Driving Mine Headings in Rock Tunnels

Driving Mine Headings in Rock Tunnels

This paper deals specifically with heading-driving as distinguished from the broader term tunnel-driving. A heading is a pilot or path-finder for the main tunnel. Some headings are complete tunnels in themselves; that is, conditions at times warrant driving a heading the full diameter of the proposed tunnel. A tunnel 10 ft. in diameter might be driven to advantage through a single heading, provided the nature of the material admits, but it usually pays to drive rock tunnels of large diameter through the heading-and-bench system. Much depends upon the material, as in soft ground tunnels of much larger diameter are driven in a single heading.

In mines, the tunnels, drifts, crosscuts, adits, etc., are usually of small diameter, hence the driving of headings applies to completed tunnels in mining-work, except, of course, in cases like drainage-tunnels and main entries leading into coal-mines, where conditions call for tunnels of large diameter, approximating those driven for railway-service. In building aqueducts the tunnels are driven from 6 to 8 ft. in diameter up to 15 or 20 ft., while in railway-service the dimensions vary from 18 to 30 ft. in diameter.

There is no department of rock-excavation so difficult as that of heading-driving. This work also belongs to the most expensive class in rock-excavation. A heading is driven directly in the solid. There are no lines of lesser resistance towards which to direct the energy of the blast, but the material must be blown out by main force. The completion of the heading simplifies all the rest of the work of excavation. It is easy, for instance, to enlarge a heading either from above, below or on the sides, by breaking towards the open face through holes approximately at right angles to the axis of the heading; or the enlargement sometimes takes place by holes driven outside of the heading running approximately parallel with it, the rock being broken into the heading. All of this is bench-work, or stoping; it involves little difficulty, and a minimum of explosive is required, because there is an open end or face towards which the energy of the blast is directed.

plan-showing-directions-of-holes-in-tunnel-heading

Fig. 1 shows the usual arrangement of heading-holes in tunnel-driving. After blasting the center-cut holes and the first of the side-round holes, the appearance of the heading in elevation will be about as shown in Fig. 2. The latter illustration shows the second set of side-round holes connected up by wires ready for firing.method-of-shooting-side-cut-holes-in-tunnel-heading

Fig. 3 shows in plan and elevation typical conditions where tunnel-headings are to be enlarged. This is stoping-work, and because of the advantageous position which the holes can be given, the removal of the rock may be accomplished at much less expense per cubic yard than in a case of heading-driving.

Driving seems to be the fitting term to use for tunnel-construction because, from the beginning to the end, every other consideration is sacrificed to progress. The tunnel is worthless until completed, and the capital involved in its construction is locked up and unprofitable. During the construction of the Simplon tunnel in the Alps premiums were offered exceeding $1,000,000 for increased speed in driving. This tunnel is the longest railway-tunnel in the world, being 12.25 miles long, and it holds the record for speed in heading-progress.method-of-enlarging-tunnel-heading

Progress in heading-driving is dependent, first, upon an efficient boring-apparatus and system, and following closely upon this, and of perhaps equal importance, is the system of blasting. The nature of the rock has a great deal to do with heading-progress. A hard rock which shatters easily is usually more favorable than a tough, soft material. The direction of the dip or of the cleavage of the rock has a good deal to do with progress. Fissures, pressure in the rock, water-bearing strata, heat, gas, and soft spots, resulting in falls, are hindrances to progress.

Driving a heading in solid rock is, of course, primarily a job for the rock-drill. Where the best records are made the natural inquiry is as to drill employed, yet the importance of this is frequently overestimated. The system is of equal importance. The best rock-drill, or the one which drills fastest, might be handicapped if insufficiently mounted, since the holes driven in headings are invariably of moderate depths, so that much of the time is employed in changing the drill from one hole to another, changing steels, etc.

