in the main tunnel, with such exceptions as these short lengths make possible. [Table III]. gives the thickness of lining used for side tunnels, the figures being taken from European practice.
Culverts.
—The purpose of culverts in tunnels is to collect the water which seeps into the tunnel from the walls and shafts. The culvert is usually located along the center line of the tunnel at the bottom. In soft-ground tunnels it is built of masonry, and forms a part of the invert, but in rock tunnels it is the common practice to cut a channel in the rock floor of the excavation. Both box and arch sections are employed for culverts. The dimensions of the section vary, of course, with the amount of water which has to be carried away. The following are the dimensions commonly employed:
| Kind of Culvert. | Height in Feet. | Width in Feet. | Thickness of Walls in Feet. | Thickness of Covering in Feet. |
|---|---|---|---|---|
| Box culvert | 1 to 1.5 | 1 to 1.5 | 0.8 to 1.2 | 0.3 |
| Arch culvert | 1 to 1.5 | 1 to 1.5 | 0.8 to 1.2 | 0.4 |
It should be understood that the dimensions given in the table are those for ordinary conditions of leakage; where larger quantities of water are met with, the size of the culverts has, of course, to be enlarged. To permit the water to enter the culvert, openings are provided at intervals along its side; and these openings are usually provided with screens of loose stones which check the current, and cause the suspended material to be deposited before it enters the culvert. In cases where springs are encountered in excavating the tunnel, it is necessary to make special provisions for confining their outflow and conducting it to the culvert. In all cases the culverts should be provided with catch basins at intervals of from 150 ft. to 300 ft., in which such suspended matter as enters the culverts is deposited, and removed through covered openings over each basin. At the ends of the tunnel the culvert is usually divided into two branches, one running to the drain on each side of the track.
Fig. 48.—Refuge Niche in St. Gothard Tunnel.
Niches.
—In short tunnels niches are employed simply as places of refuge for trackmen and others during the passing of trains, and are of small size. In long tunnels they are made larger, and are also employed as places for storing small tools and supplies employed in the maintenance of the tunnel. Niches are simply arched recesses built into the sides of the tunnel, and lined with masonry; [Fig. 48] shows this construction quite clearly. Small refuge niches are usually built from 6 ft. to 9 ft. high, from 3 ft. to 6 ft. wide, and from 2 ft. to 3 ft. deep. Large niches designed for storing tools and supplies are made from 10 ft. to 12 ft. high, from 8 ft. to 10 ft. wide, and from 18 ft. to 24 ft. deep, and are provided with doors. Refuge niches are usually spaced from 60 ft. to 100 ft. apart, while the larger storage niches may be located as far as 3000 ft. apart. The niche construction shown by [Fig. 48] is that employed on the St. Gothard tunnel.
Entrances.
—The entrances, or portals, of tunnels usually consist of more or less elaborate masonry structures, depending upon the nature of the material penetrated. In soft-ground tunnels extensive wing walls are often required to support the earth above and at the sides of the entrance; while in tunnels through rock, only a masonry portal is required, to give a finish to the work. Often the engineer indulges himself in an elaborate architectural design for the portal masonry. There is danger of carrying such designs too far for good taste unless care is employed; and on this matter the writer can do no better than to quote the remarks of the late Mr. Frederick W. Simms in his well-known “Practical Tunneling”:
“The designs for such constructions should be massive to be suitable as approaches to works presenting the appearance of gloom, solidity, and strength. A light and highly decorated structure, however elegant and well adapted for other purposes, would be very unsuitable in such a situation; it is plainness combined with boldness, and massiveness without heaviness, that in a tunnel entrance constitutes elegance, and, at the same time, is the most economical.”
Fig. 49.—East Portal of Hoosac Tunnel.
[Fig. 49] is an engraving from a photograph of the east portal of the Hoosac tunnel, which is an especially good design. The portals of the Mount Cenis tunnel were built of samples of stone encountered all along the line of excavation. The stones were cut and dressed and utilized for walls and voussoirs. The only ornament that is usually allowed on the portals is the date of the opening of the tunnel prominently cut in the stone above the arch.
Table II.
