Accidents After Construction.

Fig. 153.—Tunneling through Caved Material by Drifts.

When the fallen débris fills only a part of the section, the first thing to provide against is the occurrence of any further caving; and this is usually done by building a protecting roof above the line of the future roof masonry.Figs. 154and155show two methods of constructing this temporary roof, which it will be noticed is filled above with cordwood packing. As soon as the temporary roof is completed, the lining masonry is constructed.

Figs. 154and155.—Filling in Roof Cavity Formed by Falling Material.

Fig. 156.—Timbering to Prevent Landslides at Portal.

(4) Landslides which close the tunnel entrance are repaired in a variety of ways.Fig. 156shows a common method of preventing the extension of a landslide which has been started by the excavation for the entrance masonry.Fig. 157shows a method often adopted when the slope is quite flat and the amount of sliding material is small. It consists essentially of removing the fallen material and building a new portal farther back; that is, the open cut is extended and the tunnel is shortened. When the amount of the sliding material is very large, the contrary practice of lengthening the tunnel and shortening the open cut, as shown byFig. 158, may be adopted.

Fig. 157.—Shortening Tunnel Crushed by Landslide at Portal.

—Accidents after the completion of the tunnel may be divided into two classes: first, those which entirely obstruct the passage of trains, of which the collapse of the roof is the most common; and second, those which allow traffic to be continued while the repairs are being made,such as the bulging inward of a portion of the lining without total collapse. In the first case the first duty of the engineer is to open communication through the fallen débris, so that passengers at least may be transferred from one part of the tunnel to the other and proceed on their way. This is done by driving a heading, and strongly timbering it to serve as a passageway. If the tunnel is single tracked this heading is afterwards enlarged until the whole section is opened. In double-track tunnels the method generally adopted is to open first one side of the section and timber it strongly, so as to clear one track for traffic. While the trains are running through this temporary passageway the other half of the section is opened and repaired; the traffic is then shifted to the new permanent track, and the temporary structure first employed is replaced with a permanent lining. When the accident is such that the repairs can be made without obstructing traffic entirely, various modes of procedure are followed. In all cases great care has to be exercised to prevent accident to the trains and to the tunnel workmen. The work should be done in small sections so as to disturb as little as possible the already troubled equilibrium of the soil; the strutting should be placed so as to give ample clearing space to passing trains, and the trains themselves should be run at slow speeds past the site of the repairs. To illustrate the two kinds of accidents and the methods of repairing them, which have been mentioned, the accidents at the Giovi tunnel in Italy and at the Chattanooga tunnel in America have been selected.

Fig. 158.—Extending Tunnel through Landslide at Portal.

—In September, 1869, at a point about220 ft. from the south portal of the Giovi tunnel, a disturbance of the masonry lining for a length of about 52 ft. was observed. Accurate measurements showed that the lining was not symmetrical with respect to the vertical axis of the sectional profile. It was concluded that owing to some disturbance of the surrounding soil unsymmetrical vertical and lateral pressures were acting on the masonry. Close watch was kept of the distorted masonry, which for some time remained unchanged in position. In 1872, however, new crevices were observed to have developed, and shortly afterwards, in January, 1873, the injured portion of the masonry caved in, obstructing the whole tunnel section. The fallen material consisted chiefly of clay in a nearly plastic state. The surface of the ground above was observed to have settled. Investigation showed also that the cause of the caving was the percolation of water from a nearby creek. The water had soaked the ground, and decreased its stability to such an extent that the masonry lining was unable to withstand the increased vertical and lateral pressures.

