EXCAVATION BY DRIFTS: THE SIMPLON AND MURRAY HILL TUNNELS.
Fig. 55.—Diagram Showing Sequence of Excavations in Drift Method of Tunneling Rock.
General Description.
—The method of tunneling through hard rock by drifts is preferred by European engineers. All the great Alpine tunnels, from the Mont Cenis tunnel to the Simplon, are examples of tunneling by drifts. In this method the sequence of excavation is shown diagrammatically by [Fig. 55]. The work begins by excavating a drift close to the floor of the proposed tunnel (as shown in the center of the figure) and far in advance of the excavation of any other part. The section marked 2 is next removed and still later the portions marked 3. Then with the removal of the parts marked 4 the whole section of the tunnel will be open.
The drift is usually strutted by means of side posts carrying a cap-piece placed at intervals, and having a ceiling of longitudinal planks resting on the successive caps. In hard rock the roof of the section does not, as a rule, require regular strutting, occasional supports being placed at intervals to prevent the fall of isolated fragments: When the rock is disintegrated or full of seams, a regular strutting may be necessary, and this may be either longitudinal or polygonal in type. When longitudinal strutting is employed, a sill is laid across the roof of the drift, and upon this are set up two struts converging toward the top and supporting a cap-piece close to the roof. On this cap-piece are placed the first longitudinal crown bars carrying transverse poling-boards. Additional props standing on the sill and radiating outward are inserted as parts No. 3 are excavated. These radial props carry longitudinal bars which in turn support transverse poling-boards. When polygonal strutting is used, it may take the form of three or five segment arches of heavy timbers.
In hard rock tunnels, as a rule, there is no danger of caving in because of heavy pressures, and the whole section is left open for some time before it is lined. The lining may be of concrete masonry, but in many long tunnels, excavated through hard rock, the side walls are lined with rubble masonry and the arch with brick, and, in some instances, even the arch has been lined with rubble masonry. With skilful laborers at hand the rubble masonry lining has proved most efficient and economical, because the rock is utilized as it is excavated without any further operation. Concrete, however, is more extensively employed for lining tunnels than any other material.
Tunnels excavated by drifts enable simple means of hauling to be employed, and this is one of the reasons why the method finds so much favor with European engineers. The tracks are laid along the floor of the drift, and carry all the spoil from parts Nos. 2, 3, and 4, as well as from the front of the drift itself. As fast as the full section is completed, this single track in the drift is replaced by two tracks running close to the sides of the tunnel, or by a broad-gauge track with a third rail.
THE SIMPLON TUNNEL.[8]
Before entering upon a description of the constructive details of this, the longest railway tunnel in the world, it may be well to give a general idea of the undertaking. Many schemes for the connection of Italy and Switzerland by a railway near the Simplon Road Pass have been devised, including one involving no great length of underground work, the line mounting by steep gradients and sharp curves. The present scheme, put forward in 1881 by the Jura-Simplon Ry. Co., consists broadly of piercing the Alps between Brigue, the present railway terminus in the Rhone Valley, and Iselle, in the gorge of the Diveria, on the Italian side, from which village the railway will descend to the existing southern terminus at Domo d’Ossola, a distance of about 11 miles.
[8] Abstract from a paper read before the Institution of Civil Engineers by Charles B. Fox, Jan. 26, 1900.
In conjunction with this scheme a second tunnel is proposed, to pierce the Bernese Alps under the Lötschen Pass from Mittholz to a point near Turtman in the Rhone Valley; and thus, instead of the long détour by Lausanne and the Lake of Geneva, there will be an almost direct line from Berne to Milan via Thun, Brigue, and Domo d’Ossola.
Starting from Brigue, the new line, running gently up the valley for 11⁄4 miles, will, on account of the proximity of the Rhone, which has already been slightly diverted, enter the tunnels on a curve to the right of 1050 ft. radius. At a distance of 153 yards from the entrance, the straight portion of the tunnel commences, and extends for 12 miles. The line then curves to the left with a radius of 1311 ft. before emerging on the left bank of the Diveria. Commencing at the northern entrance, a gradient of 1 in 500 (the minimum for efficient drainage) rises for a length of 51⁄2 miles to a level length of 550 yards in the center, and then a gradient of 1 in 143 descends to the Italian side. On the way to Domo d’Ossola one helical tunnel will be necessary, as has been carried out on the St. Gothard. There will be eventually two parallel tunnels having their centers 56 ft. apart, each carrying one line of way; but at the present time only one heading, that known as No. 1, is being excavated to full size, No. 2 being left, masonry lined where necessary, for future developments. By means of cross headings every 220 yds. the problems of transport and ventilation are greatly facilitated, as will be seen later. As both entrances are on curves, a small “gallery of direction” is necessary, to allow corrections of alinement to be made direct from the two observatories on the axis of the tunnel.
