Confined groundwater zone and slope instability in weathered igneous rocks in Hong Kong

Abstract

In Hong Kong, slope failure in weathered granite and volcanic rocks has been studied extensively, but the impact of hydrogeological conditions on slope stability has received much less attention. It is customary in Hong Kong to assume that the hydraulic conductivity ( K) of weathered igneous rocks decreases with depth or as the rock mass becomes less weathered. Generally studies of groundwater for determining slope stability treat the saprolite above the rockhead as an aquifer and the rockhead as an impermeable boundary. This paper examines direct and indirect aquifer hydraulic information scattered in various sources and demonstrates that such a model may be inappropriate. Evidence is cited to show that, at least in some places, a relatively high hydraulic conductivity ( K) zone (HKZ) may exist at depth, either in the lower saprolite or at the rockhead. The completely decomposed igneous rocks, if significantly kaolinized, may have low permeability and behave as a confining zone for the HKZ. The groundwater flow regime in such an HKZ may be confined, resulting in a higher rise in water pressure in response to rainfall than might otherwise be expected. These high pressures may result in a significant reduction in slope stability. An examination of the cross-sections of some well-known landslides in Hong Kong reveals a chair-shaped rockhead profile. Numerical models coupling saturated and unsaturated subsurface flow are used to investigate the saturation process in the presence of an HKZ in slopes in response to typical rainstorms. The simplified Bishop's method is used to calculate changes in the factor of safety for slopes during such rainstorms. The saturation process and pressure distribution in a slope are complicated by the presence of an HKZ. An HKZ that is blocked at the toe represents the worst hydraulic condition for slope stability. The paper concludes that, although slope stability studies in Hong Kong have tended to focus on perching within the regolith, a critical hydraulic boundary condition may exist locally at the base of the regolith.