In the tunnel a traveling platform was constructed on two standard gage flat cars so coupled that a platform 100 ft. long and slightly narrower than the clear space between side wall forms was obtained. Connecting the end of the platform toward the mixing plant was a rampe or inclined platform mounted on wheels. The Koppel car tracks from the mixer were carried up the incline and the full length of the level platform. The cars were hauled to the foot of the incline by a light locomotive. A cable was then hooked to them; this cable was run through a block on the level platform, its free end coming back to the locomotive, which thus pulled the cars up the incline by moving back toward the mixer. On the level platform the cars were pushed by hand and dumped on the floor, whence the concrete was shoveled into the forms.
The platform construction deserves mention in the particular that it provided for adjusting the platform vertically. At each corner of the car a vertical post some 7 or 8 ft. high was set up. The side stringers of the platform carried two vertical posts at each end; these two posts were spaced just far enough apart to slide over the corner post, one on each side of it. A block at the top of the corner posts with the hoist line connected to the bottoms of the platform posts and the lead line going to a winch head, thus made it possible to lift the platform any distance within the height of the vertical post guide and hold it there by blocking under the posts. The arrangement is shown roughly by the sketch, Fig. 141. There was block and tackle for each corner post and a winch at each end of the car. The vertical movement of the platform was between 6 and 7 ft.
The floor was cemented first, then the side walls and finally the roof arch. Floor construction was begun at the portal farthest from the mixing plant. Koppel car tracks were laid through the tunnel and the concrete was dumped from them directly on the ground. The cars were hauled by a light locomotive. As the concreting advanced the dump car track was raised and suspended from timbers across tunnel so that the concrete could be placed under it. As fast as the floor hardened the permanent standard gage track was laid and a temporary third rail placed to give also a dump car track.
Fig. 141.—Sketch Showing Telescopic Support for Concreting Platform, Burton Tunnel.
When the floor had been finished the side walls were constructed, using the traveling platform and beginning at the far portal. The wall forms consisted of 4×6-in. studs, spaced 3 ft. apart and carrying 2×12-in. lagging. A 6×6-in. waling outside the studs at about mid-height held the studs to the timbering by lag bolts reaching through the wall to the 10×12-in. posts. A strip of plank nailed across wall between stud and post held the form at the top. Wall forms were erected for 100 ft. of wall at a time. These forms required about 45 ft. B. M. lumber per lineal foot of form on one side or 90 ft. B. M. for both sides. Two sets of side wall forms or 200 ft. of wall forming were built, and used over and over again. The concrete was shoveled into the wall forms from the traveling platform, the lagging being placed a board at a time as the work progressed upward and the platform being elevated as required, its final position being at about springing line level. When 100 ft. of side walls had been completed the traveling platform was moved ahead for another 100-ft. section.
Fig. 142.—Sketch Showing Device for Removing Centering Ribs, Burton Tunnel.
The centers consisted of 6×12-in. ribs, made up of 3×12-in. plank. The feet of the ribs rested on folding wedges on 6×12-in. wall plates, supported by 6×6-in. posts setting close against the finished wall. The ends of the ribs were held from closing in by 6×6-in. walings, one on each side, lag-bolted through the lining to the timbering. The centering required about 315 ft. B. M. of lumber per lineal foot of center. The method of removing the centers was novel. A flat car had erected on it a narrow working platform high enough to reach well up into the arch. Along this platform at the center was erected a sort of "horse," which could be elevated and lowered by jacks. The sketch, Fig. 142, shows the arrangement. At each end and at the middle of the platform two guide posts a a were erected and braced upright. Between these guide posts set plunger posts which were raised and lowered by screw jacks. The three plunger posts carried a longitudinal timber c. The car was run under the ribs of centering to be removed and the timber c raised by working the jacks until it came to close bearing under the ribs d. The railings and the wedges at the foot of the ribs were then removed, leaving the ribs hanging on the timber c. This timber was then jacked down to clear the lining and the ribs rotated horizontally on the point of suspension as a pivot until their ends swung in over the platform. The car was then moved ahead to where the centers were to be used again; the ribs were rotated back to their normal position across tunnel; the timber c was jacked up, and the wedges and railings placed at the first of the ribs.
The concreting on the roof arch was begun at the portal. Two shifts were worked and 42 ft. of arch were concreted each shift.