Discussion of ‘Influence of rainfall intensity and duration on slope stability in unsaturated soils’ by C. W. W. Ng & Q. Shi (Quarterly Journal of Engineering Geology, 31, 105–114)

Abstract

J. McDougall writes: The authors have presented a numerical study of the influence of rainfall infiltration on unsaturated slope stability. This type of approach can provide insights into the process of infiltration and has potential as a predictive tool but its usefulness is dependent on a realistic representation of the actual groundwater conditions. One area of difficulty, which can undermine the simulation exercise, is the interpretation of boundary flows. In the case presented, the authors state that vertical boundaries FG and HI in Fig. 3 have prescribed hydraulic heads and boundary GH is impermeable. Infiltration due to rainfall is applied to boundary FI. It is, however, unclear whether this is applied over the entire exposed upper surface or some subsection of it. Since the vertical boundary heads are prescribed, any rise in main ground- water table due to applied rainfall, particularly if it is applied over the entire exposed surface, would be constrained by these boundary conditions. Are the authors able to comment on this and their conclusion that the main water table is not significantly affected by the different rainfall events considered? The adjustment of a hydraulic regime to a prescribed boundary flow is something which may be usefully explored using an unsaturated flow model. Figure 5, which presumably reports the hydraulic conditions at the end of the 24-hour infiltration period, shows recurvate suction profiles at section C-C. Are these an indication of an advancing wetting front (especially in the case of the higher rainfall intensity) and not, as