The building of monolithic sections in trenches required that the thrust
from one set of struts be taken by the concrete before the set above
could be removed, and necessarily caused slow progress, the rate at
which concrete was deposited being just sufficient to prevent one layer
from setting before the next layer above could be placed.

The concrete used was mixed in the proportions of 1 part of cement to
3 parts of sand and 6 parts of stone, in 2-bag batches, in ¾-yd. and
1-yd. Ransome portable mixers mounted with air-driven engines on the same
frame. These mixers were placed at the surface, and were charged with
barrows, the correct quantities of sand and stone for each batch being
measured in rectangular boxes previous to loading the barrows. The
concrete was discharged from the mixer into a hopper which divided into
two chutes, only one of which was used at a time, the concrete being
shoveled from the bottom of the chutes to its final position. Facing
mortar, 2 in. thick, was deposited simultaneously with the concrete, and
was kept separate from it by a steel diaphragm until both were in place,
when the diaphragm was removed and the two were spaded together. The
bottoms of the guide-planks were cut off just above the concrete as it
progressed, and, as soon as the wall had reached a strut at one end of
the section, that strut was removed, the form was built up to the next
strut, at front and back, and braced to the sheeting, so that, by the
time the entire length of the section had been carried up to the level
of the first line of struts, forms were ready at one end for the
succeeding layers. The layers of concrete never exceeded 8 in. in
height, and at times there were slight delays in the concreting while
the carpenters made ready the next lift of forms, but such delays were
rarely long enough to permit the concrete to take its initial set.

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