Too little importance is given to the question of how a rock-drill is mounted. An illustration of this is shown by comparing the drilling-capacity of a machine when used, for instance, on a tripod or a column with that of the same type and size of rock-drill mounted on a gadder-frame or quarry-bar. The usual work of a rock-drill in average material is from 50 to 100 lin. ft. of hole in a day of 10 hr. This machine, when mounted on a gadder-frame or quarry-bar, will do 350 lin. ft. of hole in the same time, the difference being made up, not in the drilling-capacity, but in the facilities afforded for rapidly changing from one hole to another.

The remarkable records in tunnel-driving made by the hydraulic drills used at the Simplon tunnel, referred to later on in this paper, have always been attributed to the efficiency of the boring-machine. These Simplon headings were driven at the rate of about 20 lin. ft. in 24 hr., and in hard rock, while American tunnel-driving seldom exceeds 10 ft. in 24 hr. when driving in hard rock, so that here we have a difference of 2 to 1 against the American system. It is a common thing to say that a single-track railway-tunnel driven from two headings will progress at the rate of a mile a year. This is our average progress under average conditions. In the Alps the tunnels are driven from two headings at the rate of about 2 miles a year. These data are, of course, general, and do not take into consideration delays due to falls, destructive pressure in the rock, hot-water streams, etc.

It will be seen later on that the remarkable records made at the Simplon have practically been equaled by the work now going on in the Loetschberg tunnel, this latter being equipped with American percussive drills mounted on carriages similar to those used at the Simplon.

It may safely be said that the discovery of gunpowder, followed by that of dynamite, is the greatest invention that has been given us in facilitating tunnel-driving. Next comes the power-drill. In ancient times rock-excavation under ground was limited to hand-tools, wedges, etc., assisted by a system of excavation known as “ fire-setting,” which consisted in heating the rock and suddenly cooling it with water, thus disintegrating it. Pliny mentions this fire-setting system, and we are told that Hannibal used it to disintegrate the rocks while crossing the Alps.

The ancient Greeks and Romans were expert engineers in tunnel-driving. Herodotus mentions a tunnel in the island of Samos cut through a mountain 900 ft. high. Its length was 4,248 ft. and its cross-section 8 by 8 ft. This tunnel was built during the sixth century B.C. We may well be astonished in studying the records of tunnel-driving by the Romans. They built tunnels for drainage, for passages, aqueducts, etc., through rock and through earth, not only in Italy, but throughout their possessions. The drainage-tunnels built by the Etruscans, from whom the Romans learned the art, are among the greatest engineering achievements of antiquity. A tunnel in use at the present day was built through the Apennines between Naples and Pozzuoli in the first century B.C. This tunnel is said to have been originally 0.75 mile in length; height, 30 ft.; width, 25 ft. A complete history of tunnel-driving “ from the reign of Rameses II. to the present time ” is given in Drinker’s classic work.

American Tunneling Records

The following are some of the best American tunneling-records, arranged progressively. In some cases the records are those of complete tunnels and in others of the heading only, but as a heading is in itself a tunnel they may all be considered in the same class as records of progress. The figures represent monthly progress:

monthly-progress-report

There is some question as to whether the Sutro record is for one heading or two, so that its place in the list is questionable; also, notwithstanding the mental allowance for the differences in working-conditions, the Raton tunnel can hardly be considered as in the same class with the others, as the material, not to call it rock, was so soft that most of the drilling was done with coal-augers, and a steam-shovel was used to excavate the bench. The tunnel was finally lined with concrete 2 ft. thick.

The record of the new Croton Aqueduct, in 1887, of 127 ft. in one week, was a deliberate drive for one week for the purpose of making a record. This was done regardless of expense or the amount of explosive consumed. I was on the ground and know that while the figures represent the true progress made, yet they are reliable only in that they show what may possibly be done, and not what ought to be done or what can be done, under similar circumstances. The week following that during which the record was made the progress fell off about one-half.

