Three-dimensional monotonic and cyclic behaviour of sand-steel interfaces: Testing and modellin

The main objective of this thesis is to investigate the effects of various stress and displacement paths on the behaviour of sand-steel interface. The study includes three parts: (i) the development of a new automated interface testing apparatus, (ii) a comprehensive experimental program, and (iii) elasto-plastic modelling of the sand-steel interface behaviour.;The interface testing apparatus is capable of applying monotonic and cyclic forces in three perpendicular directions, simultaneously. Tests can be displacement or load controlled. Two computer-controlled stepper motors generate the desired tangential load or displacement paths on the interface plane, including rotational stress or displacement paths (circular or elliptical). Either a direct shear box or a simple shear box can be used as the soil container. In the direction normal to the interface plane, three boundary conditions, i.e. constant normal stress, constant normal stiffness, and constant volume can be imposed by using a pneumatic actuator which is operated by a computer-controlled system.;The results of more than 180 tests are reported for an interface between a Silica sand and a steel plate. Experiments include two-dimensional (2-D) monotonic and cyclic tests under constant normal stress conditions, to verify the performance of the new apparatus. A series of three-dimensional (3-D) monotonic and cyclic experiments are presented to show the coupling effects between two perpendicular shear stresses on the interface plane. A series of 2-D monotonic and cyclic experiments under constant normal stiffness condition are performed to study the variations in shear stress, normal stress, and volume change response under boundary conditions rather than the conventional constant normal stress.;The effects of various parameters such as surface roughness of steel, initial relative density of sand, magnitude of initial normal stress, rate of shearing, and type of sand are investigated. The 3-D circular and elliptical paths show that rotational paths affect the variations of the shear stress and normal displacement as well as peak and residual strengths of the interface. It is demonstrated that the shear stress, normal stress, and normal displacement variations of the interface are affected significantly if a constant normal stiffness is specified as the boundary condition. The phenomenon of the degradation of the shear stress in two-way displacement controlled cyclic tests under the constant normal stiffness condition and its relation to degradation of shaft resistance of axially loaded piles are investigated.;An existing elasto-plastic constitutive model is used to make predictions for the interface behaviour. Comparisons show that the predictions for the monotonic 2-D and 3-D tests under constant normal stress and constant normal stiffness conditions agree well with their corresponding experimental results.;The results of this study may be used in the analyses of stability and load-deformation response of soil-structure systems such as piles, sheet piles, lined tunnels, offshore gravity structures, and retaining walls.