TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
CHAPTER 1
INTRODUCTION
1-1. Purpose and scope. This manual provides
(1) Intercepting seepage that would otherwise
emerge from the slopes or bottom of an excavation.
guidance for the planning, design, supervision, con-
(2) Increasing the stability of excavated slopes
struction, and operation of dewatering and pressure
relief systems and of seepage cutoffs for deep excava-
and preventing the loss of material from the slopes or
tions for structures. It presents: description of various
bottom of the excavation.
methods of dewatering and pressure relief; techniques
(3) Reducing lateral loads on cofferdams.
(4) Eliminating the need for, or reducing, air pres-
for determining groundwater conditions, characteris-
sure in tunneling.
tics of pervious aquifers, and dewatering require-
(5) Improving the excavation and backfill char-
ments; guidance for specifying requirements for de-
acteristics of sandy soils.
watering and seepage control measures; guidance for
Uncontrolled or improperly controlled groundwater
determining the adequacy of designs and plans pre-
pared by contractors; procedures for designing, install-
can, by hydrostatic pressure and seepage, cause piping,
ing, operating, and checking the performance of de-
heave, or reduce the stability of excavation slopes or
foundation soils so as to make them unsuitable for sup-
watering systems for various types of excavations; and
porting the structure. For these reasons, subsurface
descriptions and design of various types of cutoffs for
construction should not be attempted or permitted
controlling groundwater.
without appropriate control of the groundwater and
1-2. General.
(subsurface) hydrostatic pressure.
a. It will generally be the responsibility of the con-
b. Influence of excavation characteristics. The loca-
tractor to design, install, and operate dewatering and
tion of an excavation, its size, depth, and type, such as
groundwater control systems. The principal usefulness
open cut, shaft, or tunnel, and the type of soil to be
of this manual to design personnel will be those por-
excavated are important considerations in the selec-
tions devoted to selecting and specifying dewatering
tion and design of a dewatering system. For most
and groundwater control systems. The portions of the
granular soils, the groundwater table during construc-
manual dealing with design considerations should fa-
tion should be maintained at least 2 to 3 feet below the
cilitate review of the contractor's plans for achieving
slopes and bottom of an excavation in order to ensure
the desired results.
"dry" working conditions. It may need to be main-
b. Most of the analytical procedures set forth in this
tained at lower depths for silts (5 to 10 feet below sub-
manual for groundwater flow are for "steady-state"
grade) to prevent water pumping to the surface and
flow and not for "unsteady-state" flow, which occurs
making the bottom of the excavation wet and spongy.
during the initial phase of dewatering.
Where such deep dewatering provisions are necessary,
they should be explicitly required by the specifications
c. Some subsurface construction may require de-
as they greatly exceed normal requirements and would
watering and groundwater control procedures that are
not otherwise be anticipated by contractors.
not commonly encountered by construction contract-
ors, or the dewatering may be sufficiently critical as to
(1) Where the bottom of an excavation is under-
lain by a clay, silt, or shale stratum that is underlain
affect the competency of the foundation and design of
by a pervious formation under artesian pressure (fig.
the substructure. In these cases, it may be desirable to
design and specify the equipment and procedures to be
1-1), the upward pressure or seepage may rupture the
used and to accept responsibility for results obtained.
bottom of the excavation or keep it wet even though
This manual should assist design personnel in this
the slopes have been dewatered. Factor of safety con-
work.
siderations with regard to artesian pressure are dis-
cussed in paragraph 4-8.
1-3. Construction dewatering.
(2) Special measures may be required for excava-
a. Need for groundwater control. Proper control of
tions extending into weathered rock or shale where
groundwater can greatly facilitate construction of sub-
substantial water inflow can be accommodated with-
surface structures founded in, or underlain by, per-
out severe erosion. If the groundwater has not been
vious soil strata below the water table by:
controlled by dewatering and there is appreciable flow
1-1
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