Centrifugal model test of group tunneling with small spacing beneath existing tunnels

The existing metro line 1 in Shenzhen is under-crossed by four shield tunnels of metro line 7 and line 9 in close proximity. Based on the physical modeling of centrifuge tests, the influences on the stress of surrounding soil from construction of multiple tunneling in close proximity are analyzed. The deformation mechanism and stress characteristics of the existing tunnels induced by multiple under-crossing construction are investigated. The study shows that: (1) The group tunneling with small spacing can cause the ‘group-hole effect’ in the non-crossing region. The expanding of disturbed zones is the main reason for the soil pressure increasing above the new tunnels. (2) The settlement of the existing tunnels increases with each excavation process and reaches the final settlement as a result of four under-crossing superposition. The maximum settlement is above the center-line of four tunnels with the settlement trough width of more than 60 m. (3) The four shield tunnels in close proximity cause a significant increase in the longitudinal bending moment of the existing tunnels, while the circumferential moment changes a little. Therefore, segment fracture may appear after under-crossing construction. The test results are compared with the simulated ones, and they exhibit a similar change rule.