Assessment of undrained non-linear stiffness of clays using rigidity index

Abstract

Due consideration of non-linear soil stiffness is one of the essential requisites in many geotechnical applications. A considerable amount of the geotechnical literature, as well as numerous recent studies, have been directed towards enhancing our ability to formulate mathematical relationships expressing the stiffness non-linearity of fine-grained soils and to determine the controlling parameters of such relationships. At present, the parameters of the non-linear clay stiffness formulations are commonly correlated to Atterberg's plasticity index (PI), even though this is obtained from testing remoulded samples, whereas the looked-for parameters are distinctly related to the undisturbed clay condition. In this paper, a new approach is proposed to establish the link between the non-linear stiffness parameters and the undrained rigidity index, as a more pertinent factor for the undisturbed compressibility of clays than the PI. The proposed approach was used to predict the undrained non-linear behaviour of several well-reported formations (London Clay, San Francisco Bay Mud, Skå-Edeby Clay and Hachirougata Clay). Good agreement between the predicted and the measured non-linear stiffnesses was found, implying that the presented methodology can be broadly applicable to different clay formations.