3D finite element analysis of effects of twin stacked tunnels at different depths and with different construction sequence on a piled raft

Abstract

Underground transport infrastructure (tunnelling) plays a vital role to accommodate the rapid increase in urban population. Those tunnels are unavoidably constructed adjacent to high rising building supported by piled raft foundations which reduces differential settlements in the buildings. In this study, 3D numerical modelling using an advanced hypoplasticity soil model was carried out to investigate the responses of an existing (2 × 2) piled raft to twin stacked tunnelling at various depths. In each simulation, the first tunnel was constructed near the mid-depth of the pile shaft, the second tunnel was constructed either next to the toe of the piled raft (case ST), below but to one side of the piled raft (case SB) or directly beneath the pile toe (case SU). In addition, three more analyses were carried out to investigate the effects of change of construction sequence of twin stacked tunnels (i.e. cases TS, BS and US) on piled raft. The twin stacked tunnelling in case of SU caused the largest settlement but the smallest induced bending moment of the piled raft. On the contrary, the largest tilting of the pile raft took place in case SB. The first tunnel in each case caused the load transfer from the raft to the piles. On the contrary, the second tunnelling caused the increment in the load taken by the raft due to penetration of the raft into the ground. The different construction sequence of twin stacked tunnels had a substantial effect on settlement and tilting of the piled raft.