Damage Evolution of the Surrounding Ground and Cross-Sectional Shape Optimization for Shallow-Buried Mined Subway Tunnels Under Different Lateral Pressure Conditions

In dense urban environments, excavation of shallow-buried subway tunnels can generate a pronounced excavation-damaged zone (excavation-induced plastic zone), which reduces the mechanical integrity and long-term stability of the surrounding ground. This study investigates the single-track sequential excavation method tunnel between Hongmiao Station and Ciyunsiqiao Station on Beijing Rail Transit Line 22 using theoretical analysis, FLAC3D simulation, and field monitoring. The deformation response, stress redistribution, and plastic zone evolution are evaluated throughout the excavation sequence for lateral pressure coefficients of 0.3, 0.5, 0.8, and 1.0. The influence of matching the tunnel cross-sectional shape to the initial stress condition is also examined. The simulated settlement agrees with the field measurements, with deviations within 10%. As the lateral pressure coefficient increases, the dominant deformation mode changes from vertical convergence, characterized by crown settlement and invert heave, to horizontal squeezing concentrated near the haunches and wall feet. Although the far-field plastic zone contracts toward the opening, local convergence and circumferential shear demand increase. Analysis of an idealized elliptical opening shows that the circumferential stress is most uniform when the height-to-span ratio b/a approaches the lateral pressure coefficient K0. For practical horseshoe sections, a moderate reduction in b/a and smoother curvature transitions can reduce local stress concentration while preserving crown arching action. The results provide a preliminary mechanical basis for cross-sectional shape selection under different lateral pressure conditions and offer a reference for identifying locations where differentiated support measures may be considered.