Showing Thickness of Masonry Lining for Tunnels through Soft Ground.
| Character of Material. | Keystone. | Springers. | Invert. |
|---|---|---|---|
| Ft. | Ft. | Ft. | |
| Laminated clay, first variety | 2.15 to 3 | 2.75 to 3.5 | 1.6 to 2.5 |
| Laminated clay, second variety | 3 to 4.5 | 3.5 to 5.5 | 2.5 to 4 |
| Laminated clay, third variety | 4.5 to 6.5 | 5.5 to 8.1 | 4 to 4.5 |
| Quicksand | 2 to 3.28 | 2 to 4.1 | 1.33 to 2.5 |
TABLE III.
Showing Thickness of Masonry Lining for Side Tunnels through Soft Ground.
| Character of Material. | Keystone. | Springers. | Invert. |
|---|---|---|---|
| Ft. | Ft. | Ft. | |
| Laminated clay, first variety | 1.6 to 2.3 | 1.8 to 3 | 1.5 to 2 |
| Laminated clay, second variety | 2.3 to 3 | 3 to 4.1 | 2 to 2.6 |
| Laminated clay, third variety | 3 to 4 | 4.1 to 5 | 2.6 to 3.29 |
| Quicksand | 1.6 to 2.5 | 1.3 to 2 | 1.3 to 2 |
CHAPTER IX.
TUNNELS THROUGH HARD ROCK; GENERAL DISCUSSION; REPRESENTATIVE MECHANICAL INSTALLATIONS FOR TUNNEL WORK.
The present high development of labor-saving machinery for excavating rock makes this material one of the safest and easiest to tunnel of any with which the engineer ordinarily has to deal. To operate this machinery requires, however, the development of a large amount of power, its transmission to considerable distances, and, finally, its economical application to the excavating tools. The standard rock excavating machine is the power drill, which requires either air or hydraulic pressure for its operation according to the special type employed. Under present conditions, therefore, the engineer is limited either to air or water under compression for the transmission of his power. Steam-power may be employed directly to operate percussion rock drills; but owing to the heat and humidity which it generates in the confined space where the drills work, and because of other reasons, it is seldom employed directly. Electric transmission, which offers so many advantages to the tunnel builder, in most respects is largely excluded from use by the failure which has so far followed all attempts to apply it to the operation of rock drills. As matters stand, therefore, the tunnel engineer is practically limited to steam and falling water for the generation of power, and to compressed air and hydraulic pressure for its transmission.
Whether the engineer should adopt water-power or steam to generate the power required for his excavating machinery depends upon their relative availability, cost, and suitability to the conditions of work in each particular case. Where fuel is plentiful and cheap, and where water-power is not available at a comparatively reasonable cost, steam-power will nearly always prove the more economical; where, however, the reverse conditions exist, which is usually the case in a mountainous country far from the coal regions, and inadequately supplied with transportation facilities, but rich in mountain torrents, water-power will generally be the more economical. In a succeeding chapter the power generating and transmission plants for a number of rock tunnels are described, and here only a general consideration of the subject will be presented.
Steam-Power Plant.
—A steam-power plant for tunnel work should be much the same as a similar plant elsewhere, except that in designing it the temporary character of its work must be taken into consideration. This circumstance of its temporary employment prompts the omission of all construction except that necessary to the economical working of the plant during the period when its operation is required. The power-house, the foundations for the machinery, and the general construction and arrangement, should be the least expensive which will satisfy the requirements of economical and safe operation for the time required. It will often be found more economical as a whole to operate the machinery with some loss of economy during the short time that it is in use than to go to much greater expense to secure better economy from the machinery by design and construction, which will be of no further use after the tunnel is completed. The longer the plant is to be required, the nearer the construction may economically approach that of a permanent plant. As regards the machinery itself, whose further usefulness is not limited by the duration of any single piece of work, true economy always dictates the purchase of the best quality. Speaking in a general way, a steam-power plant for tunnel work comprises a boiler plant, a plant of air compressors with their receivers, and an electric light dynamo. When hydraulic transmission of power is employed, the air compressors are replaced by high-pressure pumps; and when electric hauling is employed, one or more dynamos may be required to generate electricity for power purposes, as well as for lighting. In addition to the power generating machines proper, there must be the necessary piping and wiring for transmitting this power, and, of course, the equipment of drills and other machines for doing the actual excavating, hauling, etc.
