Abstract: A laboratory testing program was carried out to investigate the mechanical behavior of a medium unsaturated expansive clay taken from Hubei.Statically compacted specimens were used in this testing program,which was performed using three newly developed double-cell triaxial systems for unsaturated soils.The stress paths adopted in the testing program included wetting under isotropic stress condition,isotropic compression and shearing under different values of constant suction.The test results showed that the recompacted expansive clay exhibited an elasto-plastic swelling upon wetting at a low confining pressure,indicating the existence of the Suction-Decrease(SD) yield locus.Within the suction range considered(0 to 200 kPa),both the yield stress and the post-yield stiffness increased with suction,suggesting a suction-hardening effect on the soil behavior.The slope of critical state hyperlines in(q,p) plane was independent on the value of suction within the suction range considered.The contribution of suction to shear strength for this expansive clay was more significant than that of a compacted kaolin.