Back analysis of cut slope instability in soft Bangkok clay

A number of slopes formed by excavation of the soft Bangkok clay had been unstable during the construction phase of a large housing complex. The mode of slope movement was the classical rotational slip surface type normally observed in clay. The slope stability when major movement was imminent was back analyzed based on the slope configuration before the movement. Measured in-situ vane shear strength was used in the total stress analysis to determine the short-term stability at the end of construction. The effective stress analysis was applied to establish the strength parameters of the soft Bangkok clay which would lead to the factor of safety of 1.0. The suitable method of analysis and the appropriate strength parameters for slope stability analysis are recommended.