Page 123 - Numerical Analysis and Modelling in Geomechanics

Chapter 4
The stability of unsaturated soil slopes

subjected to rainfall infiltration
C.W.W.Ng and Q.Shi

Abstract
Slope instability in unsaturated residual soils and loose fills has attracted
increasing attention in recent years around the world in such countries as Brazil,
South Africa, Japan and in the Far East. Rain-induced failures are the most
common failures. Rainfall leads to the development of a perched water table,
increasing the main groundwater level and 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. However, the present understanding of the
influence of transient seepage in unsaturated soils, due to water infiltration under
various boundary and ground conditions, and hydrogeological regimes on slope
stability, is still relatively poor compared with other elements of geomechanics.
To investigate the influence of various rainfall events and initial ground
conditions on transient seepage and hence slope stability, a parametric study has
been conducted using the finite element method. A typical steep unsaturated cut
slope in Hong Kong has been adopted for the parametric study. Variables
considered in the parametric study include soil isotropic and anisotropic
permeability, and rainfall intensity and duration. Pore water pressures or suctions
predicted during the transient seepage analyses are then used as input
groundwater conditions for subsequent limit equilibrium analyses of the stability
of the slope. A factor of safety is calculated using Bishop’s simplified method, with
a modified Mohr— Coulomb failure criterion to allow for shear strength
variation due to the presence of matrix suction. Infiltration due to rain-water
causes a reduction of matrix suction, but an increase in moisture content and
water permeability in unsaturated soils. A perched water table is developed
above the main water table. The factor of safety is not only governed by the
intensity of rainfall, the initial groundwater table and the anisotropic permeability
ratio, but it also depends on antecedent rainfall duration. A critical rainfall
duration can be identified, at which the factor of safety is the lowest.