The record of the Bitter Eoot Mts. tunnel, driven by Winston Bros. Co., “ is probably the record rate of progress on American railway-tunnels driven the full width of the arch in hard rock.” This tunnel was driven by the regular top-heading system. The section of the tunnel is rectangular with semi-circular roof-arch. The width is 21.83 ft.; height to springing, 15.25 ft.; height to crown, 25.92 ft.; rock, quartzite, slightly laminated. In the six months beginning with June, 1908, the averages were : East heading, 289.3 ft. per month; west heading, 281.2 ft.; both headings, 570.5 ft; bench, both ends, 632.5 ft. The average progress, both ends, for the first 11 months of 1908 was 537.6 ft. per month.

This tunnel is timbered throughout except for 1,302 ft. of the west end. The section is 18 ft. 6 in. by 25 ft. in the clear, or inside the timber. Perhaps the most gratifying feature of the work is the increase of speed as the work progressed, as shown below:

work-progressed-heading-bench

In the west heading no advance was made for the last six days of November, as a seam of very wet running ground, talc, was struck, necessitating a change of arrangements. If the rate had been maintained for these six days the total would have been 674 instead of 608 ft., thus exceeding the previous record.

As is to be always remembered, each of these tunnels differs from all the rest as to the conditions, favorable or otherwise, so that inferences from the comparing of records are not necessarily authoritative or final.

It happens that the last two on the list, the Gunnison and Elizabeth Lake tunnels, are probably as fairly comparable with each other as any, with the interesting particular that the two were driven by different and contrasting methods. Both are in granite and about 12 by 12 ft. in section. The Gunnison tunnel was made by driving the section one-half size; that is, by a smaller heading 6 by 12 ft. The Elizabeth Lake tunnel was driven full size by the lower-heading method.

European Tunneling Records

Following our list of American high records we have now a few representing the best European practice. These are arranged progressively, and after the first two or three items this list would tag on to the other, showing an increasing rate from beginning to end of the combined record, only that, unfortunately, these latter records generally antedate or are contemporaneous with the American. The highest monthly records of European tunneling show not only larger figures but a better maintenance of approximately the maximum, month after month. In some of the cases here following several successive monthly records are given:

Mont Cenis, 1857-1870; 297 ft.
St. Gothard, 1872-1881 ; 436 ft., with a year’s average of 343 ft. (This is the only Alpine tunnel driven with top heading.)
Ricken, 1903 ; 452, 461, 413, 358 ft. (Hand-drilling entirely. Work was stopped on this tunnel nearly a year on account of fire-damp.)
Bosruck, 1902-1905 ; 546, 526 ft.
Karawanken, 1902-1905; 552, 544, 553 ft.
Arlberg, 1880-1883 ; east heading, 556, 594, 610, 613, 637 ft. (Percussion-drills.)
Arlberg ; west heading, 509, 527, 625, 641 ft. (Hydraulic rotary drills.)
Albula, 1900-1902 ; 558, 607 ft. (Hydraulic rotary drills.)
Tauern ; 548 ft., with an average of nearly 525 ft.
Loetschberg; 555, 574, 538, 558, 551, 592 ft.
Simplon, 1900-1905; north heading, 682.2 ft.
Simplon ; south heading, 685.5 ft. (This is the world’s record.)

Seven or eight of the best records are so nearly alike that the differences might easily be accounted for by the varying hardness of the rock or other material conditions, without implying anything as to the superiority of the apparatus or the system employed in either case.

The Simplon and the Loetschberg tunnels are perhaps the most interesting of the list, and they are quite intimately related to each other. The Loetschberg tunnel is destined to connect directly Berne, Switzerland, with the Simplon tunnel, thus establishing a direct communication between Italy and Alsace-Lorraine, Belgium, Holland, and the Rhine provinces, greatly shortening the trip between Berne and Brigue. The total length of the tunnel will be 8.5 miles, and it was being driven from both ends—Kandersteg at the north end and. Goppenstein at the south.

