Environmental Engineering Reference
In-Depth Information
(a)
Fill
Collivium
Toe bulge
Collivium
Highly
weathered
zone
Shale with ocassional interbedded
layers of siltstone, sandstone
and limestone
FIGURE 9.110
Development of rotational failure in side-hill fill with
inadequate subsurface drainage. (a) Early failure stage:
concentric cracks show in pavement. (b) Rotational failure
of side-hill fill over thick colluvium. (Part b from Royster,
D.L.,
Bull. Assoc. Eng. Geol.,
X, 1973. With permission.)
Primary plane of failure
(b)
The movements develop finally into a rotational failure as shown in Figure 9.110b, a case
of deep colluvium.
Figure 9.111
illustrates a case of shallow residual soils.
Side-hill fills placed on moderately steep to steep slopes of residual or colluvial soils, in
particular, are prone to be unstable unless seepage is properly controlled, or the embank-
ment is supported by a retaining structure.
Stabilization
Preventive
Interceptor trench drains
should be installed along the upslope side of all side-hill fills as
standard practice to intercept flow as shown in
Figure 9.112.
Perforated pipe is laid in the
trench bottom, embedded in sand, covered by free-draining materials, and then sealed at
the surface. Surface flow is collected in open drains and all discharge, including that from
the trench drains, is directed away from the fill area.
A
free-draining blanket
should be installed between the fill and the natural slope materi-
als to relieve seepage pressures from shallow groundwater conditions wherever either the
fill or the natural soils are slow-draining, as shown in Figure 9.112. It is prudent to strip
Soil and weathered rock zone
Roadway
central line
Note position of
water table
Original ground
line
Fill
Toe bulge
FIGURE 9.111
Development of rotational failure in side-
hill fill underlain by thin formation of
residual soils and inadequate subsurface
drainage. (From Royster, D.L.,
Bull. Assoc.
Eng. Geol.,
X, 1973. With permission.)
Shale and interbedded siltstone and sandstone
