Slopes in Stiff-Fissured Clays and Shales

Case histories and probable causes of slope failures in stiff-fissured clays and shales are reviewed, and analyses are described which were performed to determine the influence of the initial stress conditions on the stresses around excavated slopes. These analyses show that the shear stresses around excavations are much larger for conditions representative of heavily overconsolidated clays (high initial horizontal stresses) than for conditions representative of normally consolidated clays (low initial stresses). Shear stresses large enough to cause failure at some points may develop even when the factor of safety calculated by the ∖a = 0 method of analysis is much larger than unity. The higher the horizontal stresses before excavation, the higher the factor of safety corresponding to development of local failure.