TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
(Modified from "Foundation Engineering," G. A. Leonards, ed., 1962, McGraw-Hill
Book Company. Used with permission of McGraw-Hill Book Company.)
Figure 1-1. Installation of piezometers for determining water table and artesian hydrostatic pressure.
1-4. Permanent groundwater control.
or significant hydrostatic pressures within the rock or
shale deposit, rock anchors, tiebacks, and lagging or
Many factors relating to the design of a temporary de-
bracing may be required to prevent heave or to support
watering or pressure relief system are equally applica-
exposed excavation slopes.
ble to the design of permanent groundwater control
(3) An important facet of dewatering an excava-
systems. The principal differences are the require-
tion is the relative risk of damage that may occur to
ments for permanency and the need for continuous
the excavation, cofferdam, or foundation for a struc-
operation. The requirements for permanent drainage
ture in event of failure of the dewatering system. The
systems depend largely on the structural design and
method of excavation and reuse of the excavated soil
operational requirements of the facility. Since perma-
may also have a bearing on the need for dewatering.
These factors, as well as the construction schedule,
tinuously without interruption, they should be con-
must be determined and evaluated before proceeding
servatively designed and mechanically simple to avoid
with the design of a dewatering system.
the need for complicated control equipment subject to
failure and the need for operating personnel. Perma-
c. Groundwater control methods. Methods for con-
nent drainage systems should include provisions for
inspection, maintenance, and monitoring the behavior
of the system in more detail than is usually required
(1) Interception and removal of groundwater from
for construction dewatering systems. Permanent sys-
the site by pumping from sumps, wells, wellpoints, or
tems should be conservatively designed so that satis-
drains. This type of control must include consideration
factory results are achieved even if there is a rise in
of a filter to prevent migration of fines and possible
the groundwater level in the surrounding area, which
development of piping in the soil being drained.
may occur if water supply wells are shut down or if the
(2) Reduction of artesian pressure beneath the
efficiency of the dewatering system decreases, as may
bottom of an excavation.
happen if bacteria growth develops in the filter sys-
(3) Isolation of the excavation from the inflow of
tem. An example of a permanent groundwater control
groundwater by a sheet-pile cutoff, grout curtain,
system is shown in figure 1-2.
slurry cutoff wall, or by freezing.