Undrained shear strength of liquefied sands for stability analysis

The postliquefaction shear strength of sands, called the undrained critical strength or s_u(critical), is evaluated in terms of the critical strength ratio, [Formula Presents]. This allows postliquefaction stability analyses to incorporate the variation of s_u(critical) with effective vertical stress instead of using a single value as proposed elsewhere. Comparison of back-calculated critical strength ratios and the cyclic stress ratios triggering liquefaction suggests that drainage occurs in most cases during the postliquefaction flow of liquefied sands. To evaluate the stability of an existing slope that is predicted to liquefy during a dynamic event, the original slope configuration and the constant volume s_u(critical) must be used instead of the back-calculated partially drained s_u(critical) values. The proposed procedure for estimating the constant volume s_u(critical), using the results of field and/or laboratory tests, shows that the critical strength ratio is approximately one-half the yield strength ratio at the triggering of liquefaction for an earthquake magnitude of 7.5. It has been shown that the yield strength ratio at the triggering of liquefaction for a magnitude of 7.5 can be estimated by 0.011 times the equivalent clean sand blow count. Therefore, the critical strength ratio is 0.0055 times the equivalent clean sand blow count.