The Mechanism of Ground Subsidence Induced by EPB Tunneling in Sand and Cobble Stratum

Sand and cobble stratum in Chengdu Metro Line 1 presented challenging tunneling ground conditions with high water levels, and high strength large diameter cobbles that resulted in excessive wear of tunnel boring machine cutters. In general, the stratum is self-supporting for extended periods of time, making real-time monitoring for ground subsidence difficult to detect. In the process of tunneling, the traffic along roadways over the tunnel bore had to be restricted because of ground subsidence or multiple ground collapses, which have serious implications on the surrounding environment. From some ground collapse cases, the excavation parameters and real-time monitoring data were investigated to understand the ground subsidence mechanism. The research shows that: 1 Under the condition where the stratum has high water levels, non-uniform densities and higher permeability coefficients, the sand and cobble soil is in a plastic state. 2 Alone, the stratum cannot maintain earth pressure balance for EPB tunneling. 3 Additionally, once the stratum is disturbed by dynamic loads, the self-supporting nature of the ground degrades progressively. 4 Finally, the subsidence manifests at the surface as ground collapse. Our study further investigates the major factors of ground subsidence induced by EPB tunneling in such stratum. In order to prevent ground subsidence, some valid measures that were verified by previous projects were advised. The results are helpful for improving the understanding of the mechanism for ground subsidence, avoiding ground collapse and environmental impacts, and maintaining surface safety.