Modification of direct shear apparatus for soil-soil and soil-solid interface testing

The conventional direct shear apparatus has been widely used to analyze the shear behavior of the soil-soil and soil-solid interfaces. However, it has certain limitations, such as tilting of loading plate and rotation of upper shear box, which eventually lead to unfavorable shear responses and hinder the evaluation of accurate shear strength parameters. Therefore, in this study the direct shear apparatus is modified as follows: (1) application of constant vertical stress through a loading rod fixed to a circular loading plate for soil-soil tests and rectangular loading plate for interface testing; and (2) a provision of linear motion (LM) guide for smooth horizontal movement of shear box during shearing process and construction of upper and lower shear boxes for soil-solid interface testing. The modified direct shear apparatus is tested with soil-soil at different initial relative densities and interface testing using solid plate under three vertical stresses. The experimental results confirm that the vertical stress, which is imposed through the new loading system, remains constant during the shearing phase. Further analysis is conducted to establish the empirical relation of friction angle as a function of initial relative density.