Page 126 - Numerical Analysis and Modelling in Geomechanics

STABILITY OF UNSATURATED SOIL SLOPES 107
Leach and Herbert [10] developed a finite difference semi-empirically based
multi-layer model to predict and identify possible changes in piezometric
pressure resulting from extreme rainfall events with a duration of less than 1
hour. The governing two-dimensional time-variant groundwater flow equation
used to describe the changes in pressure head (h) with time in a continuous
aquifer having variable values of the specific storage (S ) and the saturated water
s
permeability (k), is:

(4.4)

In a fixed slope geometry, the response to rainfall is a direct function of the ratio
k/S . The higher the value of the ratio, the faster the heads will rise and decay,
s
and the shorter will be the response time of the system to storm events.
Parameters used in the model were obtained and adjusted from back-analysed
rainfall records. As a constant saturated permeability value is assumed, this
approach may give rise to misleading predictions in unsaturated soil regimes
which are commonly present in Hong Kong. In addition, transient water flow due
to various rainfall intensities are not considered.
Anderson and Pope [11] incorporated unsaturated flow into a hydrological
model to investigate the stability of a typical slope in Hong Kong. In the
unsaturated flow zone, water movement was assumed to be vertical (one-
dimensional), whilst in the saturated zone, water could flow in both horizontal
and vertical directions. Water permeability was no longer a constant value in the
unsaturated zone and it depended on the degree of saturation. Thus a soil-
moisture permeability relation for the specific soils was defined in their analysis.
Pore pressure and suction predicted by the model were adopted as input
parameters for limit equilibrium stability calculations. They showed that the
minimum factor of safety occurred when the storm duration just exceeded one
day, thereafter it remained approximately constant for the 4-day duration
considered. In their analyses, the flow and stability calculations were treated in a
completely uncoupled manner, similar to the approach adopted by Thomas and
Rees [14]. However, advances have been made recently to fully couple flow and
deformation analyses in unsaturated soils (Thomas and He [15]).
In practice, the ground conditions in the unsaturated region which may affect
the groundwater level at any point are generally very complex and it is difficult
to predict reliably the pore pressure distributions for stability calculations. The
current knowledge of the influence of transient seepage under various boundary
and ground conditions and hydrogeological regimes on slope stability
calculations is relatively poor compared with other elements of geomechanics. In
this paper, a finite element parametric study of transient flow has been carried out
to investigate the influence of various rainfall events, initial ground conditions
and water permeability on transient seepage in unsaturated soils and hence the soil
slope stability. A typical steep, unsaturated slope in Hong Kong has been used for