Abstract: A new apparatus called TH-20t Cyclic Shear Apparatus for Soil structure Interface (TH-20t CSASSI) is developed to investigate the monotonic and cyclic mechanical characteristics of interface between coarse grained soil and structure. The apparatus is designed to run under three normal conditions including constant stress, constant displacement and constant stiffness so that the tangential and normal stress strain relationships and their coupling effects can be examined. The deformation and stress of the interface can be measured automatically and accurately on macro and micro scales. The validity of the apparatus is verified by elementary tests. The test results show that the dilatancy of the interface is remarkable and behaves anisotropically, which imposes a significant effect on stress strain relationships of soil structure interface.