Numerical modeling of the effect of cross-sectional shape on the interaction between dip-slip faults and underground tunnels

This study investigates the interaction between normal and reverse dip-slip faults and tunnels with circular, square, and horseshoe-shaped cross-sections using validated two-dimensional finite element models, with the tunnel axis assumed to be parallel to the fault strike. The effects of cross-sectional geometry, equivalent diameter (4 and 6 m), burial depth (7 and 10 m), and tunnel position relative to the free-field rupture path were evaluated under a fault dip angle of 60° and a fault throw of 4 m. The results showed that tunnel geometry and position substantially influenced rupture propagation, ground surface displacement, lining stress, and soil–lining separation. Under the investigated faulting conditions, reverse faulting produced a more critical structural response than normal faulting because of its compression-dominated loading regime; at a burial depth of 10 m, peak lining stresses under reverse faulting were approximately 3–7 times those under normal faulting. Circular tunnels exhibited the most stable response owing to their continuous curvature and more uniform load redistribution, whereas square tunnels were the most vulnerable because of stress concentrations at their corners and the lower flexural rigidity of their flat walls. Larger tunnel diameters and greater burial depths generally increased lining stresses and intensified rupture–tunnel interaction. Their influence on rigid-body rotation was mechanism-dependent, increasing under normal faulting but decreasing under reverse faulting. Reverse faulting also caused soil–lining separation in all examined configurations and relative tunnel positions. The findings highlight the importance of cross-sectional geometry in fault-resistant tunnel design and support the use of circular sections, localized reinforcement, ground improvement, and ductile lining systems in active fault zones.