Page 134 - Numerical Analysis and Modelling in Geomechanics

STABILITY OF UNSATURATED SOIL SLOPES 115
intensity (mm/day) at the boundary. Since the thickness of colluvium and CDG
vary from place to place, a single uniform soil layer is assumed initially for the
purposes of parametric seepage and stability analysis.
The two-dimensional finite element program SEEP/W (Geo-slope [24]) has
been adopted for the parametric study. This program can be used to model both
saturated and unsaturated flows under steady-state and transient conditions.
Based on laboratory measurements of fine sand and silt (Ho [25]), the volumetric
water content function and water permeability-pore pressure characteristic
function for colluvium and CDG are derived as shown in Figures 4.2 and 4.3
respectively. Recently, experimental evidence (Ng and Pang [26]) suggests that
the volumetric water content function of CDV can be strongly affected by stress
state. The influence of stress-state dependency of volumetric water content on
the stability of unsaturated soil slopes has been investigated recently (Ng and
Pang [27]) and a lower factor of safety has been found if the influence of stress
state has been included in the analysis for some ground conditions. For
simplicity, the influence is not included in this study.
For studying the sensitivity of water permeability, the saturated water
permeability is modified according to data available in Hong Kong (see
Table 4.1). However, the shape of the curve has been kept the same. The derived
permeability functions or curves are believed to be sufficiently accurate for the
purposes of the parametric analyses.
In presenting various pore water pressure distributions after rainfall and
associated factors of safety for slope stability in this chapter, the presented
results all relate to a time immediately after completion of rainfall. It should be
noted that the factor of safety may not be always a minimum value immediate
after the rainfall. This is because water will continue to seep downwards through
the soil after the rainfall ceases, so that, if the critical slip surface is at depth, the
minimum factor of safety may not be reached until some time after the rain
stops. However, as most of slope failures in Hong Kong are shallow in depth, the
time delay for reaching the minimum factor of safety for deep-seated failures is
not relevant for the parametric study of the stability of the steep cut slope
presented in this chapter.

Influence of rainfall Intensity

Traditionally, interpretations of what rainfall conditions are needed to initiate
slope failure have frequently been based on a statistical approach. Vargas [28]
and Brand [1] correlated a number of landslides with rainfall intensity and
concluded that if the rainfall intensity of a rainstorm exceeded a threshold value
(70 mm/hr in Hong Kong), the probability of landslide was very high. Their
statistical studies do not seem to have considered the influence of initial
geological and hydrological conditions on the slope failures and the variations of
pore water pressure in the ground before and after the landslides.