The outside installations are as nearly in duplicate as circumstances will allow, and consist of the necessary offices, workshops, engine-sheds, power-houses, smithies, and the numerous buildings entailed by an important engineering scheme. Great care is taken that the miners and men working in the tunnel shall not suffer from the sudden change from the warm headings to the cold Alpine air outside; and for this purpose a large building is in course of erection, where they will be able to take off their damp working clothes, have a hot and cold douche, put on a warm dry suit, and obtain refreshments at a moderate cost before returning to their homes. Instead of each man having a locker in which to stow his clothes, a perfect forest of cords hangs down from the wooden ceiling, 25 ft. above floor-level, each cord passing over its own pulleys and down the wall to a numbered belaying-pin. Each cord supports three hooks and a soap-dish, which, when loaded with their owner’s property, are hauled up to the ceiling out of the way. There are 2000 of these cords, spaced 1 ft. 6 ins. apart, one to each man. The engineers and foremen are more privileged, being provided with dressing-rooms and baths, partitioned off from the two main halls. An extensive clothes washing and drying plant has been laid down, and also a large restaurant and canteen. At Iselle, a magazine holding 2200 lbs. of dynamite is surrounded and divided into two separate parts by earth-banks, 16 ft. high. The two wooden houses, in which the explosive is stored, are warmed by hot-water pipes to a temperature between 61° F. and 77° F., and are watched by a military patrol; but at Brigue a dynamite manufactory, started by an enterprising company at the time of the commencement of the works, supplies this commodity at frequent intervals, thereby avoiding the necessity of storing in such large quantities. This dynamite factory has been largely increased, and supplies dynamite to nearly all the mining and tunneling enterprises in Switzerland.
Geological Conditions.
—Before the Simplon tunnel was authorized, expert evidence was taken as to the feasibility of the project. The forecasts of the three engineers chosen, in reference to the rock to be encountered and its probable temperature, have, as far as the galleries have gone (an aggregate distance of nearly 21⁄2 miles), generally been found correct. At the north end, a dark argillaceous schist veined with quartz was met with, and from time to time beds of gypsum and dolomite have been traversed, the dip of the strata being on the whole favorable to progress, though timbering is resorted to at dangerous places. Water was plentiful at the commencement; in fact, one inrush has not been stopped, and is still flowing down the heading. The total quantity of water flowing from the tunnel mouth is 16 gallons per second, of which 2 gallons per second are accounted for by the drilling machines. At Iselle, however, a very hard antigorio gneiss obtains, and is likely to extend for 4 miles. Very dry and very compact, it requires no timbering, and represents no great difficulty to the powerful Brandt rock-drills, which work under a head of 3280 ft. of water.
The temperature of the rock depends not only on the depth from the surface, but largely upon the general form of that surface combined with the conductivity of the rock. Taking these points into consideration with the experience gained from the construction of the St. Gothard tunnel, 95° F. was estimated as the probable maximum temperature, owing to the height of Monte Leone (11,660 ft.), which lies almost directly over the tunnel axis.
Survey.
—After having determined upon the general position of the tunnels, taking into consideration the necessary gradients, the temperature of the rock, and a large bed of troublesome gypsum on the north side, two fixed points on the proposed center line were taken, one at each entrance of tunnel No. 1, and the bearings of these two points, with reference to a triangulation survey made in 1876, were calculated sufficiently accurately to determine, for the time being, the direction of the tunnel. In 1898, a new triangulation survey was made, taking in eleven summits, Monte Leone holding the central position. This survey was tied into that of the Wasenhorn and Faulhorn, made by the Swiss Government, and the accuracy was such that the probable error in the meeting of the two headings is only 6 cms. or 21⁄2 ins.
