Effects of principal stress direction and intermediate principal stress on undrained shear behavior of sand.

ABSTRACT Undrained monotonic loading triaxial compression and extension tests were conducted on Toyoura sand. It was found that the shear behavior was more contractive and softer in triaxial extension than in triaxial compression. This difference suggests that the stress conditions, such as the direction of the principal stress and the magnitude of the intermediate principal stress have some effects on the undrained behavior of sand. To clarify these effects, a series of tests was performed by means of an automated hollow cylindrical torsional shear apparatus. Dry-deposited Toyoura sand was used in these tests. The angle of the maximum principal stress from the vertical to the bedding plane, α, and the intermediate principal stress coefficient, b, were fixed in each test. The intermediate principal stress was fixed to horizontal. For any density, tests with a larger α-value, namely, a larger inclination of σ1 from the vertical, and a larger intermediate principal stress coefficient b were shown to generate greater excess pore water pressure. In addition, the undrained simple shear behavior of sand under initial isotropic and anisotropic stress conditions was studied. It was made clear that triaxial compression (α = 0°, b = 0) gives the highest resistance with lowest contrac- tancy, while triaxial extension (α = 90°, b = 1) gives the opposite extreme in the assessment of flow failure. Simple shear mode of deformation was shown to exhibit an intermediate stress-strain behavior between triaxial compression and extension, which is closer to most of the field conditions.