An Hypothesis Concerning a Confined Groundwater Zone in Slopes of Weathered Igneous Rocks

In this paper an attempt is made to offer insights into local slope failure mechanisms from the perspective of hydrogeology. The hypothesis examined envisages that the hydraulic conductivity distribution in a weathered profile includes a relatively high hydraulic conductivity zone associated with the jointed rock mass near rockhead. There is evidence locally for such a condition. The groundwater flow regime in such a high hydraulic conductivity zone may be confined, in the sense of a ‘confined aquifer’, resulting in higher pore pressures during exceptionally heavy rainfall periods than might otherwise be expected. These high pore pressures may be responsible for a significant reduction in slope stability. The paper will conclude that, while slope stability studies in Hong Kong so far have tended to focus on perching within the regolith, in places a critical hydraulic boundary condition may exist at the base of the regolith.