Environmental Engineering Reference
In-Depth Information
Upstream
face
Gallery
Grout
curtain
Drains
FIGURE 8.50
Schematic of grout curtains and drains beneath
thin-arch concrete dam.
Piezometers
Cut slopes in rock masses
may require subhorizontal drains to relieve cleft-water pressures
and improve stability.
Potentially unstable or actively unstable
slopes in many cases can be stabilized by the
installation of drains to decrease pore pressures along failure surfaces. There are a number
of possibilities:
Subhorizontal drains are often the most practical and economical solution. They
consist of a 2-in. diameter or larger pipe, forced into a drill hole made at a slight
inclination upslope that extends beneath the phreatic surface for some distance.
The length and depth of the drains depend on the amount of groundwater low-
ering in the slope that is desired. Drains will be severed in moving slopes and
require reinstallation.
●
Drainage galleries are sometimes used in rock masses where ground collapse is
unlikely.
●
Vertical wells, which require continuous pumping, have been used to stabilize
large moving masses. Vertical gravity drains are effective in relieving seepage
pressures caused by perched water tables where impervious strata are underlain
by a free-draining material with a lower piezometric level.
●
Relief wells and trenches have been used to relieve pressures where seepage
emerges at the toe of a slope, at times the beginning point of instability.
●
Concrete Retaining Walls
Drainage is required to prevent the buildup of hydrostatic pressures behind concrete walls
retaining slopes. Design depends on flow quantities anticipated and can range from
buried blankets to merely weep holes. Several schemes are shown in
Figure 8.51.
Flow is
collected in longitudinal drains and carried beyond the wall, or discharged through open-
ings in the wall.