The mode of procedure decided upon for repairing the damage was: (1) To open at least one track for the temporary accommodation of traffic; (2) To remove permanently the causes which had produced the collapse; (3) To build a new and much stronger lining. Close to the western side wall, which was still standing, the débris was removed, and the opening strongly strutted in order to allow the laying of a single track to reëstablish communication. At the same time a shaft was sunk from the surface above the caved portion of the tunnel, for the double purpose of facilitating the removal of the fallen material and of affording ventilation. The depth of the surface above the tunnel was 41.6 ft., which made the construction of the shaft a comparatively easy matter. The shaft itself was 61⁄2ft. wide and 18 ft. long, with its longer dimensions parallel to the tunnel, and it was lined with a rectangular horizontal frame and vertical-poling board construction. After temporarycommunication had been opened on the western track of the tunnel, the remainder of the fallen earth was removed and the excavation strutted. The new masonry lining was then built.

To remove permanently the cause of the cave-in, which was the percolation of water from a close-by stream, this stream was diverted to a new channel constructed with a concrete bed and side walls.

The failure of the original lining occurred by cracks developing at the crown, haunches, and springing lines. The new lining was made considerably thicker than the original lining, and at the points where failure had first occurred in the original arch cut-stonevoussoirswere inserted in the brickwork of the new arch as described inChapter XIII.

—The Western & Atlantic Ry. passes through the Chattanooga mountains by means of a single-track tunnel 1,477 ft. long, constructed in 1848-49. The lining consisted of a brickwork roof arch and stone masonry side walls. After the tunnel had been opened to traffic, this lining bulged inward at places, contracting the tunnel section to such an extent that it was decided to reconstruct the distorted portions. After careful surveys and calculations had been made, it was decided to take down and reconstruct about 170 ft. of the lining.

Owing to contracted space in the tunnel, it was necessary to remove all men, tools, and material, whenever trains were to pass through; and in order to do this a work-train of three cars was fitted up with necessary scaffolds, and supplied with gasoline torches for lighting purposes. Mortar was mixed on the cars, and all material remained on them until used. Débris torn out of the old wall was loaded on the cars, and hauled to the waste dump. A siding was built near the West end of the tunnel for the use of this train, and a telephone system was installed between the entrances and the working-train. On account of the contracted working-space and the greaterease with which brick could be handled, it was decided to rebuild the walls out of brick instead of stone.

In tearing out the old wall a hole was first cut through the three bottom courses of the arch and gradually widened. When the opening became four or five feet long, a small jack was placed near the center of it and brought to a bearing against the arch to sustain it. After cutting the opening to a length of from 7 to 10 ft. depending on the stability of the earth backing, the jack was removed and a piece of 8×16 in. timber placed under the arch and brought up to a bearing with jacks. One end of the timber rested on the old wall, the other on a seat built into the adjoining section of new wall. Wedges were then driven under the ends of timber and the jacks removed. With this timber in place, the old wall could be taken down with ease, the only trouble being that small stones and earth fell in from above and behind the arch. This was obviated by placing a 2 in. plank across the opening and just back of the 8×16 in. timber. At several points, however, the earth backing was saturated with water, and it became necessary to put in lagging as the old wall was removed. This timbering would be taken out as the new work was built up.

A suitable foundation for the new wall was secured at a depth from 2 to 4 ft., and a concrete footing was used. The section of the new wall was then built up as near as possible to the 8×16 in. timber; the timber was then removed and the new wall built up and keyed under the arch.

The new wall had a minimum width of 21⁄2ft. at the top, and 4 ft. at the base of rail, and was provided with weep holes at intervals. To facilitate matters, work was carried on simultaneously at two or three different places, the intention being to get one place torn out and ready for the bricklayers by the time they completed a section of the new wall at another place.

In rebuilding the arch, sections extending from the springing line up as far as was necessary to obtain the desired clearance,and from 21⁄2to 4 ft. in length, were removed. Near the sides, the earth above the arch was a stiff clay, which was self-sustaining; but near the center there occurred a stratum of gravel and clay saturated with water. This gave considerable trouble, falling through almost continuously until timbering could be placed. One end of this timber rested on the old arch, the other on the adjoining section of the new work. As the new work was to be set 6 to 13 ins. back from the old, it was necessary to block up this distance on top of the old arch, to carry the end of the lagging timber, in order that the timber should be clear of the new arch.

