Page 191 - Moving the Earth_ The Workbook of Excavation

BASEMENTS

BASEMENTS 4.31

FIGURE 4.25 Bracing.

Unstable Face. If the soil is so unstable that it cannot be trusted to stand even in short sections,
the sequence shown in Fig. 4.27 may be followed. A trench is dug with the outer edge at the dig-
ging line. This is braced with sheeting, whalers, and sheeting jacks in the manner described in
Chap. 5, except that planks are left out of the sheeting on the inner wall at regular intervals.
Additional trenches are now dug into the excavation at right angles to the edge ditch. Heels are
placed in them at or below floor level, and breast timbers run from the heels to the whalers on the out
wall, through the spaces in the inner sheeting. The dirt between the breast timbers is now dug out, usually
with a clamshell with laborers assisting, and the sheeting jacks and inner wall bracing removed.
Steel sheet piling may be driven along the digging line without the ditch. Breast timber ditches
are dug in the same manner as described. A ditch is then dug on the inner side of the steel piling,
and a whaler and breast timbers are placed. Digging is then carried down to the level of the next
whaler, which is placed and braced.
Steel piling does not require as close spacing of the whalers as wood sheeting. A single whaler
near the top is often sufficient, and in some cases it is not braced at all.
Cofferdams. When dry excavation is carried a considerable distance below the water table without
dewatering the area, the heavy walls constructed to keep out soil and water are called cofferdams.