An apparatus has been developed to carry out two- and three-dimensional tests on interfaces between soils and structural materials. The soil container facilitates both the sliding displacement at the interface and the shear deformation in the soil mass. The apparatus has the capability of maintaining the normal stiffness constant during shearing. In constant normal stiffness tests, the normal stress is adjusted continuously by a computer-controlled system to keep the normal stiffness at a desired value. In addition to constant normal stiffness tests, conventional types of tests can be performed during which the average normal stress acting on the interface is kept constant. In all types of three-dimensional tests, the interface can be subjected to tangential loading in two orhtogonal directions simultaneously. Experiments were conducted to study the two- and three-dimensional behaviour of an interface between a dense sand and a rough steel surface under both constant normal stress and constant normal stiffness conditions. The influence of various stress paths on the stress–displacement relations and shear strength characteristics of the interface are investigated. The experimental results indicate that the coefficients of friction corresponding to the resultant peak and residual shear strengths are independent of stress paths. The shear stress – tangential displacement and volume change behaviour of the interface, however, are significantly influenced by stress paths.Key words:interface testing, simple shear, peak and residual shear strength, three-dimensional, constant normal stiffness, stress pat