The stability of the excavation face is of paramount importance for the safe construction of large-diameter shield tunnels. There are two main forms of overall instability: cave-in failure and blow-out failure. These instability modes correspond to two different ultimate support pressures. Determining the ultimate support pressure during the failure process is a major challenge in the safety control of shield tunnel construction. Previous researchers have conducted extensive studies on this issue, but most of them focused on analytical solutions and model experiments, without considering complex geological conditions and actual construction scenarios. Additionally, existing studies have not proposed methods to establish the correlation between ultimate support pressure and ground loss ratio for different soil parameters. This limitation has hindered engineers from applying existing analytical solutions to set excavation face support pressures for practical engineering projects.
Our group, starting from a microscopic scale, has revealed that the collapse mechanism of non-cohesive layers in the excavation face occurs due to the shallow burial depth of the tunnel. This prevents the formation of the "bearing arch effect" in the ground layers, leading to overall failure. For sandy gravel layers, the critical burial depth ratio for arch effect formation is C/D=2.0. Building upon the foundation of the "continuous velocity field" analytical method, we have innovatively proposed a calculation framework for "ultimate support pressure-ground loss ratio" based on nonlinear finite element analysis. We studied the key parameter for instability determination of the excavation face, the "critical stability ratio," considering high permeability pressure. This research has addressed the previous limitation where this parameter could only be obtained through model experiments. Based on this, we have developed corresponding design tables. Engineers can use these tables to judge the critical stability ratio of the excavation face based on the shear strength index of the actual engineering soil and assess the stability of the excavation face according to the real-time ground loss ratio.
Addressing the widespread presence of late Quaternary cohesive soils in China, our group conducted a finite element parameter sensitivity analysis based on field monitoring data. We studied the stability of shield tunnel excavation faces under different burial depth ratios. Our findings indicate that the critical burial depth ratio for excavation face collapse in this specific geological condition without support is C/D=0.55. This result is significantly different from the critical burial depth ratio under Quaternary cohesive soil and non-cohesive soil conditions. This research holds crucial significance for the stability control of shield tunnel construction excavation faces in relevant areas.
Students: Mr Liufeng Pan (2018 intake, 2021 graduated), Mr Delin Zhu (2021 intake)