CHANGE OF SMALL STRAIN QUASI-ELASTIC DEFORMATION PROPERTIES DURING UNDRAINED CYCLIC TORSIONAL SHEAR AND TRIAXIAL TESTS OF TOYOURA SAND

JUNICHI KOSEKI, SADAHIRO KAWAKAMI, HIROSHI NAGAYAMA and TAKESHI SATO

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% .

Key words: liquefaction, sand, shear modulus, torsional shear, triaxial test, Young's modulus (IGC: D6/D7)