The dilatant behaviour of sand–pile interface subjected to loading and stress relief

Property and behaviour of sand–pile interface are crucial to shaft resistance of piles. Dilation or contraction of the interface soil induces change in normal stress, which in turn influences the shear stress mobilised at the interface. Although previous studies have demonstrated this mechanism by laboratory tests and numerical simulations, the interface responses are not analysed systematically in terms of soil state (i.e. density and stress level). The objective of this study is to understand and quantify any increase in normal stress of different pile–soil interfaces when they are subjected to loading and stress relief. Distinct element modelling was carried out. Input parameters and modelling procedure were verified by experimental data from laboratory element tests. Parametric simulations of shearbox tests were conducted under the constant normal stiffness, constant normal load and constant volume boundary conditions. Key parameters including initial normal stress ($$ \sigma_{{{\text{n}}0}}^{\prime } $$), initial void ratio (e0), normal stiffness constraining the interface and loading–unloading stress history were investigated. It is shown that mobilised stress ratio ($$ \tau /\sigma_{\text{n}}^{\prime } $$) and normal stress increment ($$ \Updelta \sigma_{\text{n}}^{\prime } $$) on a given interface are governed by $$ \sigma_{{{\text{n}}0}}^{\prime } $$ and e0. An increase in $$ \sigma_{{{\text{n}}0}}^{\prime } $$ from 100 to 400 kPa leads to a 30 % reduction in $$ \Updelta \sigma_{\text{n}}^{\prime } $$. An increase in e0 from 0.18 to 0.30 reduces $$ \Updelta \sigma_{\text{n}}^{\prime } $$ by more than 90 %, and therefore, shaft resistance is much lower for piles in loose sands. A unique relationship between $$ \Updelta \sigma_{\text{n}}^{\prime } $$ and normal stiffness is established for different soil states. It can be applied to assess the shaft resistance of piles in soils with different densities and subjected to loading and stress relief. Fairly good agreement is obtained between the calculated shaft resistance based on the proposed relationship and the measured results in centrifuge model tests.