Failure and Deformation of Sand in Torsional Shear

Abstract

The deformation and strength characteristics of sand was studied by means of the torsional shear tests on hollow cylindrical samples with a height of 20 cm, an outer radius of 5 cm and an inner radius of 3 cm. It was found that in order to obtain accurate results it is essential both to very precisely measure the axial load, the torque and the effective confining pressure σc′ and to correct measured stress values for the membrane forces. The methods for this purpose are described in detail. Each test was performed at a constant confining stress σc′ between 0.3 ~ 2.0 kgf/cm2 (29.4 ~ 196.2 kN/m2) and σ3′ at failure ranged between 0.08 ~ 0.75 kgf/cm2 (7.9 ~ 73.7 kN/m2). It was found by measuring local strain distributions throughout the sample height that the deformation is rather uniform when shear distortion γat is less than approximately 5%. Correspondingly the pre-peak stress ratio-strain relationship is rather independent of σc′ or σ3′. It was also found that with increasing σc′ a shear band or shear bands start to appear at a smaller γat and the degree of post-peak strain softening becomes more pronounced. The dependency of φ on σ3′ in the torsional shear tests for the range of σ3′ in this study was found small as well as in the triaxial and plane strain compression tests. A tendency was obtained that the value of is slightly influenced by the deformation pattern at failure. The pre-peak stress and strain relationship by the torsional shear test was found similar to that by the plane strain compression test under similar values of b =(σ2′− σ3′)/(σ1′− σ3′) and the angle of bedding plane δ to the σ1′-direction. The value of φ by the torsional shear test was found slightly larger than or similar to the value of φ by the plane strain compression test under similar values of b and δ depending on the stress level at which the value of φ is compared.