Reservoirs.
—When water-power is employed, a reservoir has to be formed by damming some near-by mountain stream at a point as high as practicable above the tunnel. The provision of a reservoir, instead of drawing the water directly from the stream, serves two important purposes. It insures a continuous supply and constant head of water in case of drought, and also permits the water to deposit its sediment before it is delivered to the turbines. The construction of these reservoirs may be of a temporary character, or they may be made permanent structures, and utilized after construction is completed to supply power for ventilation and other necessary purposes. In the first case they are usually destroyed after construction is finished. In either case, it is almost unnecessary to say, they should be built amply safe and strong according to good engineering practice in such works, for the duration of time which they are expected to exist.
Canals and Pipe Lines.
—For conveying the water from the reservoirs to the turbines, canals or pipe lines are employed. The latter form of conduit is generally preferable, it being both less expensive and more easily constructed than the former. It is advisable also to have duplicate lines of pipe to reduce the possibility of delay by accident or while necessary repairs are being made to one of the pipes. The pipe lines terminate in a penstock leading into the turbine chamber, and provided with the necessary valves for controlling the admission of water to the turbines.
Turbines.
—There are numerous forms of turbines on the market, but they may all be classed either as impulse turbines or as reaction turbines. Impulse turbines are those in which the whole available energy of the water is converted into kinetic energy before the water acts on the moving part of the turbine. Reaction turbines are those in which only a part of the available energy of the water is converted into kinetic energy before the water acts on the moving vanes. Impulse turbines give efficient results with any head and quantity of water, but they give better results when the quantity of water varies and the head remains constant. Reaction turbines, on the contrary, give better results when the quantity of water remains constant and the head varies. These observations indicate in a general way the form of turbine which will best meet the particular conditions in each case. The number of turbines required, and their dimensions, will be determined in each case by the number of horse-power required and the quantity of water available. The power of the turbines is transmitted to the air compressors or pumps by shafting and gearing.
Air Compressors.
—An air compressor is a machine—usually driven by steam, although any other power may be used—by which air is compressed into a receiver from which it may be piped for use. For a detailed description of the various forms of air compressors the reader should consult the catalogues of the several makers and the various text-books relating to air compression and compressed air. Air compressors, like other machines, suffer a loss of power by friction. The greatest loss of power, however, results from the heat of compression. When air is compressed, it is heated, and its relative volume is increased. Therefore, a cubic foot of hot air in the compressor cylinder, at say, 60 lbs. pressure, does not make a cubic foot of air at 60 lbs. pressure after cooling in the receiver. In other words, assuming pressure to be constant, a loss of volume results due to the extraction of the heat of compression after the air leaves the compressor cylinder. To reduce the amount of this loss, air compressors are designed with means to extract the heat from the air before it leaves the compressor cylinder. Air compressors may first be divided into two classes, according to the means employed for cooling the air, as follows: (1) Wet compressors, and (2) dry compressors. A wet compressor is one which introduces water directly into the cylinder during compression, and a dry compressor is one which admits no water to the air during compression. Wet compressors may be subdivided into two classes: (1) Those which inject water in the form of spray into the cylinder during compression, and (2) those which use a water piston for forcing the air into confinement.
The following brief discussion of these various types of compressors is based on the concise practical discussion of Mr. W. L. Saunders, M. Am. Soc. C. E., in “Compressed Air Production.” The highest isothermal results are obtained by the injection of water into the cylinders, since it is plain that the injection of cold water, in the shape of a finely divided spray, directly into the air during compression will lower the temperature to a greater degree than simply to surround the cylinder and parts by water jackets which is the means of cooling adopted with dry compressors. A serious obstacle to water injection, and that which condemns this type of compressor, is the influence of the injected water upon the air cylinder and parts. Even when pure water is used, the cylinders wear to such an extent as to produce leakage and to require reboring. The limitation to the speed of a compressor is also an important objection. The chief claim for the water piston compressor is that its piston is also its cooling device, and that the heat of compression is absorbed by the water. Water is so poor a conductor of heat, however, that without the addition of sprays it is safe to say that this compressor has scarcely any cooling advantages at all so far as the cooling of the air during compression is concerned. The water piston compressor operates at slow speed and is expensive. Its only advantage is that it has no dead spaces. In the dry compressor a sacrifice is made in the efficiency of the cooling device to obtain low first cost, economy in space, light weight, higher speed, greater durability, and greater general availability.
