Failure Analysis of an Instrumented Stiff Clay Slope

Site characterization for a proposed 30± m (98 ft) cut slope indicated slickensided discontinuities in claystone rock cores, laterally discontinuous strata, and erratic topography that suggested the possible occurrence of pre-existing deep bedding plane shear surface(s). An instrumentation program was developed that consisted of inclinometers and piezometers to monitor slope movement and water levels during construction. The inclinometer data indicated movement along distinct shear planes which generally ceased upon completion of blasting and excavation operations. Approximately 8-months after construction, a 6,200± m³ (8,000 yd³) slide occurred in the overlying stiff clay with no apparent indication of prior inclinometer movement. A finite element model was developed to analyze the failed stiff clay slope and help understand the influence of horizontal stresses and how they increase the probability of progressive failure in heavily overconsolidated clay and shale slopes. These slopes tend to fail by a progressive mechanism as the soil in various portions of the slip surface is successively strained beyond the peak. The model predicts the development of a shear plane along the clay and claystone interface, as identified by the inclinometers in the actual slope, although the magnitude of movement is different. Stress concentrations exist at the tips of fissures and cause propagation that is inclined to the direction of the shear band. Fracture mechanics concepts can be used to study progressive failure of these fissures in stiff clay slopes.