A comparison of shear modulus of unsaturated sand in triaxial and simple shear tests under different strains and suctions

The shear modulus (G) in sands changes when subjected to variations in the degree of saturation. The matric suction in unsaturated sands increase the effective stress and consequently the shear modulus. The trend of G/ Gsat variations is complicated as it is dependent on a number of parameters including stress state and therefore, requires more in-depth evaluations. Several isotropic cyclic triaxial as well as simple shear tests (K0 condition) are carried out under various degrees of saturation and strain amplitudes on a fine angular to sub-angular silica sand. The results show greater values of G from triaxial tests as compared to simple shear tests under similar vertical stress and void ratio, mainly due to a higher mean effective stress at isotropic triaxial condition. Besides, the two different stress states of isotropic and K0 conditions resulted in different curves of G/Gsat at the examined range of suction. Based on the measured values, G/Gsat variations versus matric suction cannot be well predicted solely by contribution of suction in the effective stress, but the variations of the degree of saturation with suction should be taken into account.