On the top of each summit is placed a signal, consisting of a small pillar of masonry founded on rock, and capped with a sharp pointed cone of zinc, 1 ft. 6 ins. high. An observatory was built at each end of the tunnel in such a position that three of the summits could be seen, a condition very difficult to fulfill on the south side owing to the depth of the gorge, the mountains on either side being over 7000 ft. high. Having taken the angles to and from each visible signal, and therefrom having calculated the direction of the tunnel, it was necessary to fix, with extreme accuracy, sighting-points on the axis of the tunnel, in order to avoid sighting on to the surrounding peaks for each subsequent correction of the alinement of the galleries. To do this, a theodolite 24 ins. long and 23⁄8 ins. in diameter, with a magnifying power of 40 times, was set up in the observatory, and about 100 readings were taken of the angles between the surrounding signals and the required sighting-points. In this manner the error likely to occur was diminished to less than 1′. Thus at the north end two points were found about 550 yds. before and behind the observatory, while on the south side, owing to the narrowness of the gorge, the points could only be placed at 82 yds. and 126 yds. in front. One of these sighting-points consists of a fine scratch ruled on a piece of glass fixed in an iron frame, behind which is placed an acetylene lamp,—corrections of alinement are always done by night,—the whole being rigidly fixed into a niche cut in the rock and protected from climatic and other disturbing agencies by an iron plate.
Method of Checking Alinement.
—The direction of heading No. 1 is checked by experts from the Government Survey Department at Lausanne about three times a year, and for this purpose a transit instrument is set up in the observatory. A number of three-legged iron tables are placed at intervals of 1 mile or 2 miles along the axis of tunnel No. 1, and upon each of these is placed a horizontal plane, movable by means of an adjusting screw, in a direction at right angles to the axis, along a graduated scale. On this plane are small sockets, into which the legs of an acetylene lamp and screen, or of the transit instrument, can be quickly and accurately placed. The screen has a vertical slit, 3 ins. in height, and variable between 13⁄16 in. and 3⁄16 in. in breadth, according to the state of the atmosphere, and at a distance shows a fine thread of light. The instrument, having first been sighted on to the illuminated scratch of the sighting-point, is directed up the tunnel, where a thread of light is shown from the first table. With the aid of a telephone this light is adjusted so that its image is exactly coincident with the cross hairs, and the reading on the graduated scale is noted. This is done four or five times, the average of these readings being taken as correct, and the plane is clamped to that average. The instrument is then taken to the first table and is placed quickly and accurately over the point just found (by means of the sockets), and the lamp is carried to the observatory. After first sighting back, a second point is given on the second table, and so on. These points are marked either temporarily in the roof of the heading by a short piece of cord hanging down, or permanently by a brass point held by a small steel cylinder, 8 ins. long and 3 ins. in diameter, embedded in concrete in the rock floor, and protected by a circular casting, also sunk in cement concrete, holding an iron cover resembling that of a small manhole. From time to time the alinement is checked from these points by the engineers, and after each blast the general direction is given by the hand from the temporary points. To check the results of the triangulation survey, astronomical observations have been taken simultaneously at each end. With regard to the levels, those given on the excellent Government surveys have been taken as correct, but they have also been checked over the pass.
Details of Tunnels.
—In cross-section, tunnel No. 1 is 13 ft. 7 ins. wide at formation level, increasing to 16 ft. 5 ins., with a total height of 18 ft. above rail-level, and a cross-sectional area of about 250 sq. ft. This large section will allow of small repairs being executed in the roof without interruption of the traffic, and will also allow of strengthening the walls by additional masonry on the inside. The thickness of the lining, never wholly absent, and the material of which it is composed, depend upon the pressure to be resisted, and only in the worst case is an invert resorted to. The side drain, to which the rock floor is made to slope, will be composed of half-pipes of 7 to 1 cement concrete. The roof is constructed of radial stones.
Tunnel No. 2, being left as a heading, is driven on that side nearest to No. 1, to minimize the length of the cross-headings, and measures 10 ft. 2 ins. wide by 6 ft. 7 ins. high. Masonry is used only where necessary, and in that case is so built as to form part of the lining of the tunnel when eventually completed. Concrete is put in to form a foundation for the side wall, and a water channel. The cross-headings, connecting the two parallel headings, occur every 220 yds., and are placed at an angle of 56° to the axis of the tunnel, to avoid sharp curves in the contractors’ railway lines. They will eventually be used as much as possible for refuges, chambers for storing the tools and equipment of the platelayers, and signal-cabins. The refuges, 6 ft. 7 ins. wide by 6 ft. 7 ins. high and 3 ft. 3 ins. deep, occur every 110 yards, every tenth being enlarged to 9 ft. 10 ins. wide by 9 ft. 10 ins. deep and 10 ft. 2 ins. high, still larger chambers being constructed at greater intervals.
Method of Excavation.
