Environmental Engineering Reference
In-Depth Information
stretching and not rupturing during deformation, and are resistant to clogging.
Installation proceeds with a disposal plate attached to the end of a length of wick drain
that is inserted into a drive pipe. The pipe, which can be wire line drill rod, is pushed into
the slope with a bulldozer or backhoe. Additional lengths of wicks and pipe are attached
and driven into the slope. When the final length is installed, the drive pipe is extracted.
Drainage galleries
are very effective for draining large moving masses but their installa-
tion is difficult and costly. They are used mostly in rock masses where roof support is less
of a problem than in soils. Installed below the failure zone to be effective, they are often
backfilled with stone. Vertical holes drilled into the galleries from above provide for
drainage from the failure zone into the galleries.
Interceptor trench drains or slots
are installed along a slope to intercept seepage in a cut or
sliding mass, but they must be sufficiently deep. As shown in
Figure 9.107,
slotted pipe is
laid in the trench bottom, embedded in sand, and covered with free-draining material,
then sealed at the surface (see
Section 8.4.4).
The drain bottom should be sloped to provide
for gravity drainage to a discharge point. Interceptor trench drains are generally not prac-
tical on steep, heavily vegetated slopes because installation of the drains and access roads
requires stripping the vegetation, which will further decrease stability.
Relief trenches or slots
relieve pore pressures at the slope toe (see Section 8.4.4, Discussion
of Earth Dams). They are relatively simple to install. Excavation should be made in sec-
tions and quickly backfilled with stone so as not to reduce the slope stability and possibly
cause a total failure. Generally, relief trenches are most effective for slump slides (
Figure
9.25)
where high toe seepage forces are the major cause of instability.
Electro-osmosis
(see
Section 8.4.3)
has been used occasionally to stabilize silts and clayey
silts, but the method is relatively costly, and not a permanent solution unless operation is
maintained.
Bench as required for equipment
access to excavate. Backfill with
compacted sand and gravel
Backfill sand and gravel
max. size 3in. with less than
5% passing #200 sieve or
equivalent crushed stone
compact in 12 in. lifts
Compact each lift with
2 passes of plate vibrator.
Geofilter fabric (nonwoven
mirafi 140, BIDIM. or equal)
6 in. dia. slotted PV C pipe
slot size 0.1 in.
Note:
Slope trench and PVC
pipe at a minimumm
of 6 in. per 100 ft.
4" min. clearance
all sides
4 ft (max.)
0
3
6
FIGURE 9.107
Typical slope trench drain.
Scale (ft)
