Cohesion intercept in effective stress-stability analysis

The cohesion intercept in the Coulomb shear-strength equation has a significant influence on the location of the slip surface and the factor of safety. The cohesion intercept results from a linear approximation of a segment of a nonlinear shear strength against effective normal stress relationship. Shear strength on a slip surface, at the instant of a global instability, may range from residual strength to peak strength for stiff clays, and from large strain strength to peak strength for soft clays. Peak strength controls local development of slip surface, but it may not be available during global failure. The critical slip surface for an initial slide is determined using the peak shear strength. However, a mobilized strength equal to or less than the peak strength is used to define the factor of safety. Effective stress-stability analysis of slope failures has resulted in a reduction factor to the peak strength in order to obtain the mobilized strength for first time slides. For over 60 slope failures in soft and stiff clays, the values of c′ (mob) are in the range of 0–26 kPa as compared to 1–150 kPa for c′ of the clays involved in the failures.