Strength mobilization in clays and silts

A large database of 115 triaxial, direct simple shear, and cyclic tests on 19 clays and silts is presented and analysed to develop an empirical framework for the prediction of the mobilization of the undrained shear strength, c(u), of natural clays tested from an initially isotropic state of stress. The strain at half the peak undrained strength (gamma(M=2)) is used to normalize the shear strain data between mobilized strengths of 0.2c(u) and 0.8c(u). 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 gamma(M=2). Multi-linear regression analysis shows that gamma(M=2) is a function of c(u), plasticity index I(p), and initial mean effective stress p'(0). Of the 97 stress-strain curves for which c(u), I(p), and p'(0) were available, the observed values of gamma(M=2) fell within a factor of three of the regression; this additional uncertainty should be acknowledged if a designer wished to limit immediate foundation settlements on the basis of an undrained strength profile and the plasticity index of the clay. The influence of stress history 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.