Abstract: A series of systematic tests was performed to study the monotonic behavior of interface between coarse-grained soil and structure with a new apparatus called TH-20t Cyclic Shear Apparatus for Soil-Structure Interface (TH-20t CSASSI). The behavior of the interface was recorded and analyzed through both macro- and micro-approaches and the mechanism of the interface was investigated. As a result, new behaviors of the interface between coarse-grained soil and structure were discovered, including: 1) the thickness of the interface that is five to six times the average grain size of the soil; 2) the obvious normal displacement due to shearing, which is affected by normal stress; 3) two shear deformation components due to sliding and constraint of the structural surface relative to the soil respectively, which forms the deformation of the interface; 4) the main factors affecting behavior of the interface including roughness of structural surface, behavior of soil and normal stress.