Owing to the small clearance between the car roof and the arch, a special form of centering was required, one that would occupy as small space as possible. Bar iron 1 in. thick, 4 ins. wide, and 20 ft. long was curved to a radius of 61⁄2ft., and on the underside of this was riveted a 6-in. plate1⁄4in. thick. This plate projected 1 in. on the sides of the centering, and carried the ends of the 1 in. boards used for lagging. The rivets were counter-sunk on the outside of the centering to present a smooth surface next the arch.

In keying up a section of the new work, a space about 18 ins. square had to be left open for the use of the workmen. As soon as the next section had been torn out, this space was built up. In building up the last section, this space had to be filled from below, which proved to be a tedious undertaking. The opening was gradually reduced to a size of 10 × 18 in., and the top ring then completed and keyed up, the adhesion of mortar holding the bricks in place until the key could be driven home. The next ring was treated in a similar manner, and so on to the face ring. Altogether 412 lin. ft. of the walls and 178 lin. ft. of the arch were taken down and rebuilt, amounting in all to 607 cu. yds. of masonry at the total cost of $7,440, or about $12.25 per cu. yds.

The regular trains arrived so frequently at the tunnel that slightly over two hours was the longest working-time betweenany two trains, and usually less than one hour at a time was all that it could be worked. In addition to the regular trains, a large number of extra trains, moving troops, had to be accommodated. Work was in progress eight months, and during that time there was no delay to a passenger train. The repairs were completed in August, 1899. The work was under the direction of Mr. W. H. Whorley, engineer of the Western & Atlantic R. R., and foreman of construction, A. H. Richards. A recent examination failed to reveal any sign of settlement cracks at the junction points of the new and old work.

The original construction of many American railway tunnels with a timber lining to reduce the cost and hasten the work has made it necessary to reline them, as time has passed, with some more permanent material. In most cases the work of removing the old lining and replacing it with the new masonry has had to be done without interfering with the running of trains, and a number of ingenious methods have been developed by engineers for accomplishing this task. Three of these methods which have been employed, respectively, in relining the Boulder tunnel on the Montana Central Ry., in Montana, the Mullan tunnel on the Northern Pacific Ry., in Montana, and the Little Tom tunnel on the Norfolk & Western R. R., in Virginia, have been selected as fairly representative of this class of tunnel work.

—This tunnel penetrates a spur of the main range of the Rocky Mountains, at an elevation at the summit of grade of 5,454 ft., and is 6,112 ft. in length. Its alignment is a tangent, with the exception of 150 ft. of 30′ curve at the north end. The material penetrated is blue trap-rock with seams for 4,950 ft. from the north end, and syenitic boulders with the intervening spaces filled with disintegrated material for the remaining 1,160 ft. The dimensions and character of the old timber lining and of the new masonry lining replacing it are shown inFigs. 159and160.

The form of masonry adopted consisted of coarse rubble side walls of granite, 13 ft. 8 ins. high, and generally 20 ins. thick,with a full center circular arch of four rings of brick laid in rowlock form. When greater strength was needed the thickness of the side walls was increased to 30 ins. and that of the arch to six rings of brick.

Cross Section.Longitudinal Section.Cross Section.Cross Section.Figs. 159and160.—Relining Timber-Lined Tunnel.

Cross Section.Longitudinal Section.

Cross Section.

Longitudinal Section.

Cross Section.Cross Section.

Cross Section.

Figs. 159and160.—Relining Timber-Lined Tunnel.

