Centrifuge modelling of tunnel-soil-pile interaction considering the presence of rigid caps

: Previous research on tunnelling beneath piles has assumed that caps (or other elements such as grade beams) above the pile head are elevated from the ground surface. This neglects the possible mobilisation of contact stresses beneath the foundation elements at the surface, which may occur as a result of pile settlements larger than ground surface movements caused by tunnelling. This paper presents a centrifuge study that compares the response of capped piles (where the cap is relatively stiff or rigid and is in contact with the soil) and simple piles (no cap) to tunnelling in dry silica sand. The paper includes results from 5 centrifuge tests, including both pile loading and tunnelling beneath piles. To consider a critical scenario, both simple and capped piles, subjected to a constant external load during tunnelling, are located directly above the tunnel. For strain measurement and estimation of internal forces, model piles are instrumented with fibre Bragg gratings (FBGs) along their shafts, while the external vertical and pile head settlement are also measured. The responses of simple and capped piles are compared by considering tunnelling-induced pile settlements and axial forces. Results show that simple pile settlements exceeded surface movements up to a point of pile failure (due to the loss of pile base capacity), whereas the cap restrained the pile settlement and satisfied equilibrium by mobilising the soil surface contact stresses (transferring the load from the pile to the cap). The paper highlights the role and potential benefits of considering the presence of pile caps in tunnel-pile interaction analyses