Environmental Engineering Reference
In-Depth Information
Longitudinal drains at
≈
2%
Upslope
drainage
Interceptor drains
along crest
Collector drain
end of cut
Downslope
collector
FIGURE 9.106
Sketch of slope face in
Figure 9.105
showing
system of longitudinal and downslope drains to
control erosion. See
Figure 8.9
and
Figure 8.10.
Monitoring
As the drains are installed, the piezometric head is monitored by piezometers and the effi-
ciency of the drains is evaluated. The season of the year and the potential for increased
flow during wet seasons must be considered, and if piezometric levels are observed to rise
to dangerous values (as determined by stability analysis or from monitoring slope move-
ments), the installation of additional drains is required.
Cut Slopes
Systems to relieve seepage forces in cut slopes are seldom installed in practice, but they
should be considered more frequently, since there are many conditions where they would
aid significantly in maintaining stability.
Failing Slopes
The relief of seepage pressures is often the most expedient means of stabilizing a moving
mass. The primary problem is that, as mass movement continues, the drains may be cut
off and cease to function; therefore, it is often necessary to install the drains in stages over
a period of time. Installation must be planned and performed with care, since the use of
water during drilling could possibly trigger a total failure.
Deep wells
have been used to stabilize many deep-seated slide masses, but they are costly
since continuous or frequent pumping is required. Check valves normally are installed so
that when the water level rises, pumping begins. Deep wells are most effective if installed
in relatively freed-raining material below the failing mass.
Vertical, cylindrical gravity drains
are useful in perched water-table conditions, where an
impervious stratum overlies an open, free-draining stratum with a lower piezometric
level. The drains permit seepage by gravity through the confining stratum and thus relieve
hydrostatic pressures (see
Section 9.2.7,
discussion of the Pipe Organ Slide). Clay strata
over granular soils, or clays or shales over open-jointed rock, offer favorable conditions for
gravity drains where a perched water table exists.
Subhorizontal drains
is one of the most effective methods to improve stability of a cut slope
or to stabilize a failing slope. Installed at a slight angle upslope to penetrate the phreatic
zone and permit gravity flow, they usually consist of perforated pipe, of 2 in. diameter or
larger, forced into a predrilled hole of slightly larger diameter than the pipe. Subhorizontal
drains have been installed to lengths of more than 300 ft (100 m). Spacing depends on the
type of material being drained; fine-grained soils may require spacing as close as 10-30 ft
(3-8 m), whereas, for more permeable materials, 30-50 ft (8-15 m) may suffice.
Santi et al. (2003) report on recent installations of subhorizontal
wick drains
to stabilize
slopes. Composed of geotextiles (polypropylene) they have the important advantages of
