Page 124 - Numerical Analysis and Modelling in Geomechanics

STABILITY OF UNSATURATED SOIL SLOPES 105
Introduction
Slope instability in unsaturated residual soils and loose fills is attracting
increasing attention in many countries around the world such as Brazil, Italy,
South Africa, Japan and in the Far East. The causes of landslides in these slopes
are attributed to a number of factors. Rain- induced failures are the most
common ones around the world (Brand [1], Fukuoka [2], Premchitt et al. [3],
Wolle and Hachich [4], Fourie [5], Lim et al. [6]). For instance, the annual number
of landslides caused by rainfall in Japan is in excess of 10,000 and could be as
high as 100,000 (Fukuoka [2]). Pore water pressure (u ) in the shallow depth of
w
these residual soil and loose fill slopes is generally negative with respect to the
atmospheric pressure (u ). The presence and the magnitude of matrix suction (u a
a
−u ) have been found to be absolutely crucial to the stability of unsaturated soil
w
slopes (Fredlund and Rahardjo [7]).
Infiltration of rain-water or ingress of a wetting front (Lumb [8]) leads to the
development of a perched water table, rising in the main groundwater level and
washing out (soil erosion due to concentrated water flow), resulting in an
increase in pore water pressure or a reduction in soil matrix suction. This, in
turn, results in a decrease in shear strength on the potential failure surface to a point
where equilibrium can no longer be sustained in the slope and then failures
occur. Slope failure mechanisms found in these landslides generally consist of
both shallow and deep-seated slips, depending mainly on the thickness of these
residual soils and loose fills. Deep-seated static soil liquefaction occurred in
some loose fill slopes under intense rainfall (Brand [1]).
The physical processes of infiltration of rainwater into the ground and its
seepage through the soil stratum have been studied by hydrogeologists, soil
scientists and geotechnical engineers. Equations and numerical models have been
derived and developed for use (Lumb [9], Leach and Herbert [10], Anderson and
Pope [11], Lam et al. [12], Pradel and Raad [13]). However, several important
limitations on the use of these equations and models are discussed in the
following paragraphs.
Lumb [9] derived an expression for the advance of the “wetting front”, with
the assumption that diffusion is negligible at the end of an intensive rainfall:

(4.1)

where n is porosity, k is saturated water permeability, t is time, and S and S are
o
s
f
the initial and final degree of saturation respectively. The soil will only be fully
saturated near the surface, but will be wet (degree of saturation, S=0.8 to 0.9)
down to a depth h (see Figure 4.1). The method was frequently used in the 1970s
and 1980s to design the water table for slopes by superimposition of the depth of
wetting front onto the main groundwater table at the end of a wet season.
However, this equation does not take account of sloping ground conditions,