Air compressors are also distinguished as double acting and simple acting. They are simple acting when the cylinder is arranged to take in air at one stroke and force it out at the next, and they are double acting when they take in and force out air at each stroke. In form compressors may be simple or duplex. They are simple when they have but one cylinder, and duplex when they have two cylinders. A straight line or direct acting compressor is one in which the steam and air cylinders are set tandem. An indirect acting compressor is one in which the power is applied indirectly to the piston rod of the air cylinder through the medium of a crank. Mr. W. L. Saunders writes in regard to direct and indirect compression as follows:—
“The experience of American manufacturers, which has been more extensive than that of others, has proved the value of direct compression as distinguished from indirect. By direct compression is meant the application of power to resistance through a single straight rod. The steam and air cylinders are placed tandem. Such machines naturally show a low friction loss because of the direct application of power to resistance. This friction loss has been recorded as low as 5%, while the best practice is about 10% with the type which conveys the power through the angle of a crank shaft to a cylinder connected to the shaft through an additional rod.”
Receivers.
—Compressed air is stored in receivers which are simply iron tanks capable of withstanding a high internal pressure. The purpose of these tanks is to provide a reservoir of compressed air, and also to allow the air to deposit its moisture. From the receivers the air is conveyed to the workings through iron pipes, which decrease gradually in diameter from the receivers to the front.
Rock Drills.
—The various forms of rock drills used in tunneling have been [described] in [Chapter III]., and need not be considered in detail here except to say that American engineers usually employ percussion drills, while European engineers also use rotary drills extensively. A comparison between these two types of drills was made in excavating the Aarlberg tunnel in Austria, where the Brandt hydraulic rotary drill was used at one end, and the Ferroux percussion drill was used at the other end. The rock was a mica-schist. The average monthly progress was 412 ft., with a maximum of 646 ft., with the rotary drills, and an average of 454 ft. with the percussion drill.
Excavation.
—Since considerable time is required to get the power plant established, the excavation of rock tunnels is often begun by hand, but hand work is usually continued for no longer a period than is necessary to get the power plant in operation. Generally speaking, the greatest difficulty is encountered in excavating the advanced drift or heading. Based on the mode of blasting employed, there are two methods of driving the advanced gallery, known as the circular cut and the center cut methods. In the first method a set of holes is first drilled near the center of the front in such a manner that they inclose a cone of rock; the holes, starting at the perimeter of the base of the cone, converge toward a junction at its apex. Seldom more than four to six holes are comprised in this first set. Around these first holes are driven a ring of holes which inclose a cylinder of rock, and if necessary succeeding rings of holes are driven outside of the first ring. These holes are blasted in the order in which they are driven, the first set taking out a cone of rock, the second set enlarging this cone to a cylinder, and the other sets enlarging this cylinder to the required dimensions of the heading. The number of holes, however, varies with the quality of rock and they are seldom driven deeper than 4 or 5 ft. This method of excavating the heading, which is commonly followed by European engineers, is illustrated in [Figs. 50] to [52]. In these figures are indicated the number of holes in each round and the sequence of rounds for the soft, medium and hard rock, as used in the Turchino tunnel of the Genova Ovada Asti line of the Mediterranean Railway of Italy. The heading was about 9 ft. square, and five sets of holes were used in blasting, the depths being 3.91, 4.26 and 4.6 ft. for soft, medium and hard rock, respectively, and the amount of dynamite consumed was 2.38, 3.91 and 5.1 pounds per cubic yard for the three classes of rock.
in Soft Rock
in Medium Rock
in Hard Rock
Figs. 50 to 52.—Arrangement of Drill Holes in the Heading of Turchino Tunnel.
Figs. 53 and 54.—Arrangement of Drill Holes in the Heading of the Fort George Tunnel.
