Experimental study on face stability of shield tunnel in water-rich inclined composite strata considering different inclination angles

Shield method has developed rapidly in the construction of underground or subsea tunnels. The stability of the tunnel excavation face is essential for the safe construction of tunnels, particularly in water-rich and inclined strata. The present study conducted shield model tests in water-rich inclined strata to analyze the surface settlement, ground earth pressure, areas of instability, and failure mechanism during the process of instability. The results indicate a decrease in settlement of the longitudinal monitoring points as the backward distance from the tunnel boundary line increases. Additionally, the settlement analysis of transverse monitoring sections reveals that the most significant settlement occurs directly above the tunnel. When equidistant from the tunnel excavation face, the surface settlement exhibits a significantly greater magnitude in front of it compared to behind it. The extent of influence is maximized when the inclination angle is negative, regardless of whether it pertains to the collapse in width or height on the surface. The application of DIC image analysis reveals that distinct failure mechanisms are observed for varying stratum inclination angles. The shear strain concentration becomes more pronounced with an increase in the backward distance, resulting in predominant distribution of shear bands in front of and above the tunnel. Compared to water-rich horizontal stratification or homogeneous soil layers, there are distinct variations in surface collapse magnitude, stratum disturbance range, and the instability mechanism of stratum instability in water-rich inclined strata. Therefore, when confronted with engineering projects involving inclined strata, it is imperative to consider the influence of geological factors.