SINKING AND JOINING TOGETHER SECTIONS OF TUNNELS BUILT ON LAND. THE SEINE. THE DETROIT RIVER TUNNELS.
In the year 1896, Mr. Erastus Wyman secured a patent for building subaqueous tunnels close to the river, by sinking and joining together small sections of tunnels previously built on land. Each section would have been provided with a long vertical tube for the air-lock when compressed air was to be admitted to expel the water and permit the construction of the lining within the sunken shell. Thus each section of the tunnel would have acted as a pneumatic caisson; being, however, an improvement on Professor Winkler’s suggestion inasmuch as the caisson was a portion of the tunnel itself, instead of a simple inclosure for facilitating the construction of the shield. Mr. Wyman proposed to use this method in the construction of a tunnel between South Brooklyn and Stapleton, Staten Island; a charter was granted him but the tunnel was never built.
The Tunnel under the Seine River.
—The caisson method of building tunnels under water was used at Paris, France, in the construction of the Metropolitan Railroad under the Seine River.
The caissons designed by Mr. L. Chagnaud were for a double track line. They were sunk, ends to ends, and formed a portion of the tunnel lining which was enveloped by a framework of metal embedded in concrete. Built-up frames carried a shell of steel plating on the sides, from toes to springing lines, and on the sides and roof of the working-chamber. A temporary plate diaphragm closed the open ends. This construction formed a vessel capable of floating with a very light draft.
The method of sinking the caissons was as follows: The caisson was erected on the river bank and when completed it was launched and towed into position between pile stagings which served the double purpose of guiding the descent at the beginning of the sinking and of forming a working platform. The caisson when launched and, consequently, before the cast-iron lining had been put in place within it, weighed 280 metric tons; but, beyond some difficulty in taking it under the bridges in the way, the towing was accomplished without serious trouble.
Fig. 149.—Transversal Section of the Caissons for the Tunnel under the Seine River.
Previous to placing the caisson in position between the stagings, the portion of the river bed it was to rest upon had been leveled by dredging. Once in position, the first work was the erecting of the cast-iron lining segments within the framework. Work was then begun by filling the annular space between the lining and the shell with concrete; this additional weight gradually sunk the caisson to the river bottom. The working shafts, made up of steel cylinders, were placed as the sinking progressed to this point.
Section A-B.
Section C-D.
Plan at Joint.
Fig. 150.—Showing the Joining of the Caissons at the Pont Mirabeau Tunnel under the Seine River.
After the caissons had been sunk to the required place and in continuation of one another, a space of nearly 5 ft. was left between them. The construction of the tunnel within the bank of earth separating the two caissons was as follows: A cofferdam was built around this space. It was formed by two diaphragms closing the ends of the tunnel, and by two longitudinal walls sunk as temporary caissons, one on each side of the tunnel and inclosing their ends. This cofferdam was covered with a metal working-chamber whose lower edges rested on top of the four walls of the cofferdam. The joints were made tight by means of rubber or packed clay. The water in the cofferdam was then pumped out, the earth excavated, and the masonry built in continuation of the two ends of the tunnel sections. The submerged sections of the tunnel which were allowed to remain full of water to render them more stable and to save effort in pumping them, were now made dry; the diaphragms were removed from the ends of the caisson tunnels and the work made continuous. [Fig. 149] shows the cross-section of the caissons.
At the Pont Mirabeau crossing of the Seine, a slightly different method was used, described in “Eng. News,” May 18, 1911. The caissons were sunk to the required line and grade with an intervening longitudinal space of 153⁄4 ins. between two adjoining caissons. At each end of this space, which was filled with the river marl, was sunk against the edges of the caissons a hollow cylinder 20 ins. outside diameter. The interior of these cylinders was excavated and filled with concrete, thus forming a continuous wall on both sides of the two adjoining caissons. The earth from the intervening space was then removed and concrete deposited from bottom opening tremies up to the level of the top of the caisson. After nearly one month the tunnel was entered from the shaft and an opening the shape and size of the tunnel section cut through the diaphragms of the 153⁄4-in. wall and the concrete tunnel lining made continuous between the two sections. [Fig. 150] shows the method of joining the caissons.
The Detroit River Tunnel.
