Concrete Lining.
The placing of concrete inside the iron tube was done by an organization entirely separate from the tunneling force. A mixing plant was placed in each of the five shafts. The stone and sand bins discharged directly into mixers below, which, in turn, discharged into steel side-dump concrete cars. All concrete was placed in normal air.
The first step, after the iron lining was scraped clean and washed down and all leaks were stopped, was the placing of biats, marked B on [Plate LXXIV]. These were made up of a 6 by 12-in. yellow pine timber, 17 ft. long, with two short lengths of the same size spliced to its ends by pieces of 12-in. channels, 3 ft. 9 in. long, clamped upon the sides. These biats were placed every 5 ft. along the tunnel in rings having side keys. Next, a floor, 13 ft. wide, was laid on the biats and two tracks, of 30-in. gauge and 6-1/2-ft. centers, were laid upon the floor. There were three stages in the concreting. Fig. 2, [Plate LXXIV], shows the concrete in place at the end of the first, and Fig. 3, [Plate LXXIV], at the end of the second stage. The complete arch above the bench walls was done in the last operation.
Two 3 by 10-in. soldiers (SS in Figs. 1 and 2, [Plate LXXIV]) were fastened to each biat and braced across by two horizontal and two diagonal braces. To each pair of soldiers a floor template, T, was then nailed. The form for the center drain was then suspended as shown in Fig. 1, [Plate LXXIV]. Three pieces of shuttering, FFF, 20 ft. long, were then nailed to the bottom of the soldiers. One is all that would have been needed for the first concrete placed, but it was easier to place them at this stage than later, when there was less room. Three rough shutters were also nailed to the curved portion for the floor template. Opposite each biat, a bracket, bb, was then nailed, which carries a set of rough boards which formed the risers for the duct steps. Everything was then ready for concreting except that, where refuge niches occurred, a form for the portion of the niche below the seat was nailed to the shuttering. This form is shown at R in Fig. 1, [Plate LXXIV].
The concrete was dumped down on each side from side-dump cars standing on the track, and, falling between the risers for the duct steps, ran or was shoveled under the forms and down into the bottom. The horizontal surface on each side the center drain was smoothed off with a shovel. The workmen became very skillful at this, and got a fairly smooth surface. This concrete was usually placed in lengths of 45 or 60 ft. After setting for about 24 hours, the brackets, bb, were removed, together with the shuttering on the steps. The triangular pieces, t in Fig. 1, [Plate LXXIV], were not removed until later. Instead, a board was laid upon this lower step on which the duct layers could work. This and the triangular piece were not removed until just before the bench concrete was placed. This was important, as otherwise the bond between the old and new concrete would be much impaired by dirt ground into the surface of the old concrete. The ducts were then laid, as shown in Fig. 2, [Plate LXXIV].
The remaining shutters for the face of the bench walls were then placed. The remainder of the forms for the refuge niches, RR, in Fig. 1, [Plate LXXIV], were nailed to the shutters, the steel beam over the niche was laid in place, the forms for the ladders, L in Fig. 2, [Plate LXXIV], which occur every 25 ft., were tacked to the shutters, the shutters and forms were given a coat of creosote oil, and then all was ready for placing the bench concrete.
The specifications required a 2-in. mortar face to be placed on all exposed surfaces and the remainder to be smoothed with a trowel and straight-edge. After about 48 hours, the biats were blocked up on the bench, and all forms between the bench walls below the working floor were removed.
The centering for the arch concrete consisted of simple 5 by 3-1/2 by 5/16-in. steel-angle arch ribs, curved to the proper radius, spaced at 5-ft. intervals. Each rib was made up of two pieces spliced together at the top. Two men easily handled one of these pieces. After splicing, the rib was supported by four hanger-bolts fastened to the iron lining as shown in Fig. 3, [Plate LXXIV].