In the center-cut method, which is the one commonly employed in America, the holes are arranged in vertical rows, and are driven from 8 to 10 ft. deep. [Fig. 53] shows the arrangement of the holes, and the method of blasting them, as used in the excavation of the heading for the Fort George tunnel of the New York rapid transit. The two center rows of holes converge toward each other so as to take out a wedge of rock; others are bored straight, or parallel, with the vertical plane of the tunnel. Those bored around the perimeter are driven either outward or upward, according as they are located, close to the sides or roof of the tunnel. In this case, the holes of the center cut were driven 9 ft. deep, while all the other holes were bored to a depth of 8 ft.
The width of the advanced gallery or heading depends upon the quality of the rock. In hard rock American engineers give it the full width of the tunnel section; but this cannot be done in loose or fissured rock, which has to be supported, the headings here being usually made about 8 × 8 ft. The wider heading is always preferable, where it is possible, since more room is available for removing the rock, and deeper holes can be bored and blasted.
The important rôle played by the power plant and other mechanical installations in constructing tunnels through rock has already been mentioned. In some methods of soft-ground tunneling, and particularly in soft-ground subaqueous tunneling, it is also often necessary to employ a mechanical installation but slightly inferior in size and cost to those used in tunneling rock. It is proposed to describe very briefly here a few typical individual plants of this character, which will in some respects give a better idea of this phase of tunnel work than the more general descriptions.
Rock Tunnels.
—The tunnels selected to illustrate the mechanical installations employed in tunneling through rock are: The Mont Cenis, Hoosac Tunnel, the Cascade Tunnel, the Niagara Falls Power Tunnel, the Palisades Tunnel, the Croton Aqueduct Tunnel, the Strickler Tunnel in America, and the Graveholz Tunnel and the Sonnstein Tunnel in Europe. In addition there will be found in another chapter of this book a description of the mechanical installations at the St. Gothard, Pennsylvania and other tunnels.
Mont Cenis Power Plant.
—The mechanical installation consisted of the Sommeilier air compressors built near the portals. The Sommeilier compressors, Mr. W. L. Saunders says, were operated as a ram, utilizing a natural head of water to force air at 80 lbs. pressure into a receiver. The column of water contained in the long pipe on the side of the hill was started and stopped automatically by valves controlled by engines. The weight and momentum of the water forced a volume of air with such a shock against the discharge valve that it was opened, and the air was discharged into the tank; the valve was then closed, the water checked; a portion of it was allowed to discharge, and the space was filled with air, which was in turn forced into the tank. Only 73% of the power of the water was available, 27% being lost by the friction of the water in the pipes, valves, bends, etc. Of the 73% of net work, 49.4 was consumed in the perforators, and 23.6 in a dummy engine for working the valves of the compressors and for special ventilation.
The compressed air was conveyed from each end through a cast-iron pipe 75⁄8 in. in diameter, up to the front of the excavation. The joints of the pipes were made with turned faces, grooved to receive a ring of oakum which was tightly screwed and compressed into the joint. To ascertain the amount of leakage of the pipes, they and the tanks were filled with air compressed to 6 atmospheres, and the machines stopped; after 12 hours the pressure was reduced to 5.7 atmospheres, or to 95% of the original pressure.
Sommeilier’s percussion drilling machines were used in the excavation of this tunnel. They were provided with 8 or 10 drills acting at the same time, and mounted on carriages running on tracks. These were withdrawn to a safe place during the blasting, and advanced again after the broken rock was removed from the front and the new tracks laid.
Machine shops were built at both ends of the tunnel for building and repairing the drilling machines, bits, tools, etc. A gas factory was built at each end for lighting purpose.
Hoosac Tunnel.
—The Hoosac tunnel on the Fitchburg R.R. in Massachusetts is 25,000 ft. long, and the longest tunnel in America. The material through which the tunnel was driven was chiefly hard granitic gneiss, conglomerate, and mica-schist rock. The excavation was conducted from the entrances and one shaft, the wide heading and single-bench method being employed, with the center-cut system of blasting which was here used for the first time. The tunnel was begun in 1854, and continued by hand until 1866, when the mechanical plant was installed. Most of the particular machines employed have now become obsolete, but as they were the first machines used for rock tunneling in America they deserve mention. The drills used were Burleigh percussion drills, operated by compressed air. Six of these drills were mounted on a single carriage, and two carriages were used at each front. The air to operate these drills was supplied by air compressors operated by water-power at the portals and steam-power at the shaft. The air compressors consisted of four horizontal single-acting air cylinders with poppet valves and water injection. The compressors were designed by Mr. Thomas Deane, the chief engineer of the tunnel.
