Effects of stress path on the flow rule of sand in triaxial compression

To investigate the viscous effects on the flow characteristics of sand, several creep tests were performed on Toyoura sand at a selected stress state during otherwise monotonic anisotropic triaxial compression at a constant stress ratio (i.e., R= σ v ’/σ h ’= 3.8) and monotonic shearing along several stress paths with positive rates of stress ratio. The direction of irreversible strain increment vector was noticeably different between anisotropic compression and shearing. When the creep deformation started following anisotropic compression, the direction of irreversible strain increment started rotating towards the one during monotonic shear loading. Such a noticeable rotation as above did not take place as a creep test commenced following monotonic shear loading. The test results suggest that the so-call plastic potential concept, which gives a unique direction of plastic strain increment vector that is independent of stress path and strain rate, is not relevant to describe the actual flow characteristics of sand. It is argued that such behaviour as observed in the present study is a result of an complicated interaction between two types of yielding (shear and compression), which may be subjected to different viscous effects.