COHESION INTERCEPT IN EFFECTIVE STRESS-STABILITY ANALYSIS

The cohesion intercept in the Coulomb shear-strength equation has a si gnificant 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 s tress relationship. Shear strength on a slip surface, at the instant o f a global instability, may range from residual strength to peak stren gth for stiff clays, and from large strain strength to peak strength f or soft clays. Peak strength controls local development of slip surfac e. but it may not be available during global failure. The critical sli p surface for an initial slide is determined using the peak shear stre ngth. However, a mobilized strength equal to or less than the peak str ength is used to define the factor of safety. Effective stress-stabili ty analysis of slope failures has resulted in a reduction factor to th e peak strength in order to obtain the mobilized strength for first ti me slides. For over 60 slope failures in soft and stiff clays. the val ues 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.