SLOPE INSTABILITY CAUSED BY SMALL VARIATIONS IN HYDRAULIC CONDUCTIVITY

Variations in hydraulic conductivity can greatly modify hillslope grou nd-water flow fields, effective-stress fields, and slope stability. In materials with uniform texture, hydraulic conductivities can vary ove r one to two orders of magnitude, yet small variations can be difficul t to determine. The destabilizing effects caused by small (one order o f magnitude or less) hydraulic conductivity variations using ground-wa ter flow modeling, finite-element deformation analysis, and limit-equi librium analysis are examined here. Low hydraulic conductivity materia ls that impede downslope ground-water flow can create unstable areas w ith locally elevated pore-water pressures. The destabilizing effects o f small hydraulic heterogeneities can be as great as those induced by typical variations in the frictional strength (approximately 4 degrees -8 degrees) of texturally similar materials. Common ''worst-case'' ass umptions about ground-water flow, such as a completely saturated ''hyd rostatic'' pore-pressure distribution, do not account for locally elev ated pore-water pressures and may not provide a conservative slope sta bility analysis. In site characterization, special attention should be paid to any materials that might impede downslope ground-water flow a nd create unstable regions.