[15]—With some modifications which permitted dispensing with compressed air, the tunnel under the Detroit River was built for the Michigan Central Railroad, connecting Detroit with Windsor, Canada. The tunnel is 6625 ft. long; of this, however, only 2625 ft. are under the river, while the approach on the American side is 2000 ft. long and that on the Canadian side, 4000 ft. The tunnel consists of two parallel circular tubes 23 ft. in diameter, built up of 3⁄8-in. steel plate. They are placed 26 ft. apart, center to center, and are connected by diaphragms at 12-foot intervals.
[15] Condensed from a paper by B. H. Ryder.
Each section of the subaqueous tunnel is approximately 262 ft. long. There are ten of these sections and an eleventh a little over 60 ft. long. These tubes were built at the shipyards of the Great Lakes Engineering Works at St. Clair, about 30 miles from Detroit. After the assembling was completed, the ends of each tube were closed by temporary wooden bulkheads to make them float, and the outside sheathed horizontally with heavy timbers bolted to the diaphragms. This sheathing running lengthwise of the tube made a form or pocket, into which the inclosing jacket of concrete was placed. The sections were then launched and towed down to the tunnel site and sunk separately in a trench on the river bottom that had been previously dredged to receive them. This trench was dug to a width of 50 ft. and depth varying from 25 to 50 ft. by clamshell buckets, swung from a scow, working to a depth below the water level of 60 to 90 ft.
As a foundation for the sections, a grillage was constructed on the surface and sunk in place in the trench by derricks swung from a scow. The grillage was placed underneath each joint between the sections and built up of I-beams imbedded in concrete. This grillage is the width of the trench and about 30 ft. long, with posts projecting downward from the four corners, and these were seated into the river bottom, by means of pile drivers, to the desired grade.
Then the eleven sections of the tunnel were lowered and connected, one at a time. By the aid of air tanks placed on each section the movement was controlled until the final sinking upon the grillage in the trench. This operation called into play the greatest engineering skill and ingenuity. When it is considered that the current velocity at the river bed is about 2 ft. per second and much higher along the surface, some idea can be gained of the problems to be overcome. The movement of the enormous sections must be absolutely under control. Thirty-five-ton blocks of concrete were sunk in the river bottom up and down stream to act as anchors, and through them cables were rigged and connected back to the hoisting engines on the derrick scows. These were prevented from moving by spuds at each corner, securely driven into the river bottom at depths sometimes as great as 90 ft. Controlling cables were also run from the sections to the tremie scow to pull one structure close to the adjoining section previously sunk, and the divers made the necessary connection. [Fig. 151] shows cross-sections and plans of the tunnel as given in “Eng. Record,” March 2, 1907.
HALF CROSS SECTION Y-Y
HALF CROSS SECTION Z-Z
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HALF HORIZONTAL SECTION X-X
HALF TOP VIEW
Fig. 151.—Cross-Sections and Plans of the Detroit River Tunnel.
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Steel masts had been previously attached to each end of the sections to enable the engineers on shore to determine the alignment and locate the exact position during the sinking.
Concrete was then deposited in the pockets, completely surrounding the tubes, forming a solid monolithic structure from end to end.
This was done by means of the tremie process.
A 32-ft. by 160-ft. scow was equipped with a concrete mixing plant and the tremie pipes, three in number, through which the concrete was deposited. Each pipe is 12 ins. in diameter, of spiral riveted steel, 80 ft. long. These pipes could be raised or lowered, reaching from the receiving hoppers on the scow to the bottom of the trench. When the pipes were filled with concrete and lowered into position, a continuous flow was maintained. As fast as the concrete escaped at the bottom end of the pipe it was replenished at the top; this process continuing until the entire space surrounding the section was filled to the desired level, and under the pressure produced not only by the depth of water under which it was submerged, but also by the weight of the long column of concrete contained in the tubes. It is interesting to note that this is the first time a large amount of concrete has been deposited at a depth of 70 ft. by this method, and upon the accomplishment of this task in a measure depended the successful building of the tunnel.
Inside the tubes was placed a lining of reinforced concrete 20 ins. thick. Side walls were built up from this ring to provide ducts, which carry the electrical cables for the distribution of power, lighting, signal and telegraph wires. They also serve to provide a footwalk along the side of the tunnel.
