Threshold stress and asymptotic stiffness of UK clays in the repeated load triaxial test.

Recent research at Loughborough University has produced a draft performance specification for pavement foundations. To assess the performance parameters of stiffness and resistance to permanent deformation of the subgrade, an extensive series of repeated load triaxial tests has been performed on a range of fine grained subgrades. Assessment of the data has shown that the deviator stress at which the cumulative permanent deformation starts to increase significantly (i.e. the threshold stress) is equal to half that at failure (and occurs at approximately 1% permanent strain). The resilient stiffness of the materials tends to low asymptotic values at higher deviator stress. Comparison between the elastic and plastic behaviour has shown that the deviator stress at ‘threshold’ is coincident with the stiffness asymptote. This paper presents a suggested simplified mechanistic design approach for pavement foundations based on these findings.