A confined groundwater zone in weathered igneous rocks and its impact on slope stability

Landslides associated with hillside cuttings in igneous rock saprolites are a serious natural hazard in Hong Kong and have been extensively studied by government agencies and local universities for many years from the perspective of soil and rock engineering. A review of the case studies for some major landslides, especially deep-seated landslides, shows certain puzzling features. First, a number of failed slopes had failed again after they were cut back to a gentler angle designed to be safe based on theoretical calculations. Second, some of the failures were postulated to have been caused by the occurrence of a high groundwater table after rainfall, but weepholes in some slopes were dry and piezometers did not show much response to the rainfall. An attempt is made to offer explanations of the hydrogeological characteristics and failure features of these deep-seated landslides. A conceptual model is proposed to explain the failure mechanisms. It is believed that a confined groundwater zone may exist in the weathered igneous rock profile due to a highly fractured zone. In Hong Kong the saprolite has been treated as an aquifer while the bedrock is considered as an impermeable boundary, with the focus of research on the saprolite above the bedrock. This paper will demonstrate that, at least in some slopes, the saprolite is more like an aquitard and that the bedrock can be essentially a confined aquifer. The high pore pressure during exceptionally heavy rainfall periods may be responsible for a significant reduction in slope stability. An examination of the cross sections of some wellknown landslides in Hong Kong revealed a chair-shaped rockhead profile. The discussion in this paper has implications on slope remedial work design. The artesian water in the bedrock at times of exceptionally heavy rainfall is the key factor in slope failure and should be the focus of slope stability studies. An effective dewatering system is needed. Other traditional measures such as regrading the slope may lessen stability.