—The work at each end of the tunnel is carried on quite independently, consequently, though similar in principle, the methods vary in detail, apart from the fact that different geological strata require different treatment. Broadly speaking, the two parallel headings, each 59 sq. ft. in section, are first driven by means of drilling-machines and the use of dynamite, this work being carried on day and night, seven days in the week; No. 1 heading is then enlarged to full size by hand-drilling and dynamite. On the Italian side, where the rock is hard and compact, breakups are made at intervals of 50 yds., and a top gallery is driven in both directions, but, for ventilation reasons, is never allowed to get more than 4 yds. ahead of the break-up, which is gradually lengthened and widened to the required section. No timbering is required, except to facilitate the excavation and the construction of the side walls. Steel centers are employed for the arch; they entail fewer supports, give more room, and are capable of being used over again more frequently without damage. They consist of two I-beams bent to a template and riveted together at the crown, resting at either side on scaffolding at intervals of 6 ft.; longitudinals 12 ft. by 4 ins. by 4 ins. support the roof. Hand rock-drilling is carried out in the ordinary way, one man holding the tool and a second striking; measurements of excavation are taken every 2 or 3 yds., a plumb-line is suspended from the center of the roof, and at every half-meter (20 ins.) of height horizontal measurements are taken to each side.
At the Brigue end a softer rock is encountered, necessitating at times heavy timbering in the heading, and especially in the final excavation to full size, Fig. 56. The bottom heading, 6 ft. 6 in. high, is driven in the center, and the heading is then widened to the full extent and timbered; the concrete forming the water channel and the foundation for one side wall is put in; the side walls are built to a height of 6 ft. 6 ins., and the tunnel is fully excavated to a further height of 6 ft. 6 ins. from the first staging. The side walls are then continued up for the second 6 ft. 6 ins., and from the second floor a third height of 6 ft. 6 ins. is excavated and timbered. Finally the crown is cleared out, heavy wooden centers are put in, the arch is turned and all timbers are withdrawn except the top poling-boards, supporting the loose rock.
1
2
3
4
5
6
7
8
Fig. 56.—Sketches Showing Sequence of Work in Excavating and Lining the Simplon Tunnel.
The masonry for the side walls is obtained either from the tunnel itself or from a neighboring quarry, and varies in character according to the pressure; but the face of the arch is always of cut or artificial stones, the latter being 7 to 1 cement concrete. Where the alinement heading, or the “gallery of direction,” joins the curving portion of tunnel No. 1, the section is very much greater, and necessitates special timbering.
Transport (Italian Side).
—A small line of railway, 2 ft. 71⁄2 ins. gauge, with 40-lb. rails, enters all three portals; but since the construction of a wooden bridge over the Diveria, the route through the “gallery of direction,” across heading No. 2, to tunnel No. 1, is used exclusively; this railway leads to the face in both headings, and, where convenient, from one heading to the other by the cross-galleries. Different types of wagons are in use; but in general they are four-wheeled, non-tipping box wagons, supplied with brakes and holding 2 cu. yds. of débris. A special type of locomotive is used, designed to pass round curves of 50 ft. radius, and supplied with a specially large boiler to avoid firing in the tunnel.
Fig. 57.—General Details of the Brandt Rotary Drills Employed at the Simplon Tunnel.
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Method of Working.
—The drilling-machines employed are of the Brandt type, [Fig. 57], and are mounted in the following manner: A small four-wheeled carriage supports at its center a beam, the shorter arm of which carries the boring mechanism and the longer a counterpoise; near its center is the distributor. In the short arm is a clamp holding the rack-bar or butting column, which is a wrought-iron cylinder with a plunger constituting a ram, and is jammed by hydraulic pressure between the walls of the heading, thus forming a rigid support for the boring-machine, and an efficient abutment against the reaction of the drill. This rack-bar can be rotated on its clamp in a plane parallel to the axis of the beam. Three or four separate boring-machines can be mounted on the rack-bar, and can be adjusted in any reasonable position.
