Collapse behavior and mechanical response of the cobble stratum during the shield driving

Abstract

The cobble stratum is prone to collapse during the shield driving. It is important to understand the collapse behavior of cobble stratum for construction safety of the tunnel. Taking Tsinghuayuan Tunnel in Beijing as the background, a test of shield tunnelling in the cobble stratum is performed using the model shield with the diameter of 280 mm. A total of 5 collapse pits occur on the ground surface during the test process. The settlements at the surface and subsurface are measured to analyze the collapse process of the cobble stratum, as well as the earth pressure in the region above the tunnel crown. The quadratic partial derivative of the settlement and earth pressure are used to judge the collapse moment of the soil. The test results show that the collapse of the cobble stratum develops with a uniform speed from the bottom to up with the shield advancing, and that the collapse region extends to the surface after the cutter head passes through the center of the collapse pit 0.5D ∼ 1.0D. Meanwhile, the final earth pressure is much smaller than its initial value only in the region above the tunnel crown, indicating the collapse region concentrating in the position directly above the crown in the cross-section. In addition, the shape of surface settlement trough changes significantly with the collapse development. Development curves of two characterization physical quantities of the surface settlement trough, i.e., the maximum settlement and width coefficient, are respectively close to the “S” shape and the inverse “S” shape.