Page 132 - Numerical Analysis and Modelling in Geomechanics

STABILITY OF UNSATURATED SOIL SLOPES 113

Figure 4.4 Finite element mesh used in the seepage analysis.
Between the end of 1970s and the early 1980s, the Geotechnical Control
Office conducted some extensive pore pressure measurements at the Mid-levels
using tensiometers and piezometers (GCO [22]). Very scattered suction values,
varying from 0 to 80 kPa (practical upper limit of the tensiometers) were
measured in colluvium, CDG and completely decomposed volcanic (CDV).
These measured values depend on locations, depths and season.

Numerical experiments using the finite element method
Figure 4.4 shows the finite element mesh of a typical unsaturated hillside with a
steep cut slope in Hong Kong for the parametric study. The ground conditions at
the site comprise a 28° natural slope which consists of a 24m thick soil layer
which may be colluvium or CDG underlain by a 6 m thick bedrock. In the
middle of the hillside, a 55° cut slope has been formed for providing a 10 m wide
carriage-way. For the purposes of parametric studies, a constant hydraulic head
is specified along the boundaries FG (at 6 mPD) and HI (at 62 mPD unless stated
otherwise), whereas the bedrock is assumed to be an impermeable layer. The
appropriateness of the specified hydraulic heads at the two boundaries can be
verified by comparing the predicted and measured suction values in the slope
(see below). On the exposed sloping surface (FI), infiltration due to rainfall is
modelled by applying flux (Q) across the side of an element with varying