Environmental Engineering Reference
In-Depth Information
Uncontrolled Seepage
Many investigations concentrate on keeping an excavation dry, but the control of forces
due to seepage is equally important in the prevention of failures, which in terms of uncon-
trolled seepage forces have been divided into two categories by Cedergren (1967):
Category
I: Failure caused by migration of particles to free exits or into coarse openings
1.
Piping failures of dams and levees caused by:
(a) Lack of filter protection
(b) Poor compaction along conduits, along foundation trenches, etc.
(c) Holes in embankments caused by animals, decomposed wood, etc.
(d) Filters or drains with openings too large
(e) Open seams or joints in rock
(f) Gravel or other coarse strata in foundations or abutments
(g) Cracks in rigid drains, reservoir linings, dam cores, etc., caused by mass
deformation
(h) Any other natural or human-made imperfection in the embankment or
foundation
2.
Clogging of drains and filter systems
Category
II: Failures caused by uncontrolled saturation and seepage forces
1.
Most slope failures, including highway and other cut slopes, open-pit mines, and
reservoir slopes caused by seepage forces
2.
Deterioration and failure of roadbeds caused by insufficient structural drainage
3.
Earth embankment and foundation failures caused by excess pore pressure
4.
Retaining wall failures caused by unrelieved hydrostatic pressures
5.
Canal linings and slabs for basements, spillways, dry docks, and other buried
structures (such as partially buried tanks for sewage treatment plants, which are
often located adjacent to streams) uplifted by unrelieved pressures
6.
Most liquefaction failures of dams and slopes caused by earthquake forces
Investigation (see also
Section 2
.3.7)
Prior to Construction
Reconnaissance,
using imagery interpretation and site visits, provides an overview of water
table conditions. All springs and other seepage conditions from slopes and cuts should be
noted. Precipitation data should be gathered to provide the basis for formulating judgments
regarding existing groundwater conditions, i.e., whether the groundwater is higher than
normal, normal, or lower than normal (see
Section 9.3.4).
On important projects such data
should go back for at least 25 years to determine if the region is in a long wet or dry period.
Explorations
should be extended to depths significantly below any excavations to define
all groundwater conditions including the depth to the existing static water table and
perched and artesian conditions. Control methods may vary with the conditions. Artesian
conditions and large quantities of water flowing in open formations will be costly to con-
trol, whereas a perched water table may simply require a gravity drainage system, which
permits the water to drain to a lower, open stratum with a lower piezometric level. Major
tunnel projects and other underground constructions in rock at substantial depths are
often best explored with pilot tunnels.
In situ tests
are performed to provide data on permeabilities and drawdown, and to eval-
uate the potential for seepage loss through abutments and foundations for dams, espe-
cially in rock masses (see
Section 3.3.4).
