Direct Shear Box And Ring Shear Test Comparison: Why Internal Angle Of Friction Vary

Abstract

The direct shear box and the ring shear test as conventionally used for measuring the strength parameters of soil for use in classical stability analyses have the major disadvantage that the stress conditions in the specimen during the test are not known. The ring shear test was specially instrumented with ‘artificial shear plane’ to investigate the shear deformations and stresses acting on the sample. The ‘artificial shear planes’ were made from plane papers with ink-marks and introduced into the specimen. New data is presented from internal measurements in terms of photographs just before failure in the ring shear. An interpretation is given for the internal angle of friction and its relationship with the strain propagation in both the methods, and a comparison made. Results are presented from both the direct shear box and the ring shear test, and these are compared. Both the tests yield varying internal angle of friction when carried out on the same specimen and conditions. Results reveal that the internal angle of friction obtained from a direct shear test is lower than that obtained from the ring shear test. It is established that the ring shear test has an inherent tendency to squeeze out material from the cell due to high stress accumulations at the outer edges. The inner edge is always understressed. The direct shear box has both of its sides equally stressed and this sharing of strains and stresses enable it to register lower bound values than those from the ring shear box. The structures which appear in the direct shear box sample before and after failure indicate that the central portion of the specimen is in simple shear. A close examination of the failure mechanism in the direct shear box shows that kinking is the dominant mode of deformation, which is different from that in the ring shear. In the ring shear, the sample is very small, and there is non-equal distribution of stresses. When this non-equal distribution of stresses is accompanied by a tendency to squeeze out of material, kinking does not dominate.