Measurement and use of residual shear strength of cohesive soils

Extended testing has confirmed that residual failure envelopes are curved and that the curvature is most pronounced below about 200kPa effective normal stress and in soils with a high clay fraction. Two new terms are introduced, Complete Failure Envelope and Lowest Residual Strength. The importance of acquiring data relevant to the correct overburden pressures is emphasized. An appropriate figure should be read from the residual friction coefficient or from a straight line approximation over a selected normal stress range, preferably from the Complete Failure Envelope. Correlation charts relating residual strength to clay fraction and/or plasticity index are only meaningful if the points have been established at the same effective normal stress, and also if this stress level is relevant to the problem being considered.