The first plan adopted in putting in the masonry was to remove all the timbering; but owing to the large number of falls and slides this was abandoned, and the plan followed was to leave in the three roof segments of the timbering with the overlying cord-wood packing and débris. In carrying on the work the first step was to remove the side timbers. This was done by supporting the roof timbers, as shown inFig. 159; that is, the first and fourth arch rib of an 8-ft. section containing four arch ribs were supported by temporary posts. The intermediate arch ribs were supported against the downward pressure by 6 × 6 in. timbers, extending from the side ribs near the tops of the temporary posts to the opposite sides of the intermediate roof segments, as shown in the longitudinal section,Fig. 160. To resist the pressure from the sides, 4 × 6 in. braces were placed across the tunnel from near the center of the intermediate segments to the upper ends of the hip segments, as shown in the cross-section,Fig. 159. The hip segments were then sawed off below the notch, and the side timbering removed and the masonry built.

The stone was conveyed into the tunnel on flat cars, and laid by means of small derricks located on the cars. Two derrickswere used, one for each side wall, and the work on both walls was carried on simultaneously.

The arch was built upon a centering, the ribs of which were 51⁄2ins. less in diameter than the distance between the side walls, so as to permit the use of 23⁄4ins. lagging. Each center had three ribs, made in 1-in. or 2-in. board segments, 10 ins. thick and 14 ins. deep. These ribs were mounted on frames, which followed the opposite walls, and were 4 ft. apart, making the total length of the center out to out about 9 ft. The frames, upon which the ribs were supported, are shown inFig. 161. As will be seen, they were mounted on dollys to enable the center to be moved from one section to another. Jacks were used to raise and lower the center into its proper position.

Cross Section.Longitudinal Section.Fig. 161.—Relining Timber-Lined Tunnel, Great Northern Ry.

Cross Section.Longitudinal Section.

Cross Section.

Longitudinal Section.

Fig. 161.—Relining Timber-Lined Tunnel, Great Northern Ry.

The arch was built up from the springing lines on both sides at the same time, four masons being employed. The rings were built beginning with the intrados, which was brought up, say, a distance of about 2 ft. from the springing line. Then the back of the ring was well plastered with from3⁄8in. to1⁄2in. of mortar, and the second ring brought up to the same height and plastered on the back, and so on until the last ring was laid. After bringing the full width of the arch up some distance, new laggings were placed on the ribs for an additional height of 2 ft. and the same process was repeated. All the space between the extrados of the masonry arch and the old lining was compactly filled with dry rubble. When high enough so that the hip segments had a foot or more bearing on the masonry the segments were securely wedged and blocked up against the brickwork, and the longitudinal 4 × 6 in. timbersremoved. The remaining space was now clear for completion of the arch, and both sides were brought up until there was not sufficient space for four masons to work, when the keying was completed by two masons beginning at the completed and working back toward the toothed end. The brickwork was built from the top of a staging-car.

Cross Section.Longitudinal Section.Fig. 162.—Relining Timber-Lined Tunnel, Great Northern Ry.

Cross Section.Longitudinal Section.

Cross Section.

Longitudinal Section.

Fig. 162.—Relining Timber-Lined Tunnel, Great Northern Ry.

In a few instances where slides occurred after the removal of the slide timbering, the method of re timbering the tunnel shown inFig. 162was adopted. Two side drifts were first run 21⁄2ft. wide by 4 ft. high, and the plate timbers placed in position and blocked. Cross drifts were then run, and the roof segments placed, and the core down to the level of the bottoms of the side drifts taken out. The lower wall plates were then placed and the hip segments inserted. The bench was then taken down by degrees, the side plates being held by jacks, and the posts placed one at a time. As the masonry at the points where slides occur consists of 30-in. walls and six-ring arch, the timbering was 22 ft. wide in the clear, with other dimensions as shown inFig. 162.

Only a single crew of brick and stone masons was employed. In order to prepare the sections for these masons it was necessary to have timber and trimming crews at work throughout the whole day of 24 hours, so that an engine and two train crews were in constant attendance. The single mason crews were able to complete 8 ft. of side wall and arch in 24 hours. The number of men actually employed at the tunnel was 35. This included electric-light maintenance, and all other labor pertaining to the work. The tunnel was lighted by an Edisondynamo of 20 arc light capacity, one arc light being placed on each side of the tunnel at all working-places. Each lamp carried a coil of wire 20 or 30 ft. long to allow it to be shifted from place to place without delay.

