On the interface friction in direct shear test

Distinct element simulations for the direct shear box tests on a dense and a loose 2D sample of a mixture of binary diameter cylinders were performed. Special attention is focused on the friction between the internal surface of the shear box and the sample. In the conventional direct shear test, where the up/downward movement of the upper shear box is restrained and the lower shear box moves horizontally, the frictional force that acts on the sample at the internal surface of the upper shear box is downward for the dense sample and upward for the loose sample, causing the real shear strength to be larger for the dense sample and smaller for the loose sample than the one calculated from the externally applied normal force. Two possible improvements for the conventional direct shear test to reduce the frictional force of the shear box are introduced: one is free of the upper shear box vertically by adding the low friction Teflon rods and a platen between the upper shear box and the bearing ring that measures the shear force; the other is to pull the upper shear box with a flexible rope or wire. The distinct element method simulation and the experimental results show that both these two improvements can reduce the influences of the interface frictional force on the shear strengths.