Fig. 4 is a map showing the location of the Loetschberg tunnel in relation to the Simplon tunnel.

As we know, this tunnel has had troubles of its own, work at the north end being stopped absolutely after a frightful and unusual accident. Though the line passed under the Kander river, there was 600 ft. of covering over the tunnel at this point, 300 ft. of this being known to be bad, but the engineers seem to have taken the chances on the other 300 ft., and the chances proved to be deadly certainties against them. The entire depth caved in, forming a funnel-shaped hole from the bottom of the river, killing 26 men, causing a loss of all the machinery, and leaving a problem in tunnel-work such as, perhaps, was never encountered before.map-showing-location-of-loetschberg-tunnel

The work at the southern end of the tunnel from Goppenstein, Valais, Switzerland, is still going on at full speed, there having been an advance of nearly 2.5 miles up to the present time. The altitude of the work is about 4,000 ft. above sea-level. The tunnel is built for double tracks, with an area of cross-section of 592.5 sq. feet.

What is known as the Belgian method is employed in driving the tunnel, there being one principal bottom heading of 60 sq. ft. and an upper heading of 35 sq. ft. Every 600 ft. upraises are made from this bottom heading, and a top heading is started from each of these upraises. From the top heading the work of taking out the tunnel to its full section is carried on in bench-work. Besides the upraises, chutes about 2 ft, square are blasted out between the bottom heading and the top, where the muck can be dropped into trains of cars below. To make sure that there will be no dropping of the middle portion, this is supported by timbers as long as necessary, the timbering being successively removed and carried forward. The sides are finally trimmed out, and a concrete lining is placed.

Fig. 5 is a longitudinal section of the Loetschberg tunnel, showing the upper and lower headings, upraises, chutes, etc., with drills in position; a transverse section of the tunnel, showing the position and size of the top and bottom headings in relation to the completed tunnel; and an enlarged outline-elevation of the tunnel-carriage.

The responsibility for the total advance thus always remains with the bottom heading, and here is the most interesting feature of the work. There is a single track, about 30-in. gauge, in the bottom heading, this heading being widened out where the upraises are to come, and the track being turned out at these points. On this track travels a special ear, which carries a normally horizontal, tilting, counterbalanced steel beam, upon the forward end of which is secured a horizontal bar, and upon this bar are four drills. The swinging of the beam upward or downward, and the placing of the drills above or below the bar, gives them all the positions for the 12 or 14 holes usually required for each round.

The original car and mounting designed for this service may be described as too much of a good thing, and it was never used. On this car were two of the balanced, vertically-swinging beams, each carrying its shaft or tunnel-bar, and on each of these three or four drills would have been mounted. The ends of these two bars were telescoped, and it was designed to clamp and hold the bars in working position by forcing the ends, out against the sides of the heading by hydraulic pressure. An important and conspicuous feature of the car was, therefore, an air-operated duplex water-pump to furnish and maintain the required pressure.

The car actually used was a much simpler affair, arid of this I have only a photograph taken from the rear, with the apparatus in working-position, in the heading. Fig. 6 is a view of the lower heading in the Loetschberg tunnel with the drills at work, showing the weight on the rear end of the tunnel-carriage. This car was built by the Ingersoll-Rand Co. It carries only a single balanced, vertically-swinging beam with a single bar pivoted to swing horizontally above the forward end of it, and the bar is secured against the sides of the heading by the usual jack-screws, so that the water-pump is dispensed with. The bar is pivoted above the beam so that when the car is run backward or forward the bar may be swung around, above and parallel with the beam, leaving no interfering projections to catch the sides of the heading. Four 3 5/8-in. Ingersoll-Rand drills are mounted on the bar, each, of course, with a separate air-connection, but all connecting by a manifold with a single hose at the rear of the car.

