Analysis for face stability of the underwater shield tunnel traversing longitudinally heterogeneous strata

Maintaining tunnel face stability during underwater shield tunneling through longitudinally heterogeneous strata remains a critical challenge, with limited theoretical research addressing this issue. This study presents an improved limit equilibrium model to assess tunnel face stability, specifically considering the failure zones on both sides of the interface. The model's validity is demonstrated through comparative verification with prior studies and numerical simulations, and a comprehensive parametric analysis reveals critical insights into tunnel face stability. The findings reveal the limit support pressure (LSP) increases when the shield machine approaches a low-strength stratum from a high-strength one and decreases during the reverse process. Neither water depth nor burial depth do not affect the evolution of the LSP but contribute to its increase as these parameters increase. The support ratio (K) rises with water depth increases while decreases with deeper burial depth. Furthermore, the tunnel face-interface distance at which LSP initiates modification (L_m) demonstrates an inverse correlation with stratum strength, while being positively influenced by burial depth and water depth, with stratum strength dominating over the latter two parameters.