Role of normal boundary condition in interface shear test for the determination of skin friction along pile shaft

This paper presents numerical results from a two-dimensional discrete element method (DEM) simulation study on the influence of lateral boundary condition on shaft resistance of a pile driven into a crushable sand. The study was conducted by comparing simulation results from the pile penetration test and the interface shear test employing parallel-bonded agglomerates for modeling of the particle breakage. The interface shear test was performed under three different types of normal boundary conditions; namely, constant normal load (CNL), constant normal stiffness (CNS), and constant volume (CV) boundary conditions. For the pile penetration test, a series of sampling windows were identified on the initial ground configuration to monitor the stress–strain, volume change, and particle breakage behavior of particle groups located within the sampling windows. A detailed investigation was then undertaken by comparing the behavior of particle groups with that from the interface shear test to find out which type of normal boundary condition best describes the lateral boundary condition in the pile penetration test. It has been found that the behavior of a particle group reaches the peak state below the pile tip and the critical state after it reaches the pile shaft. The influence of normal boundary condition on the stress ratio at the critical state is not obvious. The conventional interface shear test (i.e., CNL) can provide valuable information on the determination of skin friction along the pile shaft.