Effects of piggyback twin tunnelling on a pile group: 3D centrifuge tests and numerical modelling

The effects of tunnel construction on existing single piles have been extensively investigated, but the influence of twin tunnel advancement on an existing pile group is seldom reported in the literature. In this study, a series of three-dimensional (3D) centrifuge model tests and numerical simulations using an advanced hypoplastic soil model were carried out to investigate the response of an existing 2 x 2 pile group to piggyback (i.e. vertically aligned) twin tunnelling at various depths. Soil parameters for 3D numerical back-analyses were obtained from stress-path triaxial tests. In each twin tunnelling simulation in a centrifuge, the first tunnel was advanced three-dimensionally by controlling volume loss in-flight near the mid-depth of the pile shaft, before the second tunnel was constructed either next to the toe of the pile group (test ST), below but to one side of the pile group (test SB) or directly beneath the pile toe (test SU). The piggyback twin tunnelling in test ST resulted in the largest transverse tilting (of 0.2%) but the smallest settlement of the pile group under a working load. This is because the second tunnelling caused significant non-uniform change in vertical effective stress underneath the four piles in the group. On the contrary, the tunnelling directly beneath the pile group (i.e. test SU) caused the smallest tilting but the largest settlement of the pile group (4.6% of pile diameter) and substantial mobilisation of shaft resistance. This is attributable to the most significant and uniform loss of toe resistance of each pile in the group resulting from stress relief from the second tunnelling. Two distinct load transfer mechanisms can be identified in the pile group, namely downward load transfer in test ST and upward load transfer in tests SB and SU.