Either 12 or 14 holes are drilled in each round. These are 4 ft. deep and 2 in. at the bottom, in four vertical rows, the inner rows running nearly parallel with the tunnel axis, the same as the rest. As soon as the holes are all drilled the bar is swung around straight and the car is run back to the last turnout until the blasting is done. Before the blasting, a 3/8-in. steel plate, 6 ft. 6 in. by 3 ft. 3 in. in area, is laid down just ahead of the end of the track, and after the blasting is over a cut is quickly made through the center of the muck-pile down to the plate, and the tunnel-carriage is run forward on this, the bar is set again, and the drills begin the top row of holes. Mucking-out continues during the drilling.

The use of this carriage facilitates the mucking to a considerable degree. After the blast in the usual American tunnel there is a mass of muck reaching nearly to the roof piled up in front and close to the face of the heading. This muck is shoveled away only to a sufficient degree to allow the men to climb over it and dig holes close to the face for the purpose of placing the columns which carry the rock-drills. The use of this carriage, with the bar which carries the drills projecting forward some 12 ft. or more from the truck, requires that the first mucking be done on the steel plate on the floor, and only sufficient to make a trench through the center of the heading, so that the tunnel-carriage may reach close enough to enable the drills to begin operations in the top holes over the muck.

longitudinal and transverse sections of loetscherberg tunnel

heading-with-drills-at-work

rock-drills-in-headings-of-college-hill

radialax-channeler-cutting

Here we have the drills at work earlier than with the American system, and the muckers have a better opportunity to load the cars, because the material is scattered on each side of the heading instead of being piled up only at the face.

Where the rock-drills are mounted on a bar carried by a tunnel-carriage it is easier to keep them in condition, free from muck and grit, than with the American system, where they are detached from the column-arms and laid on the floor. This is an important point, not only in reducing maintenance-expense and decreasing wear in the cylinder and other moving parts, but also in lessening the difficulty in keeping the stuffing-boxes tight, resulting in a smaller leakage of air.

Blasting is done by fuse-and-cap method, the fuses being all fired at the same time, but the length of the fuses is such that the center holes are fired first. By this system time is saved; but there is some danger of missed holes, and to minimize this three fuses are used in the bottom holes and two in all the others. The explosive used is 60-per cent, dynamite, made at Brieg. The center holes are charged with 2.7 kg. (6 lb.) each, and the average total charge for the 12 or 14 holes is from 24 to 26 kg. (53 to 57 lb.). The holes are not very deep, not exceeding 4 ft., starting 2.5 in. and finishing 2 in., to take cartridges up to 50 mm., or nearly 2 in., in diameter.

To accomplish the rapid rate of advance which is maintained the number of men employed is large. Night is, of course, the same as day, and the work is continuous. There are three 8-hr. shifts, and each shift is expected to drill two rounds and to shoot twice, making about 7 ft. per shift, or from 18 to 24 ft. per day. A liberal bonus is paid to the men all around—Italians from the Northern provinces—for all speed above a certain rate. Each separate drill has 2 men, and 2 additional helpers handle the steels, 10 men clear away after the shot, and all the 20 men work together for the placing of the truck and the laying of its side track, located about every 600 feet.

Experience has shown that the truck is indispensable if the rapid advance is to be maintained, though it is not without its objectionable features. The fact that all the four drills on the bar are in the same horizontal plane makes it difficult to give the upwardly-inclined holes just the same angle they could have if the drills were on separate columns, and to get a clean break to the bottom of the hole more explosive must be used.