—This tunnel is 3,850 ft. long, and crosses the main range of the Rocky Mountains, about 20 miles west of Helena, Mont. The tunnel is on a tangent throughout, and has a grade of 20% falling toward the east. The summit of the grade, west of the tunnel, is 5,548 ft. above sea level, and the mountain above the line of the tunnel rises to an elevation of 5,855 ft. Owing to the treacherous nature of the material through which the tunnel passed, it had been a constant menace to traffic ever since its construction in 1883, and numerous delays to trains had been caused by the falls of rock and fires in the timber lining. For these reasons it was finally decided to build a permanent masonry lining, and work on this was begun in July, 1892.

With Wall Plates.Without Wall Plates.Old Timber Sections.Minimum Section.Average Section.Permanent Work.Fig. 163.—Relining Timber Lined Tunnel, Great Northern Ry.

With Wall Plates.Without Wall Plates.Old Timber Sections.

With Wall Plates.

Without Wall Plates.

Old Timber Sections.

Minimum Section.Average Section.

Minimum Section.

Average Section.

Permanent Work.

Fig. 163.—Relining Timber Lined Tunnel, Great Northern Ry.

The original timbering consisted of sets spaced 4 ft. apartc.toc., with 12 × 12 in. posts supporting wall plates, and a five-segment arch of 12 × 12 in. timbers joined by 11⁄2-in. dowels. The arch was covered with 4-in. lagging, and the space between this and the roof was filled with cordwood. Except where the width had been reduced by timbering placed inside the original timbering to increase the strength, the clear width was 16 ft., and the clear height 20 ft. above the top of the rail.Fig. 163shows the timbering and also the formof masonry lining adopted. The side walls are of concrete and the arch of brick. This new masonry, of course, required the removal of all the original timbering. The manner of doing this work is as follows: A 7-ft. section,A B,Fig. 164, was first prepared by removing one post and supporting the arch by struts,S S. After clearing away any backing, and excavating for the foundation of the side wall, two temporary posts,F F, were set up, and fastened by hook bolts.Fig. 146,L, and a lagging was built to form a mold for the concrete. Several of these 7-ft. sections were prepared at a time, each two being separated by a 5-ft. section of timbering.

Section, with Concrete Car.With Wall Plate.Without Wall Plate.Longitudinal Section.Fig. 164.—Construction of Centering Mullan Tunnel.

Section, with Concrete Car.

With Wall Plate.Without Wall Plate.

With Wall Plate.

Without Wall Plate.

Longitudinal Section.

Fig. 164.—Construction of Centering Mullan Tunnel.

The mortar car was then run along, and enough mortar (1 cement to 3 sand) was run by the chute into each section to make an 8-in. layer of concrete. As the car passed along to each section, broken stone was shoveled into the last preceding section until all the mortar was taken up. The walls were thus built up in 8-in. layers, and became hard enough to support the arches in about 10 to 14 days. The arches were then allowed to rest on the wall, and the posts of the remaining 5-ft. sections were removed, and the concrete wall built up in the same way as before.

The average progress per working-day was 30 ft. of side wall, or about 45 cu. yds.; and the average cost, including all work required in removing the timber work, train service, lights and tools, engineering and superintendence, and interest on plant, was $8 per cubic yard.

Fig. 165.—Centering Mullan Tunnel.

The centering used for putting in the brick arches is shown inFig. 165. From 3 ft. to 9 ft. of arch was put in at a time, the length depending upon the nature of the ground. To remove the old timber arch, one of the segments was partly sawed through; and then a small charge of giant powder was exploded in it, the resulting débris, cordwood, rock, etc., being caught by a platform car extending underneath. From this car the débris was removed to another car, which conveyed it out of the tunnel. The center was then placed and the brickwork begun, the cement car shown inFig. 164being used for mixing the mortar. The size of the bricks used was 21⁄2+ 21⁄2+ 9 ins., four rings making a 20-in. arch and giving 1.62 cu. yds. of masonry in the arch per lin. ft. of tunnel. The bricks were laid in rowlock bond, two gangs, of three bricklayers and six helpers each, laying about 12 lin. ft. per day. The brickwork cost about $17 per cu. yd. The total cost of the new lining averaged about $50 per lin. ft.

