Cyclic triaxial tests on clay subgrades for analytical pavement design

To introduce a performance specification, pavement foundations must be designed using analytical methods incorporating the laboratory measured parameters of resilient elastic modulus and resistance to permanent deformation of the subgrade and foundation materials. This paper presents results from a program of repeated load triaxial tests performed on a range of fine-grained subgrades prepared in a number of states to evaluate these parameters for various design conditions. The results highlight several difficulties in measuring small strains on "undisturbed" soils over a large strain range and in predicting and modeling long-term behavior. However, testing at higher strains has shown that the deviator stress at which the cumulative permanent deformation starts to increase significantly, termed the "threshold stress," approximates to 50% of the deviator stress at failure. In addition, the resilient modulus of the soils is shown to approach a low asymptotic value at higher deviator stress. Comparison between elastic and plastic behavior shows that the deviator stress at "threshold" coincides with the stiffness asymptote. Using these correlations a simplified mechanistic design method for pavement foundations is proposed.