TM 5-818-4/AFM 88-5, Chap. 5
CHAPTER 4
EARTHWORK: EXCAVATION AND PREPARATION FOR FOUNDATIONS
4-1. Excavation.
that the contractor or the contracting office often do
a. General. In general, excavation for subsurface
not possess, and the assistance of specialists in this
structures will consist of open excavation and shaft
field should be obtained.
and tunnel excavation. Where excavation to great
(b) Groundwater without significant seepage
depths is required, a variety of soils and rock may be
flow can also be a problem since excess hydrostatic
encountered at a single site. Soils may range through a
pressures can develop below relatively impervious
wide spectrum of textures and water contents. Rock
strata and cause uplift and subsequent foundation or
encountered may vary from soft rock, very similar to a
slope instability. Excess hydrostatic pressures can also
firm soil in its excavation requirements, to extremely
occur behind sheet pile retaining walls and shoring
hard rock requiring extensive blasting operations for
and bracing in shaft and tunnel excavations. Visual ob-
removal. Groundwater may or may not be present. The
servations should be made for indications of trouble,
groundwater conditions and the adequacy of ground-
such as uncontrolled seepage flow, piping of material
water control measures are important factors in exca-
from the foundation or slope, development of soft wet
vation, in maintaining a stable foundation, and in
areas, uplift of ground surface, or lateral movements.
backfilling operations. The extent to which ground-
(c) Accurate daily records should be kept of the
water can be controlled also influences the slopes to
quantity of water removed by the dewatering system
which the open excavation can be cut, the bracing re-
and of the piezometric levels in the foundation and be-
quired to support shaft and tunnel excavation, and the
neath excavation slopes. Separate records should be
handling of the excavated material.
kept of the flow pumped by any sump-pump system re-
quired to augment the regular dewatering system to
b. Good construction practices, and problems. A
note any increase of flow into the excavation. Flow-
majority of the problems encountered during excava-
meters or other measuring devices should be installed
tion are related to groundwater conditions, slope sta-
on the discharge of these systems for measurement
bility, and adverse weather conditions. Many of the
purposes (TM 5-818-5/NAVFAC P-418/AFM 88-5,
problems can be anticipated and avoided by precon-
Chap. 6). These records can be invaluable in evaluating
struction planning and by following sound construc-
"Changed Condition" claims submitted by the contrac-
tion practices.
tor. The contractor should be required to have "stand-
(1) Groundwater. Probably the greatest source of
by" equipment in case the original equipment breaks
problems in excavation operations is groundwater. If
down.
the seepage of groundwater into an excavation is ade-
quately controlled, other problems will generally be
(2) Surface water. Sources of water problems
minor and can be easily handled. Several points should
other than groundwater are surface runoff into the ex-
be recognized that, if kept in mind, will help to reduce
cavation and snow drifting into the excavation. A pe-
problems attributable to groundwater. In some in-
ripheral, surface-drainage system, such as a ditch and
stances, groundwater conditions can be more severe
berm, should be required to collect surface water and
than indicated by the original field exploration investi-
divert it from the excavation, In good weather there is
gation since field explorations provide information
a tendency for the contractor to become lax in main-
only for selected locations and may not provide a true
taining this system and for the inspection personnel to
picture of the overall conditions.
become lax in enforcing maintenance. The result can
(a) If groundwater seepage begins to exceed the
be a sudden filling of the excavation with water during
capacity of the dewatering system, conditions should
a heavy rain and consequent delay in construction. The
not be expected to improve unless the increased flow is
surface drainage system must be constantly main-
known to be caused by a short-term condition such as
tained until the backfill is complete. Drifting snow is a
heavy rain in the area. If seepage into the excavation
seasonal and regional problem, which can best be con-
becomes excessive, excavation operations should be
trolled by snow fences placed at strategic locations
halted until the necessary corrective measures are de-
around the excavation,
termined and effected. The design and evaluation of
(3) Slope integrity. Another area of concern dur-
dewatering systems require considerable experience
ing excavation is the integrity of the excavation
4-l
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