Environmental Engineering Reference
In-Depth Information
grouted, whereas dams with a pool depth of 200 ft (30 m) or more, have had foundations
grouted (Sherard et al., 1963). Conditions normally requiring grouting include badly frac-
tured zones such as those from faulting, pervious sandstones, and vesicular and cav-
ernous rocks. The grouting of limestones is a common but often unsuccessful practice; a
difficult problem to assess is the depth of grouting required when limestone extends for
substantial depths. In pervious rock masses, grout curtains usually extend for some dis-
tance beyond the abutments to contain seepage losses.
Either a
concrete wall
or a
slurry wall
(Figure 8.49e)
provides an effective cutoff. Slurry
walls have been used with success even in highly pervious gravel and sand formations to
depths of 80 ft (24 m) (Sherard et al., 1963). It is necessary to consider whether there are
any possible sources of embankment settlement.
Relief wells or trenches
, installed at the downstream toe (Figure 8.49f), are relatively inex-
pensive and usually highly effective in relieving seepage pressures. They have the advan-
tage that they can be installed after construction, if necessary. Well spacing depends on the
amount of seepage to be controlled and often 50 to 100 ft (15-30 m) on centers is adequate
to reduce seepage pressures to acceptable limits in soils. It is advantageous that the wells
penetrate to the full depth of the pervious stratum. Inside pipe diameter should be 6 in. or
larger if heavy flows are anticipated. The wells usually consist of perforated pipe of metal
or wood, surrounded by a gravel pack, although in recent years pipe of plastic, concrete,
or asphalt-coated galvanized metal has been used. Installation should be approached with
the same care as with water wells, i.e., the wells should be drilled by a method that does
not seal the pervious stratum with fines, then surged with a rubber piston to remove
muddy drilling fluid. Seepage water from wells is discharged at ground surface in a hor-
izontal pipe connected to a lined drainage ditch running along the embankment toe. Relief
wells have the disadvantages of decreasing the average seepage path and, therefore,
increasing the underseepage quantity; they are subject to deterioration and require peri-
odic inspection and maintenance. Unless protected, metal pipes corrode, and wooden
pipes rot and are attacked by organisms.
Concrete Dams
Grout curtains
are sometimes used to reduce uplift pressures beneath foundations and
reduce pore pressures in the abutments of concrete dams founded in rock masses.
Drain holes
should be installed as common practice in rock masses to assure that seep-
age pressures are relieved since complete cutoff with a grout curtain is seldom achieved.
A typical configuration of grout curtains and a relief-well system beneath a thin-arched
concrete dam is shown in
Figure 8.50.
Drain spacing
ranges from 10-30 ft (3-10 m) on centers. The depth extends to perhaps
75% of that of the grout curtain. Drains should be inspected periodically to assure that
plugging is not occurring, either from the deposition of minerals carried in groundwater
or minerals from the grout itself.
Piezometers
should be installed to monitor pore- or cleft-water pressures. An indication
of high pressures requires that the installation of additional drains be considered.
Slopes
See also
Sections 9.4.3
and
9.4.4.
Cut slopes in soils
are often provided with vegetation to pre-
vent erosion and deter infiltration, longitudinal surface drains upslope of the cut and along
benches to carry away runoff, and transverse surface drains downslope to direct runoff on
long cuts. In some cases, subhorizontal drains are installed to relieve pore pressures along
potential failure surfaces. In silts, temporary relief may be obtained by electro-osmosis.
