Abstract: Maintaining the stability of the excavation face is key for ensuring the safety of underwater shield tunnel construction. However, the majority of current studies on the stability of excavation face focus on the homogeneous strata, with limited research conducted on the upper loose and lower dense strata. Active instability tests are conducted in this study, in concert with the digital image correlation (DIC) technique, to investigate the effects of different water pressure ratios in upper loose and lower dense water-rich strata. The accuracy of these model tests is verified using numerical simulations. The results indicate that as water pressure ratio decreases, there is an increase in both the peak displacement of surface settlement and the seepage path range of water ahead of the excavation face expands. In contrast, decreasing water pressure ratio will break the limit equilibrium state of the strata faster, cause the earth pressure on the cutterhead to change more rapidly, and increase the instability range of the strata.

Key words: Shield tunnel; Upper loose and lower dense strata; Excavation face stability; Water pressure ratio

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