Palisades Tunnel.
—The Palisades tunnel was constructed to carry a double track railway line through the ridge of rocks bordering the west bank of the Hudson River and known as the Palisades. It was located about opposite 116th St. in New York City. The material penetrated was a hard trap rock very full of seams in places, which caused large fragments to fall from the roof. The excavation was made by a single wide heading and bench, employing the center-cut method of blasting with eight center holes and 16 side holes for the 7 × 18 ft. heading. Ingersoll-Sergeant 21⁄2 in. drills were used, four in each heading and six on each bench, and 30 ft. per 10 hours was considered good work for one drill.
The power-plant was situated at the west portal of the tunnel, and the power was transmitted by electricity and compressed air to the middle shaft and east portal workings. The plant consisted of eight 100 H. P. boilers, furnishing steam to four Rand duplex 18 × 22 in. air compressors, and an engine running a 30 arc light dynamo. The compressed air was carried over the ridge by pipes, varying from 10 ins. to 5 ins. in diameter, to the shaft and to the east portal, and was used for operating the hoisting engines as well as the drills at these workings. Inside the tunnel, specially designed derrick cars were employed to handle large stones, they being also operated by compressed air. This car ran on a center track, while the mucking cars ran on side tracks, and it was employed to lift the bodies of the cars from the trucks, place them close to the front, being worked where large stone could be rolled into them, and return them to the trucks for removal. In addition to handling the car bodies the derrick was used to lift heavy stones. The hauling was done first by horse-power, and later by dummy locomotives.
Croton Aqueduct Tunnel.
—In the construction of the Croton Aqueduct for the water supply of New York City, a tunnel 31 miles long was built, running from the Croton Dam to the Gate House at 135th St. in New York City. The section of the tunnel varies in form, but is generally either a circular or a horseshoe section. In all cases the section was designed to have a capacity for the flow of water equal to a cylinder 14 ft. in diameter. To drive the tunnel, 40 shafts were employed. The material penetrated was of almost every character, from quicksand to granitic rock, but the bulk of the work was in rock of some character. The excavation in rock was conducted by the wide heading and bench method, employing the center-cut method of blasting. Four air drills, mounted on two double-arm columns were employed in the heading. The drills for the bench work were mounted on tripods. Steam-power was used exclusively for operating the compressors, hoisting engines, ventilating fans and pumps; but the size and kind of boilers used, as well as the kind and capacity of the machines which they operated, varied greatly, since a separate power-plant was employed for each shaft with a few exceptions. A description of the plant at one of the shafts will give an indication of the size and character of those at the other shafts, and for this purpose the plant at shaft 10 has been selected.
At shaft 10 steam was provided by two Ingersoll boilers of 80 H. P. each, and by a small upright boiler of 8 H. P. There were two 18 × 30 in. Ingersoll air compressors pumping into two 42 in. × 10 ft. and two 42 in. × 12 ft. Ingersoll receivers. In the excavation there were twelve 31⁄2 in. and six 31⁄8 in. Ingersoll drills, four drills mounted on two double arm columns being used on each heading, and the remainder mounted on tripods being used on the bench. Two Dickson cages operated by one 12 × 12 in. Dickson reversible double hoisting engine provided transportation for material and supplies up and down the shaft. A Thomson-Houston ten-light dynamo operated by a Lidgerwood engine provided light. Drainage was effected by means of two No. 9 and one No. 6 Cameron pumps. At this particular shaft the air exhausted from the drills gave ample ventilation, especially when after each blast the smoke was cleared away by a jet of compressed air. In other workings, however, where this means of ventilation was not sufficient, Baker blowers were generally employed.
Strickler Tunnel.
