The stability of strata in shield tunnel construction is not only closely related to the excavation face but also a crucial factor in the synchronous grouting process at the shield tail. On February 7, 2018, a serious water seepage accident occurred at the shield tail during the construction of Line 2 of the Foshan Metro, resulting in 11 deaths and 8 injuries, with severe social consequences. This incident sounded an alarm for engineers: the construction quality of synchronous grouting in shield tunnels is directly related to the overall construction and operational safety of the entire tunnel. However, due to the complexity of geological conditions and the unpredictability of synchronous grouting, the diffusion and filling process of grout in the shield tail gap is still not clear. Current related studies often rely on analytical derivations based on fluid mechanics theories, and experimental research primarily focuses on soft soils. There is relatively limited research on complex coarse-grained soil layers such as sandy gravel.
Our research team approached the issue from practical engineering perspectives and conducted experimental studies on synchronous grouting in sandy gravel strata. We revealed that the fundamental reason for inadequate filling of the shield tail gap in this stratum was the collapse of upper soil obstructing the diffusion path of the grout. Grouting in the upper part of the shield tail mainly follows a "permeation-diffusion" process, while in the lower part, it primarily follows a "diffusion-filling" process. Addressing the asymmetric grouting conditions often encountered in practical engineering due to clogging in grouting pipelines, we innovatively proposed simulating the pressure of faulty grouting holes using reduction coefficients. These reduction coefficients were calibrated based on ground loss rates. Building on this, we elucidated the surface settlement patterns and development laws under asymmetric grouting conditions and proposed a functional expression linking asymmetric grouting reduction coefficients and surface settlement trough shifts.
Regarding synchronous grouting pressure distribution under different burial depth conditions, our team, in collaboration with Shanghai Tunnel Engineering Co., Ltd., conducted reduced-scale model tests using a ground-access shield tunnel as a basis. We explored synchronous grouting pressure distribution patterns for typical soft clay conditions under negative cover and shallow cover scenarios, recommending a grouting rate range of 120%-140%. It was also concluded that using bottom two-hole grouting is feasible for negative cover conditions.
Students: Mr Liufeng Pan (2018 intake, 2021 graduated), Mr Delin Zhu (2021 intake), Mr Wei Chen (2023 intake)
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