ON THE DETERMINATION OF STRESS STATE IN THE SIMPLE SHEAR APPARATUS

The simple shear apparatus is one of the few commonly available laboratory apparatus that permits the application of controlled rotations of the principal axes of stress and strain to soil samples. However, because of the boundary conditions in the apparatus the soil sample does not respond as a single element, and this should be reflected in the analysis of test results. In the Cambridge University simple shear apparatus, the sample is surrounded by an array of load cells (contact stress transducers) that measure the complete distribution of boundary stresses throughout a test. For simple shear test results to be presented in terms of useful stress parameters, a procedure for computing the stress state from the load cell measurements is required. Such a procedure is described, making use of the concept of an average stress tensor to determine a representative stress state in the central part of the sample, which is least influenced by the ends of the apparatus. Less complex and expensive apparatus exist that can only measure the average normal and shear stresses applied to the top and bottom horizontal boundaries of the sample. Patterns of soil response have been determined from tests on Leighton Buzzard sand in the more elaborately instrumented Cambridge apparatus, and a method is described for using these patterns to deduce the complete stress state in the less complex apparatus.