Hydro-mechanical numerical evaluation of rainfall-induced fully coupled groundwater flow, land deformation, and failure potential in a variably saturated heterogeneous hill slope with consideration of interlinked rainfall-infiltration-seepage processes

Abstract

A series of steady- and transient-state numerical simulations is performed to evaluate rainfall-induced fully coupled groundwater flow, land deformation, and failure potential in an actual variably saturated heterogeneous hill slope with consideration of interlinked rainfall-infiltration-seepage processes. The slope is variably saturated under various rainfall rates. It is composed of colluvium underlain by weathered rock over fresh rock. As a combined methodology, the so-called mixed-type variable rainfall-infiltration-seepage flow boundary condition and constitutive mathematical equations are implemented first into a generalized fully coupled poroelastic hydro-mechanical numerical model. The resultant numerical model is then used in the numerical simulations. The steady- and transient-state numerical simulations show that both rainfall and layered heterogeneity have significant effects on spatial distributions and temporal changes of fully coupled groundwater flow, land deformation, failure potential, and stability with interlinked rainfall-infiltration-seepage processes in the slope. The steady-state numerical simulations show that, as the rainfall rate increases up to a critical rainfall rate, the slope becomes more saturated with water, and thus its overall stability deteriorates. However, under more than such a critical rainfall rate, the slope becomes fully saturated with water, and thus its hydro-mechanical responses are unchanged. The transient-state numerical simulations show that, as the time progresses under each maximum daily rainfall rate, pressure head buildup and slope unstabilization and failures initiate near the slope toe and then propagate toward the slope crest. Such trends occur faster and stronger as the maximum daily rainfall rate increases. In terms of interlinked rainfall-infiltration-seepage processes, as the rainfall rate increases up to the critical rainfall rate, or as the time progresses under each maximum daily rainfall rate, the seepage face expands from the slope toe toward the slope crest. As a result, rainwater infiltration occurs along the slope surface above the height of the seepage face, while groundwater seepage takes place along the slope surface below it.