Principal Stress Rotation: A Missing Parameter

Changes in the directions of principal stresses occur in all ground work associated with engineering works, as well as earthquakes; an important topical example is the cyclic loading from waves on the foundation soil of offshore structures. A review of the shortcomings of existing laboratory shear apparatus is followed by a description of a new plane strain device, the Directional Shear Cell, which avoids strain constraints while enabling chosen rotations of principal stress directions to be imposed on a uniformly stressed cubical sample. These rotations may be single jumps of any magnitude from 0? to 90? or continuous cyclic variations in direction up to 75—. Data from tests on sand in the Directional Shear Cell show the severe effects of some principal stress rotations on stress-strain relationships. The cyclic tests with large continuous direction change showed an unrelenting increase in cumulative strain at low mobilised shear strength.