Page 190 - Moving the Earth_ The Workbook of Excavation

BASEMENTS

4.30 THE WORK

with it and cutting minute channels that weaken the structure. The flow of water is much slower
through soil than through an open ditch, and it exerts pressure proportional to the restriction of flow.
If the water is allowed to stand in the excavation at its natural level, it will cease to carry par-
ticles out of the bank, and will exert a back pressure against the bank that will tend to hold it in
place. However, this will not prevent the part of the bank above water from creeping under soil
pressure or absorption of capillary water, and wave action set up by wind or dropping of stones
or clods will cut into the bank at water level and undermine the top.
In general, where unstable soils or abundant groundwater is expected, open excavation should
not be done until preparations have been made to build walls immediately after its completion;
and if construction is delayed, it is better not to keep it pumped dry.

Side Effects of Dewatering. Often the most serious aspect of removing water from an excavation
is the effect on adjoining property. Water makes up a substantial part of the bulk of some soils,
and its removal, even if it does not carry particles, sometimes causes shrinkage, with settlement
of the surface and overlying structures. Damage to structures may also be caused by creeping of
plastic soils from beneath them into the pit.

SHORING

Wall Bracing. Movements of soil into a pit can almost be stopped, and water intake reduced, by
the use of timber bulkheads or sheet piling. These are required by law in many cities, and are often
good, although expensive, insurance against costly repairs and underpinning.
Installing such bulkheads is a highly technical operation, involving knowledge of soil behavior,
engineering calculations, and skilled personnel. There is sufficient space available in this volume
for only a brief sketch of one method.
Another method would be to drive interlocking steel sheet piling to several feet below the pit
floor and, if needed, tiebacks into the ground the piling is holding back. The tiebacks must be
solidly embedded in stable ground, preferably bedrock.

Bracing Stable Soil. Figure 4.25 illustrates installation of thorough bracing in an excavation
where a short section of face will stand for awhile without support. A long section is cut back by
the shovel to a slope which is expected to be stable, (A) and (B). Then a short section, perhaps 10
feet, is cut and trimmed to final shape, (C). Sheeting plank, 12 by 3 inches or heavier, is placed
vertically against the dirt wall. This plank should be long enough to reach a foot or two below the
bottom of the pit, and 1 or 2 feet above the ground surface. Bottom penetration may be obtained
by ditching, or by driving the planks down with an air hammer fitted with a special head for the
thickness of plank used.
Horizontal timbers, called whalers, are placed along the face of the sheeting, being temporarily
supported on cleats nailed to the planks. The whalers should be 6 inches by 6 inches or larger, and
should not be more than 5 feet apart vertically.
Beams or plank mats called heels are placed on firm, undisturbed soil in the pit floor, sloping
down toward the wall. These are used as abutments to take the thrust of the breast timbers that
extend from the heels to the whalers. These should be 10 inches by 10 inches or larger. Each
whaler must have two or more breast timbers, spaced 5 or more feet apart. If the heels are firm,
the spacing of breast timbers can be increased by using heavier whalers.
While this bracing is being installed, an adjoining section of the wall is trimmed. This is braced
in the same manner and the work continued in successive sections.
The sections may be tied together in several ways. The breast timbers may be placed against
the whalers where they are butted together, as in Fig. 4.26(A), with or without the plate shown.
The joints in different whalers may be staggered, as in (B), or may be overlapped, as in (C).
Nailing is kept to a minimum to avoid damage to the lumber. The bracing is dismantled after
the foundation is placed and the material removed for reuse. The sheeting is usually pulled by a
crane or pile driver equipped with a special clamp for gripping the tops of the planks.