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.

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.

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.

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.

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.

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.

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.

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[322]
[323] 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.

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:

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.