STRAIN-SOFTENING AND SHEAR-BAND FORMATION OF SAND IN MULTIAXIAL TESTING

The strain softening of a granular soil under a sigma(1) theta sigma(2 ) theta sigma(3) theta condition and along a wide spectrum of paths wa s investigated experimentally, To ensure that the observed behaviour w as not an aberration due to boundary imperfections, a specially design ed multi-axial cell was used, Microprocessor control of both stress an d strain paths was incorporated into the testing programme, In particu lar, strain path testing where the strain increment ratio d epsilon(v) /d epsilon(1) 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 development of any non-homogeneity. Shear band formation is not necessarily a con sequence of boundary imperfections, but can occur as the inevitable re sponse of a sample to certain stress states and shear paths. The condi tions for shear band formation were established and three types of str ain softening identified.