Charles Prelini

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.

Figs. 50 to 52.—Arrangement of Drill Holes in the Heading of Turchino 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.