Cross Section.Longitudinal Section.Fig. 166.—Relining Timber-Lined Tunnel, Norfolk and Western Ry.Larger illustration

Cross Section.Longitudinal Section.

Cross Section.

Longitudinal Section.

Fig. 166.—Relining Timber-Lined Tunnel, Norfolk and Western Ry.

Larger illustration

—The tunnel has a total length of 1,902 ft., but only 1,410 ft. of it were originally lined with timber. This old timber lining consists of bents spaced 3 ft. apart, and located as shown by the dotted lines in the cross-section,Fig. 166. Instead of renewing this timber, it was decided to replace it with a brick lining. Although the tunnel was constructedthrough rock, this rock is of a seamy character, and in some portions of the tunnel it disintegrates on exposure to the air. In removing the timber to make place for the new lining some of the roof was found close to the lagging, but often also considerable sections showed breakages in the roof extending to a height varying from 1 ft. to 12 ft. above the upper side of the timbering. This dangerous condition of the roof made it necessary that only a small section of the timber lining should be removed at one time. It made it necessary, also, that the brick arch should be built quickly to close this opening, and finally that all details of centers, etc., should be arranged so as to furnish ample clearance to trains. The accompanying illustrations show the solution of the problem which was arrived at.

Fig. 167.—Relining Timber-Lined Tunnel, Norfolk and Western Ry.

Referring to the transverse and longitudinal sections shownbyFig. 166, it will be seen that two side trestles were built to carry an adjustable centering for the roof arch. Two sections of these trestles and centerings were used alternately, one being carried ahead and set up to remove the timbering while the masons were at work on the other. The manner of setting up and adjusting the trestles and centerings is shown byFig. 166and also byFig. 167, which is an enlarged detail drawing of the set screw and rollers for the centering ribs. The following is the bill of material required for one set of trestles and one center:

With this arrangement the progress made per day varied from 2 lin. ft. to 3 lin. ft. of lining complete. By work complete is meant the entire lining, including stone packing between the brickwork and the rock. On Feb. 23, 1900, 363 ft. of lining had been completed, at a cost of $33.50 per lin. ft. This cost includes the cost of removing the old timber, the loose rock above it, and all other work whatsoever.

In long tunnels, especially when excavated in hard rock, proper ventilation is of great importance, because the air cannot be easily renewed, and the amount of oxygen consumed by miners horses and lamps during construction is very large. The gases produced by blasting also tend to fill the head of excavation with foul air. Pure atmospheric air contains about 21% of oxygen and only 0.04% of carbonic acid; when the latter gas reaches 0.1% the fact is indicated by the bad odor; at 0.3% the air is considered foul, and when it reaches 0.5% it is dangerous. It is generally admitted that the standard of purity of the air is when it contains 0.08% of carbonic acid.

A large quantity of carbonic acid in the air is easily detected by observing the lamps, which then give out a dim red light and smoke perceptibly; the workmen also suffer from headache and pains in the eyes, and breathe with difficulty. Naturally, miners cannot easily work in foul air and, therefore, make very slow progress. It is, therefore, to the interest of the engineer to afford good ventilation, not only because of his duty to care for the safety and health of his men, but also for reasons of economy, so that the men may work with the greatest possible ease, thus assuring the rapid progress of the work.

It would be impossible to change completely the atmosphere inside a tunnel, as the gases developed from blasting will penetrate into all the cavities and gather there, but the fresh aircarried inside by ventilation has a very small percentage of carbonic acid, mixes with that which contains a greater quantity, and dilutes it until the air reaches the standard of purity. We have not here considered the gases developed from the decomposition of carboniferous and sulphuric rocks, which may be met with in some tunnels, and which render ventilation still more necessary. Tunnels may be ventilated either by natural or artificial means.

