Civil Engineering Reference
In-Depth Information
6.3.1 Influence of ground water on stability
forces
U
and
V
is given by equations (6.6)
and (6.7), respectively.
In the preceding discussion, it has been assumed
that it is only the water present in the tension
crack and along the sliding surface that influ-
ences the stability of the slope. This is equivalent
to assuming that the rest of the rock mass is
impermeable, an assumption that is certainly not
always justified. Therefore, consideration must be
given to water pressure distributions other than
those presented in this chapter. Under some con-
ditions, it may be possible to construct a flow net
from which the ground water pressure distribu-
tion can be determined from the intersection of
the equipotentials with the sliding surface (see
Figure 5.10). Information that would assist in
developing flow nets includes the rock mass per-
meability (and its anisotropy), the locations of
seepage on the face and recharge above the slope,
and any piezometric measurements.
In the absence of actual ground water pressure
measurements within a slope, the current state of
knowledge in rock engineering does not permit a
precise definition of the ground water flow pat-
terns in a rock mass. Consequently, slope design
should assess the sensitivity of the factor of safety
to a range of realistic ground water pressures, and
particularly the effects of transient pressures due
to rapid recharge (see Figure 5.11(b)).
The following are four possible ground water
conditions that may occur in rock slopes, and
the equations that can be used to calculate the
water forces
U
and
V
. In these examples, the pres-
sure distributions in the tension crack and along
the sliding plane are idealized and judgment is
required to determine the most suitable condition
for any particular slope.
(b)
Water pressure may develop in the tension
crack only, in conditions for example, where
a heavy rainstorm after a long dry spell res-
ults in surface water flowing directly into the
crack. If the remainder of the rock mass is
relatively impermeable, or the sliding surface
contains a low permeability clay filling, then
the uplift force
U
could also be zero or nearly
zero. In either case, the factor of safety of the
slope for these transient conditions is given
by equation (6.4) with
U
=
0 and
V
given
by equation (6.7).
(c)
Ground water discharge at the face may be
blocked by freezing (Figure 6.5(a)). Where
the frost penetrates only a few meters behind
the
face,
water
pressures
can
build
up
(a)
b
z
z
w
V
H
U
p
(b)
(a)
Ground water level is above the base of ten-
sion crack so water pressures act both in the
tension crack and on the sliding plane. If the
water discharges to the atmosphere where
the sliding place daylights on the slope face,
then it is assumed that the pressure decreases
linearly from the base of the tension crack to
zero at the face. This condition is illustrated
in Figure 6.3 and the method of calculating
U
h
w
z
w
p
Figure 6.5
Possible ground water pressures in plane
failures: (a) uniform pressure on slide plane for
drainage blocked at toe; (b) triangular pressure on
slide plane for water table below the base of tension
crack.
