Receivers.

“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.”

—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.

—The various forms of rock drills used in tunneling have beendescribedinChapter III., and need not be considered in detail here except to say that American engineersusually 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.

—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 inFigs. 50to52. 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 Turchinotunnel 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 Rockin Medium Rockin Hard Rock

in Soft Rockin Medium Rock

in Soft Rock

in Medium Rock

in Hard Rock

Figs. 50to52.—Arrangement of Drill Holes in the Heading of Turchino Tunnel.

Figs. 53and54.—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. 53shows 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 thiscase, 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.

—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.

—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 andstopped 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 75⁄8in. 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.

—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 andone 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.

—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 21⁄2in. 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.

—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 31⁄2in. and six 31⁄8in. 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.

—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, drivinga 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 11⁄2cu. ft. capacity, so suspended that by means of a tripping device it was automatically dumped when the proper point on the loop was reached.

—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 atshaft No. 1, and another, consisting of one 75 H. P. boiler, was located at shaft No. 0.

—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 161⁄2ft. 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.

—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.

—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× 71⁄2ft. 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 31⁄2in. 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.

—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 ashield, as described in thesucceeding chapteron 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 191⁄2in. high-pressure and two 333⁄8in. 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 two50 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.

Fig. 55.—Diagram Showing Sequence of Excavations in Drift Method of Tunneling Rock.

—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 byFig. 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 ofthe 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.

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 schemesfor 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 MilanviaThun, Brigue, and Domo d’Ossola.

Starting from Brigue, the new line, running gently up the valley for 11⁄4miles, 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 51⁄2miles 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 ventilationare 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 suchlarge quantities. This dynamite factory has been largely increased, and supplies dynamite to nearly all the mining and tunneling enterprises in Switzerland.

—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 21⁄2miles), 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.

—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 proposedcenter 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 21⁄2ins.

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 23⁄8ins. 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 protectedfrom climatic and other disturbing agencies by an iron plate.

—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 between13⁄16in. and3⁄16in. 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 thegeneral 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.

—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.

—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 thesecond 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.

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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.

—A small line of railway, 2 ft. 71⁄2ins. gauge, with 40-lb. rails, enters all three portals; but since the construction of a wooden bridge over the Diveria, the routethrough 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.

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