Abstract
Tunnels are often preferred for underground transportation systems in densely populated areas. It is almost inevitable to excavate a tunnel close to existing pile foundations in these areas.
Since many previous studies mainly focus on the responses of single piles subjected to tunnelling, the effects of the tunnelling on pile groups are not well understood. In this study, two three-dimensional centrifuge tests were carried out to investigate settlement and load transfer mechanism of a pile group subjected to tunnelling at two critical locations relative to the pile group, namely next to (Test T) and below the toe of the pile group (Test B).
Moreover, numerical back-analyses of the centrifuge tests are conducted by using a hypoplastic model, which takes small-strain stiffness into account. The tunnelling in test T 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 below the pile group toe (i.e. test B) caused the smallest tilting but the largest settlement of the pile group (4.3% 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 tunnelling. Two distinct load transfer mechanisms can be identified in the pile group, namely downward load transfer in test T and upward load transfer in test B.