The boring-machine performs the double function of continually pressing the drill into the rock by means of a hollow ram (I) and of imparting to the drill and ram a uniform rotary motion. This rotary motion is given by a twin cylinder single-acting hydraulic motor (E), the two pistons, of 27⁄8 ins. stroke, acting reciprocally as valves. The cranks are fixed at an angle of 90° to each other on the shaft, which carries a worm, gearing with a worm-wheel (Q) mounted upon the shell (R) of the hollow ram (I), and this shell in turn engages the ram by a long feather, leaving it free to slide axially to or from the face of the rock. The average speed of the motor is 150 revolutions to 200 revolutions per minute, the maximum speed being 300 revolutions per minute. The loss of power between the worm and worm-wheel is only 15% at the most; the worm being of hardened steel and the wheel of gun-metal, the two surfaces in contact acquire a high degree of polish, resulting in little wearing or heating. Taking into consideration all other sources of loss, 70% of the total power is utilized. The pressure on the drill is exerted by a cylinder and hollow ram (I), which revolves about the differential piston (S), which is fixed to the envelope holding the shell (R). This envelope is rigidly connected to the bed-plate of the motor, and, by means of the vertical hinge and pin (T), is held by the clamp (V) embracing the rack-bar. When water is admitted to the space in front of the differential piston the ram carrying the drilling-tool is thrust forward, and when admitted to the annular space behind the piston, the ram recedes, withdrawing the tool from the blast-hole. The drill proper is a hollow tube of tough steel 23⁄4 ins. in external diameter, armed with three or four sharp and hardened teeth, and makes from five to ten revolutions per minute, according to the nature of the rock. When the ram has reached the end of its stroke of 2 ft. 21⁄2 ins., the tool is quickly withdrawn from the hole and unscrewed from the ram; an extension rod is then screwed into the tool and into the ram, and the boring is continued, additional lengths being added as the tool grinds forward; each change of tool or rod takes about 15 secs. to 25 secs. to perform. The extension rods are forged steel tubes, fitted with four-threaded screws, and having the same external diameter as the drill. They are made in standard lengths of 2 ft. 8 ins., 1 ft. 10 ins., and 113⁄4 ins. The total weight of the drilling-machine is 264 lbs., and that of the rack-bar when full of water is 308 lbs. The exhaust water from the two motor cylinders escapes through a tube in the center of the ram and along the bore of the extension rods and drill, thereby scouring away the débris and keeping the drill cool; any superfluous water finds an exit through a hose below the motors and thence away down the heading. The distributor, already mentioned, supplies each boring-machine and the rack-bar with hydraulic pressure from the mains, with which connection is effected by means of flexible or articulated pipe connections, allowing freedom in all directions. The area of the piston for advancing the tool is 151⁄2 sq. ins., which, under a pressure of 1470 lbs. per sq. in., gives a pressure of over 10 tons on the tool, while for withdrawing the tool 21⁄2 tons is available. In the rock found at Iselle, namely, antigorio gneiss, a hole 23⁄4 ins. in diameter and 3 ft. 3 ins. in length is drilled, normally, in 12 mins. to 25 mins.; a daily rate of advance of 18 ft. to 19 ft. 6 ins. is made in a heading having a minimum cross-section of 59 sq. ft.; the time taken to drill ten to twelve holes, 4 ft. 7 ins. deep, is 21⁄2 hrs.
When the débris resulting from one operation has been sufficiently cleared away, a steel flooring, which is provided near the face to enable shoveling to be more easily done, and to give an even floor for the wheels of the drilling-carriage, is laid bare at the head of the line of rails, and the drilling-machines are brought up on their carriage by eight or ten men. When advanced sufficiently close to the face, the rack-bar is slewed round across the gallery and is wedged up against the rock sides; connection is made between the distributor and the hydraulic main, by means of the flexible pipe, and pressure is supplied by a small copper tube to the rack-bar ram, thereby rigidly holding the machine. Next, connections are made between the three drilling-machines and the distributor, and in 20 mins. from the time the machine was brought up all three drills are hard at work, water pouring from the holes.
The noise of the motors and grinding-tools is sufficient to drown all but shouts; and where the extension rods do not fit tightly, small jets of water play in all directions, necessitating the wearing of tarpaulins by the men directing the tools. Lighting is done wholly by small oil-lamps, provided with a hook to facilitate fixing in any crack in the rock; electricity will probably be used to light that portion of the tunnel which is completed.
Two men are allotted to each drill, one to drive the motor, the other to direct and replenish the tool, one foreman and two men in reserve completing the gang. A small hammer is freely used to loosen the screw joints of the extension rods and drill. A hole is usually commenced by a two-edged flat-pointed tool, until a sufficient depth is reached to prevent the circular tool from wandering over the face of the rock, but in many instances the hole is commenced with a circular tool. The exhaust water during this period flows away by the hose underneath the motor. In the antigorio gneiss, ten to twelve holes are drilled for each attack, three to four in the center to a depth of 3 ft. 3 ins., the remainder, disposed round the outside of the face, having a depth of 4 ft. 7 ins. The average time taken to complete the holes is 13⁄4 hr. to 21⁄2 hrs. Instead of pulverizing the rock, as do the diamond drills, it is found that the rock is crushed, and that headway is gained somewhat in the manner of a circular saw through wood. The core of rock inside the tool breaks up into small pieces, and can be taken out if necessary when the drill requires lengthening.
