Page 144 - Numerical Analysis and Modelling in Geomechanics

STABILITY OF UNSATURATED SOIL SLOPES 125

Figure 4.11 Pore water pressure distribution for various water permeabilities.

and two orders of magnitude larger than the previous two cases, shown in
Figure 4.6 and Figure 4.11a respectively. Thus, the ratio Q/k governs the
response of pore water pressure for the low permeability soil and this leads to
high pore water pressure being developed in the ground, as shown in
Figure 4.6b.
Figure 4.12 shows the variation of the factor of safety with saturated water
permeability for a given slope and rainfall intensity. As expected, the factor of
safety reduces with decreasing saturated water permeability but the rate of
reduction of the factor of safety is relatively small for high water permeability
with respect to the rainfall intensity. On the other hand, the factor of safety drops
more significantly when the saturated water permeability is of a comparative
magnitude to the rainfall intensity.
Back-analysis of subsurface water flows in slopes at the Mid-levels conducted
by Leach and Herbert [10] have concluded that it is necessary to consider
heterogeneous anisotropic flows in order for their model to predict correctly the
observed groundwater changes. The governing differential equation for their
model (see equation (4.4)) is essentially the same as equation (4.8), except that
flux was not considered.
In this chapter, heterogeneous anisotropic flow means that the permeability in
the x-direction (horizontal), k , and in the y-direction (vertical), k , are not equal,
x
y
but that the ratio (k x /k ) is a constant at any point within the soil mass. The
y
magnitude of k and k can also vary with a change of the moisture content or of
x
y