Suction and mechanical behaviours of unsaturated intact loess from constant water content triaxial tests

A series of isotropic compression and triaxial shear tests under constant water content (CW) are performed on intact loess with various initial suctions to simulate the rapid destruction of soil mass in practical projects under undrained conditions. The variation characteristics of deformation, critical state, yield and suction of intact unsaturated loess are studied under the condition of constant water content. The results show that the isotropic compression and triaxial shear stress applied have different effects on the variation characteristics of deformation and suction of unsaturated loess. The critical state lines (CSLs) at different levels of initial suction on the deviator stress versus the net mean stress (q-p) plane are parallel, and a convergence of all the critical state points towards a single line much closer to the saturated CSL can be normalized whatever suctions on the deviator stress versus effective mean stress (q-p′) plane. The CSLs on the void ratio versus the net mean stress(e-lnp) and effective mean stress plane (e-lnp′) are parallel for different suctions, whose slopes are larger than those of saturated loess and smaller than those of isotropic compression curves after yielding. For different suctions, the relationships between the quotients e/e_s between unsaturated and saturated critical state void ratios at the same effective average stress against the values of the degree of saturated of the gas phase are unique. The net mean yield stress and deviator yield stress increase with the initial suction. The shape of the initial and subsequent yield curves on the deviator stress versus the mean net stress (q-p) plane are nearly elliptical and inclined about K₀ line. Respective mobilization of the yield curve either by suction or by stress hardening is isotropic.