were never allowed to remain open at the full depth, the concreting
being started as soon after the necessary length of rock had been
uncovered as the forms and preliminary work for a section could be
prepared. Each section was a monolith, except in a few cases where
very heavy rains made it impossible to hold the laborers.

The various operations in building the concrete wall are shown on
Fig. 7. Guide-planks, "_a a_," Section "_A-A_," were securely spiked
to alternate tiers of struts for the length of the section, the face
of each guide-plank being set on line with the intended face of the
concrete wall, and 2-in. tongued-and-grooved spruce plank were laid
along the guide-plank to the height of the bottom strut and securely
braced from the front sheeting. A 4-in. brick wall was built
simultaneously on line with the back of the wall to the height of
the first step. Where the bottom strut was below that elevation, the
brickwork was left low at that immediate point and built up when the
strut was removed. The brick wall was then water-proofed on the side
toward the concrete, and loose laps of the water-proofing were allowed
to hang over the brickwork and at least 8 in. down the back. A 6-in.
vitrified pipe drain was then laid along the surface of the rock just
outside of the brick wall, the joints in the pipe being caulked with
oakum saturated in cement, and pointed with cement mortar above a line
1 in. below the horizontal diameter, the remainder of each joint being
left open. Cross-drains were laid from tees in the back drain to the
face of the wall at all low points in the rock and at least for every
25 ft. of wall length, the joints of these discharge pipes being caulked
and cemented throughout. The surface of the rock was then washed and
scraped clean, and was covered with about 1 in. of mortar, after which
the section was ready for concrete.