Strength mobilization in clays and silts

Abstract

A large database of 115 triaxial, direct simple shear, and cyclic tests on 19 clays and silts is presented and ana- lysed to develop an empirical framework for the prediction of the mobilization of the undrained shear strength, cu, of natural clays tested from an initially isotropic state of stress. The strain at half the peak undrained strength (gM=2) is used to normal- ize the shear strain data between mobilized strengths of 0.2cu and 0.8cu. A power law with an exponent of 0.6 is found to describe all the normalized data within a strain factor of 1.75 when a representative sample provides a value for gM=2. Multi-linear regression analysis shows that gM=2 is a function of cu, plasticity index Ip, and initial mean effective stress p 0 0. Of the 97 stress-strain curves for which cu, Ip, and p 0 were available, the observed values of gM=2 fell within a factor of three of the regression; this additional uncertainty should be acknowledged if a designer wished to limit immediate founda- tion settlements on the basis of an undrained strength profile and the plasticity index of the clay. The influence of stress his- tory is also discussed. The application of these stress-strain relations to serviceability design calculations is portrayed through a worked example. The implications for geotechnical decision-making and codes of practice are considered.