—It is well known that if two rooms of different temperatures are put in communication with each other, e.g., by opening a door, a draft from the colder room will enter the other from the bottom, and a similar draft at the top, but with a contrary direction, will carry the hot air into the colder room, thus producing perfect ventilation, until the two rooms have the same temperature. Now, during the construction of tunnels the temperature inside may be considered as constant, or independent of the outside atmospheric variations; hence during summer and winter, there will always be a draft affording ventilation, owing to the difference of temperature inside and outside the tunnel. In winter time the cold air outside will enter at the bottom of the entrances and headings, or along the sides of the shafts, and the hot air will pass out near the top of the headings or entrances or the center of the shafts; in summer the air currents will take the contrary direction.

Natural ventilation in tunnels is improved when the excavation of the heading reaches a shaft, because the interior air can then communicate with the exterior at two points, at different levels. In such cases a force equal to the difference in weight between a column of air in the shaft and a similar one of different density at the entrance of the tunnel, will act upon the mass of air in the tunnel and keep it in movement, thus producing ventilation. Consequently, during winter, when the outside air has greater weight than that inside, the air will come in by the headings and go out by the shaft, and in the summer it will enter at the shaft and pass out at the entrance.Sometimes to afford better ventilation shafts 8 or 12 in. in diameter are sunk exclusively for the purpose of changing the air. When the inside temperature is equal to that outside, as often happens during the spring and autumn, there are no drafts, and consequently the air in the excavation is not renewed and becomes foul; then fires are lighted under the shaft and a draft is artificially produced. The hot air going out through the shaft, as through a chimney, allows the fresh air to come in as in ordinary ventilation.

When the head of the excavation is very far from the entrances, or when the mountain is too high to allow excavation by shafts, it is quite impossible to secure good natural ventilation, especially during the spring and autumn months, and the engineer has to resort to some artificial means by which to supply fresh air to the workmen.

—Artificial ventilation in tunnels may be obtained in two different ways, known as the vacuum and plenum methods. Their characteristic difference consists in this, that in the vacuum method the air is drawn from the inside and the vacuum thus produced causes the fresh air from the outside to rush in, while the plenum method consists in forcing in the fresh air which dilutes the carbonic air produced inside the tunnel by workingmen and explosives. In the vacuum method the pressure of the atmosphere inside the tunnel is always less than the pressure outside, while in the plenum method the pressure within is always greater than that outside. Ventilation is the result of this difference of pressure, as the tendency of the air toward equilibrium produces continuous drafts. Both these methods have their advantages and disadvantages; but in the presence of hard rock, when explosives are continually required, the vacuum method is considered the best, because the gases attracted to the exhaust pipes are expelled without passing through the whole length of the tunnel, thus avoiding the trouble that a draft of foul air will give to the workmen who are within the tunnel. In both these methods itis necessary to separate the fresh air from the foul one; and this is done by means of pipes which will exhaust and expel the foul air in the vacuum method, or force to the front a current of fresh air when the plenum method is used. Artificial ventilation may also be obtained by compressed air which is set free after it has driven the machines, especially in tunnels excavated through rock, when rock drilling machines moved by compressed air are employed.

—The most common of the vacuum appliances consists in the simple arrangement of a pipe leading from the head of the tunnel out through the fire of a furnace. The air in the pipe is rarefied by the heat of the furnace and then set free from the other end of the pipe, thus creating a partial vacuum in the pipe, into which the foul air of the head rushes, the fresh air from the entrance taking its place, and thus ventilating the tunnel. A similar arrangement may be used with shafts, and the foul air may be driven out by a furnace which is placed either at the top or bottom of the shaft. Such furnaces act the same as those commonly used for heating purposes in the houses, with this difference, that, instead of fresh air being forced in, foul air is expelled. Another simple arrangement for producing a vacuum is by means of a steam jet which is thrown into the pipe, and which helps the expulsion of the air by heating it, thus producing a different density which originates a draft besides that mechanically originated by the force of the steam jet, which tends to carry out the foul air of the pipes.

