Change of small strain quasi-elastic deformation properties during undrained cyclic torsional shear and triaxial tests of toyoura sand

Abstract

Undrained cyclic torsional shear and triaxial tests were performed on hollow cylindrical specimens of dense Toyoura sand, which were consolidated isotropically after preparation by air-pluviation and saturation. At several stress states, small strain quasi-elastic deformation properties were measured by applying very small amplitude cyclic torsional and vertical loads. During the liquefaction process, the observed degree of degradation of quasi-elastic shear modulus and vertical Young's modulus, was similar to that measured during isotropic consolidation. This suggests that the soil structure was damaged during the process of liquefaction. A larger extent of degradation was observed under triaxial extension or torsional shear than under triaxial compression condition. Such a change in the quasi-elastic deformation properties should be properly considered when analyzing the liquefaction process of sands based on elasto-plastic modeling. The behaviors observed during small amplitude cyclic vertical loading under undrained condition could be explained by considering inherent and stress state-induced anisotropy in the modeling of quasi-elastic deformation characteristics, and by correcting for the effects of membrane penetration. The latter correction was necessary, although the involved strain level was as small as 0.001%.