A Cyclic Undrained Simple Shear Testing Method for Soils
An undrained simple shear testing method for soils using a torsional shear apparatus is described, in which a hollow cylindrical specimen is sheared under plane strain conditions without deforming and rotating all horizontal planes. In this testing method, the simple shear deformation mode is achieved only by several mechanical means without using an automated pressure control system. Typical results of cyclic undrained simple shear tests using uniform cyclic shear stresses on one-dimensionally consolidated saturated sand are presented. A similar undrained testing method with the same inner and outer lateral confining pressures, in which the simple shear condition is satisfied only in an approximated way, is also described. The test results show that the deviation of the stress-strain behavior in the approximate testing method from that under the exact simple shear condition increases as the shear strain increases. However, it was also found that for the purpose of evaluating cyclic undrained simple shear strength values defined by the amplitude of cyclic shear strain, the use of this approximate testing method is justified.
Some typical stress-strain relationships are also presented in detail. It is shown that despite large changes in the effective axial stress during cyclic undrained loading, the relationships between the ratio of the shear stress to the effective axial stress and the shear strain are of a smooth hyperbolic type. Further, it is shown that in a cyclic undrained simple shear test, the angles of the directions of the major principal strain increments rotate discontinuously between ± 45° relative to the axial stress direction. Consequently, the angles of the directions of the principal stress also change rather discontinuously, and at relatively large strains, it becomes almost ± 45° relative to the axial stress direction.