Foul air may also be expelled by means of exhaust fans which are connected with pipes near the entrance of the tunnel. The fan consists of a box containing a kind of a paddle wheel turned by steam or water power and arranged so as to revolve at a high speed. The air inside the pipe is forced out by blades attached to the wheel, and thus the foul air of the front is driven away and fresh air from the entrance rushes in to take its place, and perfect ventilation is obtained.

The best manner of expelling foul air from tunnels, according to the vacuum method, is by means of bell exhausters. This consists of two sets of bells connected by an oscillating beam and balancing each other. Each set consists of a movable bell, which covers and surrounds a fixed bell with a water joint. In the central part of the fixed bell there are valves which open upwards, and on the bottom of each movable bell there are valves which open from the outside. When one bell ascends, the valves at the bottom are closed, the air beneath is then rarefied, and a vacuum is produced; the valves in the central part of the fixed bell filled with water are opened, and there is an aspiratory action from the pipe leading to the headings, and the foul air is thus carried away. The apparatus makes about ten oscillations per minute, and the dimensions of the bells depend upon the quantity of air to be exhausted in a minute. In the St. Gothard tunnel, where these bell exhausters were used, they exhausted 16,500 cu. ft. of air per minute.

—Fresh air may be driven into tunnels to dilute the carbonic acid by two different ways, viz., by water blast and by fans. Water when running at a great velocity produces a movement in the air which may be sometimes usefully and economically employed for ventilating tunnels. Water falling vertically is let run into a large horizontal zinc pipe having a funnel at the outer end; into this the air attracted by the velocity of the water is forced. By an opening at the bottom the water is afterward withdrawn from the pipe, and there remains only the air which is pushed forward by the air which is being continually sucked in by the velocity of the water.

The best and most common means of ventilation by the plenum method is by fans. There are numerous varieties of these fans in the market, but they all consist of a kind of fan wheel which by rapid revolution forces the fresh air into the pipe leading to the headings of the tunnel or to the working places. Instead of a large single fan, such as is used for miningpurposes, it is better to have a number of small fans acting independently of each other, conveying the fresh air where it is needed through independent pipes.

—A new method of ventilating tunnels was devised by Mr. Saccardo for the ventilation of the Pracchia tunnel along the Bologna and Lucca Railway in Italy. At the highest end of the tunnel the mouth was contracted inward in a funnel shaped form so as to just admit a train. Immediately at this contraction, a lateral tunnel, 50 feet long, branched off from one side of the main tunnel. At the mouth of this lateral tunnel was installed a fan which forced air into the tunnel and with 70 revolutions per minute delivered 3.532 cu. ft. of air per second at a water pressure of 1 in. This air current was directed inward through a second contraction or funnel, parallel to the one at the entrance and 23 ft. beyond it. In operation the action of the artificial air current was to suck in a considerable volume of outside air, while the air pressure was sufficient to counterbalance the movement of air produced by a train moving at a velocity of 16.1 ft. per second. Mr. Saccardo’s method was employed in ventilating a tunnel on the Norfolk and Western Railway with satisfactory results.

—In the excavation of tunnels in hard rock a number of rock drilling machines are employed which are moved by compressed air at a pressure of not less than five atmospheres. At each stroke about 100 cu. ins. of compressed air are set free, and at an average of 10 strokes per minute there would be 5000 cu. ins. of air at five atmospheres or 25,000 cu. ins., or a little more than 175 cu. ft. of fresh air at normal pressure set free every minute by each of the machines employed. But the air exhausted from the drilling machine is foul.

Regarding ventilation by compressed air, Mr. Adolph Sutro, in a lecture delivered to the mining students of the University of California, said:

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