Three-dimensional centrifuge modelling of pile group responses to side-by-side twin tunnelling

• Effects of buried depths of twin tunnelling on an existing pile group were investigated.
• Tunnelling below the pile toe results in the maximum apparent loss of pile capacity (i.e., 40%).
• Tunnelling near pile toe causes the largest titling (0.2%), reaching the limit given by EC7.
• Tunnelling near pile toe leads to the maximum bending moment, exceeding its bending capacity.

Development of underground transportation systems inevitably involves tunnels, which may have to be constructed in close proximity to existing piles and pile groups in densely built areas. Since many previous studies mainly focus on the responses of single piles subjected to single tunnelling, the effects of twin tunnelling on pile groups are not well investigated and understood. In this study, a series of three-dimensional centrifuge tests were carried out to investigate the response of a pile group under a working load subjected to the advancement of twin tunnels in dry sand. Side-by-side twin tunnels (excavated one after the other on both sides of the pile group) were simulated in-flight by controlling 1% volume loss at three critical locations relative to the pile group, namely near the mid-depth of pile shaft (Test SS), next to (Test TT), and below the toe of the pile group (Test BB). In addition, an in-flight pile load test was carried out to determine the pile group capacity. Among the three tunnelling tests, twin tunnelling below the pile toe (Test BB) caused the largest settlement of the pile group (2.4% of pile diameter), resulting in an apparent loss of capacity of 40%. On the other hand, the largest induced transverse tilting of the pile cap of 0.2% reached the limit suggested by Eurocode 7 and the most significant bending moment exceeded the ultimate capacity by 6% were measured near the pile head due to the advancement of the first tunnel near the pile toe. In spite of the severely bending moments induced by tunnelling, no more than 15% increase in axial force was observed in piles in all the tests.