Investigation Into the Effects of the Thickness of a Hollow-Cylinder Soil Specimen on the Stress Distributions in Triaxial Torsional Shear Testing

To investigate the effect of the thickness of a hollow cylinder on the stress distributions in a torsional triaxial shear test of clay, an equation for calculating the circumferential shear stress on a cross section of the cyclider is derived based on a linear elastic constitutive model and elastic mechanics. The "stress error" parameter and "average stress error" parameters are defined to assess the non-uniformity of stresses. The parameter ranges are discussed for specimens with various thicknesses. Torsional triaxial shear testing of specimens with different thicknesses is simulated in using a three-dimensional finite-element method (3D FEM) with Duncan-Chang's non-linear elastic model. The results show that the circumferential shear stress non-uniformity increases with increasing thickness. The maximum, minimum and average "stress error" values are analyzed for commonly used specimen dimensions. When the soil's stress-strain is linear elastic, the "average stress error" is 12.8 % for an outer radius = 50 mm and a thickness = 20 mm and 9.0 % using Duncan-Chang's model. If the "average stress error" is limited to < 10 %, the ratio of the inner radius over the outer radius is 0.68.