There are cross passages in the tunnel every 200 ft., and also various niches for the different equipment needed in connection with the signaling, telephone and fire alarm system. The tunnel is lighted with 800 16-candle-power incandescent lights.
The track construction is new. There is no ballast used, the ties being laid in concrete. A ditch in the center of each track carries the rainfall that will flow down from the summits to sumps which are drained by centrifugal pumps.
One remarkable feature of its construction is that compressed air was not used in the building of the subaqueous tunnel, but it was necessary in building the approach tunnels. This is contrary to the usual program where compressed air is required in subaqueous work, and not ordinarily used in approach or land tunnel construction.
The trains are operated by very heavy electric locomotives, operated by the third-rail system.
The tunnel was constructed under the supervision of W. S. Kinnear, Chief Engineer of the Detroit River Tunnel Co.; Butler Bros. of New York were the general contractors.
CHAPTER XXII.
ACCIDENTS AND REPAIRS IN TUNNELS DURING AND AFTER CONSTRUCTION.
In the excavation of tunnels it often happens that the disturbance of the equilibrium of the surrounding material by the excavation develops forces of such intensity that the timbering or lining is crushed and the tunnel destroyed. To provide against accidents of this kind in a theoretically perfect manner would require the engineer to have an accurate knowledge of the character, direction and intensity of the forces developed, and this is practically impossible, since all of these factors differ with the nature and structure of the material penetrated. The best that can be done, therefore, is to determine the general character and structure of the material penetrated, as fully as practicable, by means of borings and geological surveys, and then to employ timbering and masonry of such dimensions and character as have withstood successfully the pressures developed in previous tunnels excavated through similar material. If, despite these precautions, accidents occur, the engineer is compelled to devise methods of checking and repairing them, and it is the purpose of this chapter to point out briefly the most common kinds of accidents, their causes, and the usual methods of repairing them.
Accidents During Construction.
—Accidents may happen both during or after construction, but it is during construction, when the equilibrium of the surrounding material is first disturbed, and when the only support of the pressures developed is the timber strutting that they most commonly occur.
Causes of Collapse.
—Collapse in tunnels may be caused: (1) by the weight of the earth overhead, which is left unsupported by the excavation; (2) by defective or insufficient strutting; and (3) by defective or weak masonry.
(1) The danger of collapse of the roof of the excavation is influenced by several conditions. One of these is the method of excavation adopted. It is obvious that the larger the volume of the supporting earth is, which is removed, the greater will be the tendency of the roof to fall, and the more intense will be the pressures which the strutting will be called upon to support. Thus the English and Austrian methods of tunneling, where the full section is excavated before any of the lining is placed, and where, as the consequence, the strutting has to sustain all of the pressures, present more likelihood of the roof caving in than any of the other common methods.
The character and structure of the material penetrated also influence the danger of a collapse. A loose soil with little cohesion is of course more likely to cave than one which is more stable. Rock where strata are horizontal, or which is seamy and fissured, is more likely to break down under the roof pressures than one with vertical strata and of homogeneous structure. Soft sod containing boulders whose weight develops local stresses in the roof timbering is likely to be more dangerous than one which is more homogeneous. A factor which greatly increases the danger of collapse, especially in soft soils, is the presence of water. This element often changes a soil which is comparatively stable, when dry, into one which is highly unstable and treacherous. The liability of the material to disintegration by atmospheric influences and various other conditions, which will occur to the reader, may influence its stability to a dangerous extent, and result in collapse.
(2) Collapse is often the result of using defective or insufficient strutting. Of course, in one sense, any strutting which fails under the pressures developed, however enormous they may be, can be said to be insufficient, but as used here the term means a strutting with an insufficient factor of safety to meet probable increases or variations in pressure. Insufficient strutting may be due to the use of too light timbers, to the spacing of the roof timbers too far apart, to the yielding of the foundations, to insufficient bearing surface at the joints, etc. Collapse is often caused by the premature removal of the strutting during the construction of the masonry. The masons, to secure more free space in which to work, are very likely, unless watched, to remove too many of the timbers and seriously weaken the strutting.