The lowest holes, inclined downwards, are full of water; consequently two detonators and two fuses are inserted, but apart from this, water has little effect on the charge. The fuses of the central holes are brought together and cut off shorter than those of the outer holes, in order that they may explode first to increase the effect of the outer charges. All portable objects, such as drills, pipe connections, tools, etc., have meanwhile been carried back; the steel flooring is covered over with a layer of débris to prevent injury from falling rock, and to the end of the hydraulic main is screwed a brass plug pierced by five holes; and immediately the explosions occur a valve is opened in the tunnel, and five jets of water play upon the rock, laying the dust and clearing the air. The necessity for this was shown on one occasion when this nozzle was broken by the explosion and the water had to be turned off immediately to avoid useless waste; on reaching the face, the atmosphere was found to be so highly charged with dust and smoke that it was impossible to distinguish the stones at the feet, although a lamp had been placed on the ground; and despite the fact that the air tube was in full blast, the men experienced great difficulty in breathing. A truck is now brought up, and four men clear a passage in front, through the heap of débris, two with picks and two with shovels, while on either side and behind are as many men as space will permit. The stone is thrown either to the sides of the heading or into the wagon, shoveling being greatly aided by the steel flooring, which, before the explosion, had been laid over the rails for nearly 10 yds. down the tunnel to receive the falling rock. These steel plates are taken up when cleared, and the wagon is pushed forward until the drilling-machine can be brought up again, leaving the remaining débris at the sides to be handled at leisure during the next attack. The roof and side walls are, of course, carefully examined with the pick, to discover and detach any loose or hanging rock. The times taken for each portion of the attack in this particular antigorio gneiss are as follows: Bringing up and adjustment of drills, 20 mins.; drilling, between 13⁄4 hr. and 21⁄2 hrs.; charging and firing, 15 mins.; clearing away débris, 2 hrs.; or for one whole attack, between 41⁄2 hrs. and 51⁄2 hrs., resulting in an advance of 3 ft. 9 in., or a daily advance of nearly 18 ft.
From this it appears that the time spent in clearing away the débris equals that taken up in drilling, and it is in this clearing that a saving of time is likely to be effected rather than in the process of drilling. Many schemes have been tried, such as a mechanical plow for making a passage; at Brigue, “marinage,” or clearing by means of powerful high-pressure water-jets, directed down the tunnel, was tried, but the idea is not yet sufficiently developed.
Another series of experiments has been tried at Brigue with regard to the utilization of liquid air as an explosive agent instead of dynamite; and for this purpose a plant has been laid down, consisting of one ammonia-compressor, two air-compressors, and two refrigerators, furnishing 1⁄10 gallon of liquid air per hour at an expenditure of 17 H. P. The system used is that of Professor Linde, who himself directs the experiments. The great difficulty experienced is that of shortening the interval of time that must elapse between the manufacture of the cartridge and its explosion. The liquid oxygen, with which the cartridge, containing kieselguhr (silicious earth) and paraffin, is saturated, evaporates very readily, losing power every moment; hence the effect of each cartridge cannot be guaranteed, and though it is an exceedingly powerful explosive when used immediately after manufacture, no practical result has yet been obtained.
Power Station.
—Water is abundant at either end, and therefore hydraulic power is the motive force employed. On the Italian side, a dam 5 ft. high has been thrown across the Diveria at a point near the Swiss frontier, about 3 miles above the site of the installations. A portion of the water thus held back enters, through regulating doors and gratings, a masonry channel leading to two parallel settling tanks, each 111 ft. by 16 ft., whence, after dropping all its sand and solid matter, the now pure water passes into the water-house, and, after flowing over a dam, through a grating and past the admission doors, enters a metallic conduit of 3-ft. pipes. Each of the settling tanks and the approach canal are provided with doors at the lower end leading direct to the river, through which all the sand and solid matter deposited can be scoured naturally by allowing the river-water to rush freely through. For this purpose the floor of the basins is on an average gradient of 1 in 30. For a similar reason the river-bed just outside the entrance to the approach canal is lined with wooden planks, from which the stones collecting behind the dam can be scoured by allowing an iron flap, hinged at the bottom, to change its position from the vertical to the horizontal in a gap left purposely in the dam, so causing a rushing torrent to sweep it clean.
The chief levels are:
| Level | of water at dam | 794.00 | meters | above | sea | level. |
| „ | in water-house | 793.70 | „ | „ | „ | „ |
| „ | at turbines | 618.50 | „ | „ | „ | „ |
giving a total fall of 175.20 ms. or 570 ft., and a pressure of 17.52 atmospheres.
