HYDROGEOMECHANICAL NUMERICAL SIMULATION OF IMPACTS OF RAINFALL RATE ON GROUNDWATER FLOW AND LAND DEFORMATION IN AN UNSATURATED HETEROGENEOUS SLOPE AND ITS STABILITY

A series of numerical simulations using a fully coupled hydrogeomechanical numerical model COWADE123D (Kim, 1995, 2006) was performed to analyze groundwater flow and land deformation in an unsaturated heterogeneous slope and its stability under various rainfall rates. The results of the numerical simulations show that the rainfall rate has significant impacts on groundwater flow and land deformation in the unsaturated slope and its stability. The slope becomes more saturated, and thus its overall stability deteriorates, especially in the weathered rock and weathered soil (colluvium) layers, as the rainfall rate increases. However, the slope becomes fully saturated, and thus its hydrogeomechanical responses are almost identical under more than a critical rainfall rate. From the viewpoint of hydrogeology, the pressure head, and hence the hydraulic head increase as the rainfall rate increases. As a result, the groundwater table rises, the unsaturated zone reduces, the seepage face expands from the slope toe toward the slope crest, and the groundwater flow velocity increases along the seepage face. From the viewpoint of geomechanics, the horizontal displacement increases, while the vertical displacement decreases toward the slope toe as the rainfall rate increases. This may result from the buoyancy effect associated with the groundwater table rise as the rainfall rate increases. As a result, the overall land deformation intensifies toward the slope toe, and the unstable or potential failure zones, in which the factors of safety against shear and tension failures are less than unity, become thicker near the slope toe and propagates from the slope toe toward the slope crest. These results of the numerical simulations also suggest that heterogeneity cannot always be ignored if it is observed in actual slope systems, and thus it must be properly considered when more rigorous predictions of groundwater flow and land deformation in a slope and its stability under various rainfall rates are to be obtained. This work was supported by the Under Ground Safety (UGS) Convergence Research Department Project of the Electronics and Telecommunications Research Institute (ETRI) funded by the National Research Council of Science and Technology (NST), Ministry of Science, ICT and Future Planning, Republic of Korea.