Abstract:

For overlapped tunnels construction, excavating the second tunnel is an unloading process for first tunnel. So the lining of first tunnel will deform toward second tunnel; but this is temporary. Based on Laojie ~ Shaibulu shield tunnel of Shenzhen metro line 3, 3D FEM and centrifuge model test are used to study the longitudinal displacements of first tunnel induced by second tunnel construction directly above. The results indicate that the differential displacements of first tunnel induced by second tunnel construction mainly occurs in the range between 3.5D (3.5 times the diameter of the new tunnel) in front of and 3D behind the excavation face. According to this, the countermeasures have been taken into account, mainly including temporary ballast and temporary support. In the range of (0-1)D, (1-2)D, (2-3)D adding temporary ballast of 20 t/3 m, 20 t/6 m, 20 t/9 m respectively to first tunnel; and setting temporary support in first tunnel in the range between 4D in front of and 4D behind the excavation face will well reduce the differential displacements of lower tunnel induced by upper tunnel excavation.

Key words: overlap tunnels, shield tunneling, longitudinal effect, temporary support, countermeasure