(3) The third cause of collapse is badly built masonry. Poor masonry may be due to the use of defective stone or brick, to the thinness of the lining, to poor mortar, to weak centers which allow the arch to become distorted during construction, to poor bonding of the stone or bricks, to the premature removal of the centers, to driving some of the roof timbers inside it, etc.
Prevention of Collapse.
—Tunnels very seldom collapse without giving some previous warning of the possible failure, and also of the manner in which the failure is likely to occur. From these indications the engineer is often able to foresee the nature of the danger and take steps to check it. The danger may occur either during excavation or after the lining is built. During excavation the danger of collapse is indicated beforehand by the partial crushing or deflection of the strutting timbers. If the timbers are too light or the bearing surfaces are too small, crushing takes place where the pressures are the greatest, and the timbers bend, burst, or crack in places, and the joints open in other places. The remedy in such cases is to insert additional timbers to strengthen the weak points, or it may be necessary to construct a double strutting throughout. When the distance spanned by the roof timbers is too great, failure is generally indicated by the excessive deflection of these timbers, and this may often be remedied by inserting intermediate struts or props. In some respects the best remedy under any of these conditions is to construct the masonry as soon as possible.
When collapse is likely to occur after the masonry is completed, its probability is generally indicated by the cracking and distortion of the lining. A study of the cause is quite likely to show that it is the percolation of water through the material surrounding the lining which causes cavities behind the lining in some places, and an increase of the pressures in other places. When it is certain that this water comes from the surface streams above, these streams may often be diverted or have their beds lined with concrete to prevent further percolation. When percolating water is not the cause of the trouble, a usually efficient remedy is to sink a shaft over the weak point, and refill it with material of more stable character. These, and the remedies previously suggested, are designed to prevent failure without resorting to reconstruction. When they or similar means prove insufficient, reconstruction or repairs have to be resorted to.
Repairing Failures.
—Tunnels may collapse in several ways: (1) The front and sides of the excavation may cave in; (2) the floor or bottom may bulge or sink; (3) the roof may fall in; (4) the material above the entrances may slide and fill them up.
(1) One of the most common accidents is the caving of the front and sides of the excavation. This may often be prevented by taking care that the face of the excavation follows the natural slope of the material instead of being more or less nearly vertical. When, however, caving does occur it may usually be repaired by removing the fallen material, strongly shoring the cavity, and filling in behind with stone, timber, or fascines.
(2) The bulging or rising of the bottom of the tunnel may usually be considered as a consequence of the squeezing together of the side walls. It usually occurs in very loose soils, and is chiefly important from the fact that the reconstruction of the side walls is made necessary. The sinking of the tunnel bottom is a more serious occurrence. It seldom happens unless there is a cavity beneath the floor, due either to natural causes or to the fact that mining operations have gone on in the hill or mountain penetrated by the tunnel. When the bottom of the tunnel sinks, three cases may be considered: (a) when the sinking is limited to the middle of the tunnel floor; (b) when only a portion of the foundation masonry is affected; and, (c) when the entire lining is disturbed. In the first case repairs are easily made by filling in the cavity with new material. In the second case the unimpaired portion of the masonry is temporarily supported by shoring while the injured portion is removed and rebuilt on a firm foundation. The remaining cavity is then filled. In the case of the complete failure of the lining, the method of repairing employed when the roof falls, and described below, is usually adopted.
(3) The most dangerous of all failures is the falling of the tunnel roof. In such casualties two cases may be considered: (a) When the falling mass completely fills the tunnel section, and (b) when it fills only a portion of the section.
Fig. 152.—Tunneling through Caved Material by Heading.