The quantity of water capable of being taken from the Diveria in winter, when the rivers which are dependent upon the mountain snows for their supply are at their lowest, is calculated to be 352 gallons per second. Thus, taking the fall to be diminished by friction, etc., to 440 ft., and the useful effect at 70%, there is obtained 2000 H. P. on the turbine shaft.
The metallic conduit varies in material according to the pressure; thus cast-iron pipes 3 ft. in diameter and 13⁄16 in. thick are used up to a pressure of 2 atmospheres, from which point they are of wrought-iron. The cast-iron portion has of late caused a good deal of trouble, owing to settlement of the piers causing occasional bursts, consequently a masonry pier has been placed under each joint of this portion. The following table gives the thicknesses and diameters, varying with the pressure:
| Water Pressure. | Thickness. | Diameter. | Weight per Yard. | |||
|---|---|---|---|---|---|---|
| Head in Feet. | Milli- meters. | Inch. | Feet. | Inches. | Lbs. | |
| 246 | 6 | 1⁄4 | 3 | 0 | 326 | |
| 311 | 7 | ... | 3 | 0 | 383 | |
| 360 | 8 | ... | 3 | 0 | 431 | |
| 393 | 9 | ... | 3 | 0 | 483 | |
| 426 | 10 | ... | 3 | 0 | 556 | |
| 476 | 12 | ... | 3 | 0 | 651 | |
| 590 | 16 | 5⁄8 | 3 | 3 | 1⁄3 | 977 |
This pipe is supported every 30 ft. on small masonry piers, on the top of which is placed a block of wood hollowed out to receive the pipe, thus allowing any movement due to the contraction and expansion of the conduit. However, to prevent this movement becoming excessive, the pipe is passed at intervals of 300 yds. to 500 yds. through a cubical block of masonry of 13 ft. side, strengthened by longitudinal tie-bars. Five bands of angle-bar riveted round the pipe, with their flanges embedded in the masonry, constitute a rigid fixed point. Straw mats are thrown over the pipe where it is exposed to the sun. The temperature of the conduit is not, however, found to vary greatly, since the pipe is kept full of water. To supply the rock-drills with water at a maximum pressure of 100 atmospheres, or 1470 lbs. per sq. in., a plant of four pairs of high-pressure pumps has been laid down, and a still larger addition is in course of erection. At present, two Pelton turbines of 250 H.P. each, running at 170 revolutions per minute, drive the pumps, by means of toothed gearing, at 63 revolutions per minute. These pumps are of very simple but strong construction, single suction and double delivery, entailing one suction and one delivery-valve, both heavy and both of small lift. The larger portion of the plunger has exactly double the cross-sectional area of the smaller portion, so that in the forward stroke half of the water taken in at the last admission is pumped into the high-pressure mains, and at the same time a fresh supply of water is sucked in. During the backward stroke half of this new supply is pumped into the mains, and the remainder enters the second chamber, to be pumped during the next forward stroke. Thus the work done in the two strokes is practically the same. The pumps are in pairs, and are set at an angle of 90°, to insure uniform pressure and uniform delivery in the mains. Their size varies; but at Iselle there are three pairs, with a stroke of 2 ft. 21⁄2 ins., and the plungers of 211⁄16 in. and 17⁄8 ins. (approximately) in diameter, supplying 1.32 gallons per second.
To avoid injury to the valves, the water to be pumped is taken from a stream up the mountain side, and is passed through filter screens. The high-pressure water, after passing an accumulator, enters the tunnel in solid drawn wrought-iron tubes, 31⁄8 ins. in internal diameter, 3⁄16 in. thick, and in lengths of 26 ft. The diameter of these mains varies with their length, so as to avoid loss of pressure. With the 1250 yds. of tunnel now driven 10 atmospheres are lost.
At Brigue the installations are, as far as possible, identical. The Rhone water, however, before reaching the water-house, is carried from the filter basins, a distance of 2 miles, in an armored canal built upon the Hennebique system,[9] the walls and supporting beams, of cement concrete, being strengthened by internal tie-bars of steel. The concrete struts, resembling balks of timber at a distance, are occasionally 35 ft. high and 1 ft. 71⁄2 ins. square. The metallic conduit is 5 ft. in diameter, with a minimum flow of 176 cu. ft. per second and a total fall of 185 ft. In case water-power should be unavailable, three semi-portable steam engines, two of 80 H.P. and one of 60 H.P., are always kept in readiness at each end of the tunnel, and are geared by belts to the turbine shaft.