This disadvantage in a horizontal bar over the vertical columns is more than compensated for by the readiness with which the bar may be set in place and jacked. One bar only has to be jacked, and that across the tunnel horizontally against the walls, which are usually rigid, while with the column system there are two, sometimes three, columns to be placed in position, each in a separate place, and each one must have a firm base, which is not always available close to the face of a heading just after the blast. But this is not all, nor is this the most imporant function of the tunnel-carriage system. By its use, and because of the facility which it affords for readily mounting

tunnel-carriage-with-old-burleigh-drills

the rock drill and moving it from one hole to another, it is possible to put in a large number of shallow holes of large diameter, as against the American system, which is a small number of deep holes of small diameter. It must be understood that when a rock-drill mounted upon a column has once begun its work in a heading, so much time has been occupied in placing it in position, getting the jack-screws tight, and pointing the hole itself, that we have fallen into the habit of keeping it there and putting in as deep a hole as we reasonably can consistent with the dimensions of the heading, the nature of the rock, and the angle of the hole.

https://www.youtube.com/watch?v=pd_GnxHeQdA

Fig. 7 is a view of the rock-drills at work in the heading of the College Hill tunnel, Providence, R. I. This is a practical working scene, showing a common method of driving a tunnel of small diameter, one column being used with two arms, with one drill on each arm.

Other things being equal, a deep hole can be drilled faster than two or more shallow holes, the combined depths of which are equal to the deep hole. The difference is made up in the time lost in changing the mounting, whether that mounting be a column or a tripod. The deeper the holes are driven the smaller is the diameter of the hole at the bottom, and we know, of course, that it is at the bottom of the holes that we want the greater amount of concentrated explosive force, and that the larger the hole is at the bottom the better is the effect of the blast.

A theoretically-perfect hole would be one larger at the bottom than at the orifice; but this is not practical in tunnel-work, hence we must suffer the loss of gauge, and by an excessive amount of explosive, by good tamping, and other means, try to make up for this handicap.

Another habit which we have fallen into is, that too small a number of holes are inserted in the heading. This is because we have found it difficult to make reasonable progress in hard rock by drilling an adequate number of holes, owing mainly to the fact that the mounting must be changed from one place to another. Having the habit of deep holes, we find it necessary to start them in a certain particular place, so that they will bottom just where we want them in order to “ break the ground.” Now, it is just in this that we see the disadvantage under which the column system suffers.

Having the horizontal bar used in the Loetschberg tunnel with four percussive drills mounted upon it, and this bar being practically balanced on a little tunnel-carriage and easily jacked in place across the walls, we begin putting in a hole with each machine. If one bottoms its hole ahead of the others it is simply swung on its radius, the center of the axis being the bar itself, and another hole put in. All of these holes are shallow, and being shallow they are naturally of large diameter. One meter is the usual depth, with a maximum of 4 ft. This is from one-half to one-third the usual depth of hole inserted in a tunnel-heading under the American column system; and because the holes are shallow it is not necessary to start them in so exact a position, but as long as we have enough of them, and use plenty of dynamite, the rock is thrown out in pieces of smaller size, and usually farther away from the face of the heading, than where the blast is from a deep hole, even though it may be directed to meet another hole, the two holes practically joining at the bottom, forming a wedge, and acting to concentrate the force by means of which the center cut is made.

This tunnel-carriage is nothing more or less than what is shown in Fig. 5, and is so simple and so heavy that it is practically indestructible.

What we have always understood as a tunnel-carriage is a cumbersome affair, quite different from that used in the Loetschberg tunnel. A carriage was the first form of mounting adopted when rock-drills were used to drive the Hoosac and other American tunnels. The carriage usually occupied the entire area of the beading, and was a hindrance to progress rather than otherwise.

Fig. 8 shows the Burleigh drill-carriage used in the Hoosac and other early tunnels. This is the original form of tunnel-carriage, and attention is drawn to a comparison between the Burleigh carriage and that used in the Loetschberg tunnel, shown in Fig. 5. The Burleigh carriage occupied too much space and was too cumbersome for practical tunnel-driving. It did not afford facilities for mucking, and too much delay occurred after a blast before the carriage could be run close enough to the heading for the drills to begin work. The Loetschberg machine, with its small truck and short wheel-base, need not approach very close to the heading because of its overhanging arm. The two to four drills, mounted on the single shaft carried by the arm, are quite sufficient for all ordinary heading-purposes.