When the whole section is filled by the fallen material, the problem may be considered as the excavation of a new tunnel of short length inside the old tunnel, and under rather more difficult conditions. The first task, particularly if men have been imprisoned behind the fallen material, is to open communication through it between the two uninjured portions of the tunnel. It is advisable to do this even when there is no danger to life because of imprisoned workmen, since it enables the work of repairing to be conducted from both directions. The excavation of a passageway through the fallen material is rendered difficult, both because the fallen material is of an unstable character, and also because it is usually filled with the lining masonry, timbering, etc. When, therefore, the accident has happened before the full section of the original material has been removed, the first heading or drift is driven through this original material rather than through the fallen débris. Any of the regular soft-ground methods of tunneling may be employed, but it is usually better to select one which allows the masonry to be built with as little excavation as possible at first. For this reason the German method of tunneling is particularly suited to repair work of this nature. The Belgian method may also be used to advantage, particularly when the caving extends to the surface of the ground above, and the upper portion of the débris is, therefore, practically the same material as that through which the original tunnel was driven. The greatest defect of the Belgian method for making repairs is that the roof arch is supported by a rather unstable mass of mingled earth, stone, and timber, which constitutes the bottom layer of the fallen material. The method of strutting the work when the German or Belgian method is used is shown by [Fig. 152]. It sometimes happens that the fallen débris is so unstable that it will not carry safely the arch masonry in the Belgian method or the strutting in the German method, and in these cases one of the full-section methods of excavation is usually adopted. The nature of the strutting employed is shown by [Fig. 153]. When the section has been opened and the new masonry built, great care should be taken to fill the cavity behind the masonry with timber or stone; and should the disturbance reach to the ground surface it is often a good plan to sink a shaft through the disturbed material, and fill it with more stable material.
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. 154] and [155] show 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. 154 and 155.—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. 156] shows a common method of preventing the extension of a landslide which has been started by the excavation for the entrance masonry. [Fig. 157] shows 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 by [Fig. 158], may be adopted.
Fig. 157.—Shortening Tunnel Crushed by Landslide at Portal.
Accidents After Construction.
—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.
Giovi Tunnel Accident.
—In September, 1869, at a point about 220 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 61⁄2 ft. 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 temporary communication 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-stone voussoirs were inserted in the brickwork of the new arch as described in [Chapter XIII].
Chattanooga Tunnel.
—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 greater ease 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 21⁄2 ft. 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 21⁄2 to 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 61⁄2 ft., and on the underside of this was riveted a 6-in. plate 1⁄4 in. 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 between any 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.
CHAPTER XXIII.
RELINING TIMBER-LINED TUNNELS WITH MASONRY.
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.
Boulder Tunnel.
—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 in [Figs. 159] and [160].
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. 159 and 160.—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 in [Fig. 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 derricks were 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 51⁄2 ins. less in diameter than the distance between the side walls, so as to permit the use of 23⁄4 ins. 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 in [Fig. 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.
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 from 3⁄8 in. to 1⁄2 in. 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. timbers removed. 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.
In a few instances where slides occurred after the removal of the slide timbering, the method of re timbering the tunnel shown in [Fig. 162] was adopted. Two side drifts were first run 21⁄2 ft. 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 in [Fig. 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 Edison dynamo 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.
Mullan Tunnel.
—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.
The original timbering consisted of sets spaced 4 ft. apart c. to c., 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. 163] shows the timbering and also the form of 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.
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 in [Fig. 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 in [Fig. 164] being 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.
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Little Tom Tunnel.
—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 constructed through 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 shown by [Fig. 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 by [Fig. 166] and also by [Fig. 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:
| Trestles: | ||||||
| Caps and sills | 8 | pieces | 8 × 8 | ins. | × 20 | ft. |
| Posts | 18 | „ | 8 × 8 | „ | × 11 | „ |
| Braces | 16 | „ | 6 × 4 | „ | × 7 | „ |
| Centerings: | ||||||
| Ribs | 27 | „ | 2 × 18 | „ | × 7 | „ |
| Bracing | 12 | „ | 2 × 8 | „ | × 7 | „ |
| Support to crown lagging | 2 | „ | 6 × 6 | „ | × 10 | „ |
| Crown lagging | 20 | „ | 3 × 6 | „ | × 2 | „ |
| Side lagging | 30 | „ | 3 × 6 | „ | × 10 | „ |
| Side strips | 2 | „ | 2 × 12 | „ | × 9 | „ |
| Blocking for rollers | 1 | „ | 5 × 8 | „ | × 12 | „ |
| 6 screw and roller castings complete with bolts and lever; 114 bolts3⁄4-ins. in diameter; 71⁄2 U. H. hexagonal nut and 2 cast washers each. | ||||||
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.