—The Strickler tunnel for the water supply of Colorado Springs, Col., is 6441 ft. long with a section of 4 ft. × 7 ft. It penetrates the ridge connecting Pike’s Peak and the Big Horn Mountains, at an elevation of 11,540 ft. above sea level. The material penetrated is a coarse porphyritic granite and morainal débris, the portion through the latter material being lined. The mechanical installation consisted of a water-power electric plant operating air compressors. The water from Buxton Creek having a fall of 2400 ft. was utilized to operate a 36 in. 220 H. P. Pelton water-wheel, which operated a 150 K. W. three-phase generator. From this generator a 3500 volt current was transmitted to the east portal of the tunnel, where a step-down transformer reduced it to a 220 volt current to the motor. The transmission line consisted of three No. 5 wires carried on cross-arm poles and provided with lightning arresters at intervals. The plant at the east portal of the tunnel consisted of a 75 H. P. electric motor, driving a 75 H. P. air compressor, and of small motors to drive a Sturtevant blower for ventilation, to run the blacksmith shop, and to light the tunnel, shop, and yards. From the compressor air was piped into the tunnel at the east end, and also over the mountain to the west portal workings. Two drills were used at each end, and the air was also used for operating derricks and other machinery. For removing the spoil a trolley carrier system was employed. A longitudinal timber was fastened to the tunnel roof, directly in the apex of the roof arch. This timber carried by means of hangers a steel bar trolley rail on which the carriages ran. Outside of the portal this rail formed a loop, so that the carriage could pass around the loop and be taken back to the working face. Each carriage carried a steel span of 11⁄2 cu. ft. capacity, so suspended that by means of a tripping device it was automatically dumped when the proper point on the loop was reached.
Niagara Falls Power Tunnel.
—The tail-race tunnel built to carry away the water discharged from the turbines of the Niagara Falls Power Co., has a horse-shoe section 19 × 21 ft. and a length of 6700 ft. It was driven through rock from three shafts by the center-cut method of blasting. In sinking shaft No. 0 very little water was encountered, but at shafts Nos. 1 and 2 an inflow of 800 gallons and 600 gallons per minute, respectively, was encountered. The principal plant was located at shaft No. 2, and consisted of eight 100 H.P. boilers, three 18 × 30 in. Rand duplex air compressors, a Thomson-Houston electric-light plant, and a sawmill with a capacity of 20,000 ft. B. M. per day. The shafts were fitted with Otis automatic hoisting engines, with double cages at shafts Nos. 1 and 2, and a single cage at shaft No. 0. The drills used were 25 Rand drills and three Ingersoll-Sergeant drills. The pumping plant at shaft No. 2 consisted of four No. 7 and one No. 9 Cameron pumps, and that at shaft No. 2 consisted of two No. 7 and two No. 9 Cameron pumps and three Snow pumps. An auxiliary boiler plant consisting of two 60 H. P. boilers was located at shaft No. 1, and another, consisting of one 75 H. P. boiler, was located at shaft No. 0.
Cascade Tunnel.
—The Cascade tunnel was built in 1886-88 to carry the double tracks of the Northern Pacific Ry. through the Cascade Mountains in Washington. It is 9850 ft. long with a cross-section 161⁄2 ft. wide and 22 ft. high, and is lined with masonry. The material penetrated was a basaltic rock, with a dip of the strata of about 5°. The rock was excavated by a wide heading and one bench, using the center-cut system of blasting. A strutting consisting of five-segment timber arches carried on side posts, spaced from 2 ft. to 4 ft. apart, and having a roof lagging of 4 × 6 in. timbers packed above with cord-wood. The mechanical plant of the tunnel is of particular interest, because of the fact that all the machinery and supplies had to be hauled from 82 to 87 miles by teams, over a road cut through the forests covering the mountain slopes. This work required from Feb. 22 to July 15, 1886, to perform. In many places the grades were so steep that the wagons had to be hauled by block and tackle. The plant consisted of five engines, two water-wheels, five air compressors, eight 70 H. P. steam-boilers, four large exhaust fans, two complete electric arc-lighting plants, two fully equipped machine-shop outfits, 36 air drills, two locomotives, 60 dump cars, and two sawmill outfits, with the necessary accessories for these various machines. This plant was divided about equally between the two ends of the tunnel. The cost of the plant and of the work of getting it into position was $125,000.
Graveholz Tunnel.
