Abstract: Being different from clay, fly ash is a kind of granular material. The stress ratio-strain relation of fly ash is related to both mean stress and void ratio. A number of consolidated-drained triaxial tests were carried out on a fly ash with different initial void ratios under different confining pressures. Relevant stress-strain relations have been obtained. Simple modifications have been made to the elastoplastic model proposed by Yao, et al., which considers the mean stress and initial void ratio. The mechanical behavior of fly ash specimens with different initial void ratios from drained triaxial tests are consistently predicted by the modified model. The application of the state parameter and hardening/softening parameters in the critical state model can predict dilatancy, hardening and
softening characteristics of the fly ash.

Key words: critical state line, isotropic compression, strain softening, triaxial test,  dilatancy, fly ash