The Alpine tunnel-carriage is a little truck with a wheel-base of only about 4 ft. and a gauge on the track which corresponds to the regular track used in the heading for conveying the material. It consists of a pair of axles with four wheels, and a cast-iron body with a central support, on which the I-beam carrying the shaft-bar is pivoted. On the opposite end of this I-beam is a heavy weight by which the bar is balanced, and by means of a vertical screw and a nut operated by a hand-wheel this I-beam is see-sawed to any position desired.

Next in importance to the system of mounting the drills is the system of blasting. Because of the larger number of holes, the greater diameter, and because the holes are not directed on lines of maximum breaking-efficiency, a great deal more explosive is used than in the American system, but dynamite is cheap—much cheaper in fact than time and labor. To blast by fuse instead of by electric battery would seem to be a step backward, and yet in this class of work it has some advantages. With the electric system the heading is wired and the center or cut holes blasted first. After this the wires leading to the side rounds are connected, and it frequently happens that the first blast has damaged the wires or has covered them so that considerable time is lost in getting ready for the side-round blast. Broken wires result in mis-fires. The use of the time-fuses means that the whole heading is fired practically in one operation, though there is a lapse of a few seconds between the discharges, owing to a difference in the lengths of the fuses, the shorter fuses being connected with the center-cut holes, and the length of fuse being increased in proportion as the side-rounds are blasted from the center. Mis-fires also occur with the fuse system, but this is minimized by employing two or three fuses and caps in each hole.

It takes less time to connect these holes by fuse and to fire them than it does to connect the wires, to see that they are properly insulated, to couple up the leading wires, and to discharge the battery, especially so when through the system of deep-cut holes it is found necessary in American tunnels to blast the center first and the side rounds alternately afterwards.

Other details of the tunnel-work as a whole need not be considered in the present paper. The upper heading is driven to keep pace with the lower one, but in the individual faces here the rate of advance is slower. For this work two 3.5-in. drills are used on 5.5-in. columns, and of course fewer men are called for. This heading has a sectional area of about 40 sq. ft., and the daily advance of each face is about 10 feet.

A large number of machines and accessories are used for the enlarging of the tunnel—3-in. drills on tripods, and a large number of hammer-drills in finishing off the walls, drilling pop- holes, etc.

The initial motive-power available is an electric current up to 2,500 h.p., developed from water-power. There are two Ingersoll-Rand air-compressors, with a total capacity of 4,250 cu. ft. of free air per minute. These discharge into four receivers at a pressure of 110 lb. The pipe-line into the tunnel is specified to be of such dimensions that the loss of pressure shall not exceed 2.2 lb. in about 6.5 miles, the maximum distance expected to be driven from this end.

Besides these compressors for driving the drills and smaller work, there are also two 4-stage compressors with a free-air capacity of 920 cu. ft. per minute, delivering at a pressure of 1,700 lb. into receivers made up of 12 tubes with a total capacity of 425 cu. feet.

There are two compressed-air locomotives of from 150 to 200 h.p., capable of hauling very heavy loads up a 3-per cent, grade and giving excellent service. These are of French manufacture, while practically all the other machinery is American.

On the line approaching this tunnel there are about 20 short tunnels, some up to a third of a mile in length, which have been driven in preliminary section for a single temporary track, some by hand and others by the use of the Electric-Air drill. These drills have worked, many of them, in places where it would have been impracticable to supply air, and, independently of this consideration, they have given excellent satisfaction.

Returning to the main tunnel, it will be realized that there has been a thorough study and a careful systemization of the entire work, for the purpose of avoiding delays and interferences, and for keeping the necessarily slower work at as rapid a pace as possible. The series of operations at the heading for the entire round have been tabulated as follows:

series-of-operation

These operations follow each other and must wait for each other, while the others not mentioned, principally the removing of the muck, can go on between times.

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