—The Graveholz tunnel on the Bergen Railway in Norway is notable as being the longest tunnel in northern Europe, and also as being built for a single-track narrow-gauge railway. This tunnel is 17,400 feet long, and is located at an elevation of 2900 feet above sea-level. Only about 3% of the length of the tunnel is lined. The mechanical installation consists of a turbine plant operating the various machines. There are two turbines of 100 H. P. and 120 H. P. taking water from a reservoir on the mountain slope, and furnishing 220 H. P., which is distributed about as follows: Boring-machines, 60 H. P.; ventilation, 30 to 40 H. P.; electric locomotives, 15 H. P.; machine shop, 15 H. P.; electric-lighting dynamo, 25 H. P.; electric drills, the surplus, or some 40 H. P. The boring-machines and electric drills will be operated by the smaller 100 H. P. turbine.
Sonnstein Tunnel.
—The Sonnstein tunnel in Germany is particularly interesting because of the exclusive use of Brandt rotary drills. The tunnel was driven through dolomite and hard limestone by means of a drift and two side galleries. The dimensions of the drift were 71⁄2 × 71⁄2 ft. The power plant consisted of two steam pressure pumps, one accumulator, and four drills. The steam-boiler plant, in addition to operating the pumps, also supplied power for operating a rotary pump for drainage and a blower for ventilation. The hydraulic pressure required was 75 atmospheres in the dolomite, and from 85 to 100 atmospheres in the limestone. The drift was excavated with five 31⁄2 in. holes, one being placed at the center and driven parallel to the axis of the tunnel, and four being placed at the corners of a rectangle corresponding to the sides of the drift, and driven at an angle diverging from the center hole. The average depths of the holes were 4.3 ft., and the efficiency of the drills was 1 in. per minute. One drill was employed at each front, and was operated by a machinist and two helpers, who worked eight-hour shifts, with a blast between shifts at first, and later twelve-hour shifts, with a blast between shifts. The 24 hours of the two shifts were divided as follows: boring the holes, 10.7 hours; charging the holes, 1.1 hours; removing the spoil, 11.7 hours; changing shifts, 0.5 hour. The average progress per day for each machine was 6.7 ft. The total cost of the plant was $17,450.
St. Clair River Tunnel.
—The submarine double-track railway tunnel under the St. Clair River for the Grand Trunk Ry. is 8500 ft. long, and was driven through clay by means of a shield, as described in the [succeeding chapter] on the shield system of tunneling. The mechanical plant installed for prosecuting the work was very complete. To furnish steam to the air compressors, pumps, electric-light engines, hoisting-engines, etc., a steam-plant was provided on each side of the river, consisting of three 70 H. P. and four 80 H. P. Scotch portable boilers. The air-compressor plant at each end consisted of two 20 × 24 in. Ingersoll air compressors. To furnish light to the workings, two 100 candle-power Edison dynamos were installed on the American side, and two Ball dynamos of the same size were installed on the Canadian side. The dynamos on both sides were driven by Armington & Sims engines. These dynamos furnished light to the tunnel workings and to the machine-shops and power-plant at each end. Root blowers of 10,000 cu. ft. per minute capacity provided ventilation. The pumping plant consisted of one set of pumps installed for permanent drainage, and another set installed for drainage during construction, and also to remain in place as a part of the permanent plant. The latter set consisted of two 500 gallon Worthington duplex pumps set first outside of each air lock, closing the ends of the river portion of the tunnel. For permanent drainage, a drainage shaft was sunk on the Canadian side of the river, and connected with a pump at the bottom of the open-cut approach. In this shaft were placed a vertical, direct-acting, compound-condensing pumping engine with two 191⁄2 in. high-pressure and two 333⁄8 in. low-pressure cylinders of 24 in. stroke, connected to double-acting pumps with a capacity of 3000 gallons per minute, and also two duplex pumps of 500 gallons capacity per minute. For permanent drainage on the American side, four Worthington pumps of 3000 gallons’ capacity were installed in a pump-house set back into the slope of the open-cut approach. For the permanent drainage of the tunnel proper two 400 gallon pumps were placed at the lowest point of the tunnel grade. Spoil coming from the tunnel proper was hoisted to the top of the open cut by derricks operated by two 50 H. P. Lidgerwood hoisting-engines. The pressure pumping plant for supplying water to the hydraulic shield-jacks at each end of the tunnel consisted of duplex direct-acting engines with 12 in. steam cylinders and 1 in. water cylinders, supplying water at a pressure of 2000 lbs. per sq. in.