Stability Analysis of the Longitudinal Slope Linear Shield Tunnel Excavation Face

In shield tunnel engineering, longitudinal slopes and other complex alignments are commonly encountered. Given the uneven stress distribution in the soil ahead of the tunnel face during excavation, studying and understanding the instability mechanisms of the excavation face is particularly crucial. In this study, a visualized transparent soil test was designed to investigate shield tunneling along a longitudinal slope. The displacement patterns of the soil in front of the excavation face were analyzed in detail. Furthermore, FLAC3D numerical simulations were employed to examine the variations in the ultimate support ratio and vertical stress of the soil under different slope conditions. The reliability of the results was also validated. The research findings reveal the stability characteristics of the excavation face and its influencing factors during shield tunneling at different slope angles. These findings provide a scientific basis for the design of shield tunnels with longitudinal slopes. The results of the study indicate that (1) the maximum destabilization width of the soil with slope i = 15% is increased by 18.2% and 36.8% compared to that with slope i = 0% and −15%, respectively; (2) the ultimate support force, as well as the horizontal and vertical displacements of the excavation face, increase significantly under an upward slope condition (i > 0) compared to those in a horizontal tunnel (i = 0) and a downward slope (i < 0); and (3) the longitudinal slope gradient i is negatively correlated with the inflection point depth, meaning that steeper slopes result in shallower loosening zone depths.