[9] Network of steel rods embedded in concrete.
Ventilation.
—In tunneling, one of the most important problems to be solved is that of ventilation, and it is for this reason that the Simplon tunnel consists of two parallel headings with cross cuts at intervals of 220 yds. At Brigue, a shaft 164 ft. deep was sunk through the overlying rock until the “gallery of direction” was encountered. Up this chimney the foul air is drawn by wood fires, the fresh air—a volume of 19,000,000 cu. ft. per day, or 13,200 cu. ft. per minute—entering by heading No. 2, penetrating up to the last cross gallery, and returning by tunnel No. 1. The entrances of No. 1 and the “gallery of direction,” besides those of all the intermediate cross galleries, are closed by doors. By this arrangement, however, fresh air does not reach the working faces; therefore a pipe, 8 ins. in diameter, is led from the fresh air in No. 2 to within 15 yds. of the face of each heading, and up this pipe a draft of air is induced by means of a jet of water, the volume to each face being 800 cu. ft. per minute. One single jet of water from the high-pressure mains, with a diameter of 1⁄16 in., is capable of supplying over 1000 cu. ft. of air per minute at the end of 160 yds. of pipe, and during the attack the men at the drills are in a constant breeze with the thermometer standing at 70° F. At Iselle, air is blown into the entrance of heading No. 2 at the rate of 14,100 cu. ft. per minute by two fans driven from the turbine shaft. This air travels from the fans along a pipe 18 ins. in diameter, till a point 15 yds. up the tunnel is reached, where beyond a door the pipe narrows to form a nozzle 10 ins. in diameter. This door is kept open to allow the outside air to be induced up the tunnel, as the headings are at present only 2500 yds. long, giving a resistance of not quite sufficient power to cause the air to return. The fresh air then travels up No. 2, crossing over the top of the “gallery of direction,” from which it is shut off by doors, to the last cross gallery, returning by No. 1, and finally leaving either by the “gallery of direction” or by No. 1. A system of cooling the air and driving it on by means of a large number of water-jets will be installed in No. 2 where that heading crosses over the “gallery of direction,” but at present there is no need for it.
The average temperature at the face is 73° F. during the drilling operation, 76° F. after firing the charges, and a maximum of 80° F., lately attaining to 86° F. on the south side, with 80° F. and 85° F. before and after firing. The temperature of the rock is taken at every 110 yds. in holes 5 ft. deep, and shows a gradual increase according to the depth of over-laying rock, to the conductivity of the rock, and to the form of the mountain surface. The maximum hitherto reached on the north side is 68° F., while on the south side, although a smaller distance has been traversed, it attains to 79° F., due to the more rapid increase in depth. Moreover, the temperature of the rock is observed at the permanent stations, 550 yds. from the entrances, in its relation to that of the tunnel and outside air, and though on the north side that of the rock varies almost as quickly as that of the tunnel air, on the south it is influenced very much less.
A few statistics may be of interest with regard to the progress of the last three months (taken from the trimestrial report of January, 1900). At Brigue, where there are three drilling-machines in No. 1 and two in the parallel heading, the total length excavated was 995 yds. or 6409 cu. yds. in 89 working days, the average cross-sectional area being 57 sq. ft. This required 507 attacks and 3066 holes, which had a total depth of 26,600 ft. and 14,700 re-sharpenings of the drilling-tool, with 44,000 lbs. of dynamite.
The average time occupied in drilling was 2 hrs. 45 mins., while charging, firing, and clearing away the débris took 6 hrs., 35 mins. At Brigue 648 men and 29 horses were employed at one time in the tunnel. At Iselle the numbers were 496 men and 16 horses, working in shifts of 8 hrs. Outside the tunnel, in the shops, forges, etc., the men work 8 hrs. to 11 hrs. per day, the total being 541 men at Brigue and 346 men at Iselle. On the Italian side, where the rock is very much harder, there were three drilling-machines in each heading; the total length excavated, with a cross-sectional area of 62 sq. ft., was 960 yds. or 6700 cu. yds. in 91 working days. This required 61,293 re-sharpened tools, 758 attacks, 7940 holes with a total depth of 33,000 ft., and 56,000 lbs. of dynamite. The average time spent in drilling was 2 hrs. 55 mins., and in charging and clearing 2 hrs. 36 mins. Thus, in the hard gneiss, to excavate 1 cu. yd. of rock required 81⁄2 lbs. of dynamite, and each tool pierced 61⁄2 ins. of rock before it required re-sharpening.