Strain softening and shear band formation of sand in multi-axial testing

The strain softening of a granular soil under a σ₁′ ≠ σ₂′ ≠ σ₃′ condition and along a wide spectrum of paths was investigated experimentally. To ensure that the observed behaviour was not an aberration due to boundary imperfections, a specially designed multi-axial cell was used. Microprocessor control of both stress and strain paths was incorporated into the testing programme. In particular, strain path testing where the strain increment ratio dε_r/dε₁ was controlled to a specified value was used extensively. A photographic technique was used to detect the initiation of shear band. Strain softening was found to be path-dependent. It can take the form of shear band formation, or it can occur without the devleopment of any non-homogeneity. Shear band formation is not neccessarily a consequence of boundary imperfections, but can occur as the inevitable response of a sample to certain stress states and shear paths. The conditions for shear band formation were established and three types of strain softening identified.