FAILURE OF SLOPES IN WEATHERED OVERCONSOLIDATED CLAY

Small slides in backslopes in overconsolidated clay are only locally reported and documented, but they constitute a large maintenance expense item in transportation department budgeting. While these failures are usually attributed to drastic changes in water content caused by saturation, there are additional factors that are conducive to the loss of stability but are not considered adequately. These factors are related to reduction in strength and weather-ability. The design of slopes uses shear-strength parameters, the values of which are conveniently obtained from triaxial compressive strength tests. However, these values reflect ultimate strength conditions whereas slope failures represent a state of reduced strength. The adjustment from ultimate to reduced strength can be effected by using the Webb technique. When this is done, a drop in the cohesion and interparticle friction values results and, in turn, a substantial reduction in the factors of safety governing slope stability. The weathering of material can be approximated in the laboratory by ultrasonic degradation tests. The data from these tests indicate that the clay content and the plasticity index of the soils in the slopes are higher than those indicated by the conventional standard tests. The slopes, acted on by water, develop higher pore water pressures and less resistance to the forces initiating sliding than predicted. Consequently, slope stability is further reduced and reaches failure or near-failure conditions. (Author)