Clay subgrades can be weak and will settle under traffic loading. Investigation of seasonal variations in subgrade resilient modulus were undertaken at three sites in Victoria and a further two sites in Northern Queensland as part of a large scale study into the performance of rail track. The findings relating to the influence of stress level and soil suction on the resilient modulus of natural soil have relevance to road as well as rail transport. This paper reviews the current state of knowledge concerning the influence of soil suction on soil stiffness and the prediction of resilient modulus. Test data are reviewed and an empirical predictive model for the asymptotic resilient modulus of natural soils is established for moduli up to 300 megapascals. (a) For the covering entry of this conference, please see ITRD abstract no. E217099.
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