The sump and pump chamber on Tunnels _C_ and _D_ differed from the one
described only in minor details; but, being wholly constructed in rock,
presented fewer difficulties and permitted a complete envelope of
water-proofing to be placed in the top.


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