Environmental Engineering Reference
In-Depth Information
Clogging
Drain performance is reduced or eliminated by clogging that results from the infiltration
of fines, or from the encrustation of water. Drain holes have been plugged by calcite
leached from grout curtains (Christensen, 1974). Geosynthetics, including geotextiles and
geocomposites, are discussed by Koerner (1986).
Design
Drain design is based on Darcy's law or on flow nets and is described in Cedergren (1967),
for example.
Blankets
Geotextiles are becoming common use as blanket drains. Blanket drains extend longitudi-
nally with or without pipes. Horizontal blankets are used in pavements, building under-
drains, and earth dams.
Inclined or vertical blankets are used in earth dams (chimney drains) or behind retain-
ing walls.
Trench Drains
Trench drains are normally vertical, extending longitudinally to intercept flow into pave-
ments or beneath side-hill fills, or transversely to provide a drainage way downslope.
Along the toe of dams or slopes they provide for pressure relief.
They contain pipes (perforated, jointed, slotted, or porous), which during installation
are placed near the bottom of the trench and surrounded with pea gravel. The trench back-
fill is of carefully selected pervious filter material designed to prevent both piping and the
infiltration of the adjacent soil, and compacted to prevent surface settlement. The surface
is sealed with clay or some other impervious material to prevent rainwater infiltration.
(For Slopes, see
Section 9.5.3.)
Triangular or Trapezoidal Drains
Triangular or trapezoidal drains extend longitudinally along the toe of earth dams, in
which case they are normally very large; in the body of the dam embankment; or behind
retaining walls where they are connected to other drains.
Circular Drains
Vertical circular drains
of relatively small diameters (4-8 in., 10-20 cm usually) are used to
relieve water pressures (1) in slopes where perched conditions overlie an open material of
lower piezometric level and drainage can occur under gravity forces (see
Section 9.4.4)
and
(2) along the toe of earth embankments or slopes (relief or bleeder wells). Larger-diameter
drains are used to relieve excess hydrostatic pressure, allowing consolidation during sur-
charging (sand drains), or to relieve pore pressures that may lead to liquefaction during
earthquakes (see
Section 11.3.3).
Horizontal or subhorizontal drains include large open galleries excavated in rock
masses to improve gravity flow and decrease seepage pressures and subhorizontal grav-
ity drains of slotted pipe, which are one of the most effective means of stabilizing moving
slopes, especially where large masses are involved (see Section 9.4.4).
