Civil Engineering Reference
In-Depth Information
Figure 6.1
Plane failure on
smooth, persistent bedding
planes in shale (Interstate 40,
near Newport, Tennessee).
(a)
(b)
(c)
s
Upper slope
Release surfaces
Slice of unit
thickness
Tension crack
Face
Slide plane
f
p
Slide
plane
For sliding
f
>
p
>
Figure 6.2
Geometry of slope exhibiting plane failure: (a) cross-section showing planes forming a plane failure;
(b) release surfaces at ends of plane failure; (c) unit thickness slide used in stability analysis.
where
z
is the depth of the tension crack,
H
is the
slope height,
ψ
f
is the slope face angle and
ψ
p
is
the dip of the sliding plane.
The following assumptions are made in plane
failure analysis:
the sliding surface, escaping at atmospheric
pressure where the sliding surface daylights
in the slope face. The pressure distributions
induced by the presence of water in the ten-
sion crack and along the sliding surface are
illustrated in Figure 6.3.
(a)
Both sliding surface and tension crack strike
parallel to the slope.
(d)
The forces
W
(the weight of the sliding
block),
U
(uplift force due to water pressure
on the sliding surface) and
V
(force due to
water pressure in the tension crack) all act
through the centroid of the sliding mass. In
other words, it is assumed that there are no
(b)
The tension crack is vertical and is filled with
water to a depth
z
w
.
(c)
Water enters the sliding surface along the
base of the tension crack and seeps along
