Underground tunnels subjected to asymmetric load or ground conditions are susceptible to experiencing uneven longitudinal bending, shearing, and torsional deformations, which further induce cross sectional flattening and warping. The intrinsic damages caused by multiple deformation modes are critical for tunnel health and safety but have long been neglected in practice. In the paper, a three-dimensional analytical model for soil-tunnel interactions was proposed with multiple-mode deformations incorporated, where the tunnel is assumed as a thin-walled pipe resting on an elastic foundation with five deformation modes: bending, shearing, torsion, warping, and flattening. Besides, a three-dimensional variable soil spring model was adopted, accounting for the strata discontinuities in longitudinal and transverse directions. A finite element solution for the proposed model was derived under arbitrary external loads using the principle of minimum potential energy. The validity of the proposed model was substantiated through three case studies. Based on the model, the coupling relationship of tunnel structure in transverse and longitudinal directions was revealed. Furthermore, parametric analysis was conducted to reveal the impact of tunnel width-to-thickness ratio, soil resistance coefficient, and composite strata on tunnel behaviors. These results significantly contribute to a deeper understanding of the intricate behaviors of tunnels, offering potential advancements for improved tunnel design methodologies.

Tunnels crossing active faults are susceptible to severe damage. This study establishes an improved semianalytical methodology for analyzing the mechanical responses of tunnels subjected to active fault dislocation. The methodology incorporates nonlinear axial, transverse, and vertical soil-tunnel interactions, shear effects, and geometric nonlinearity within its governing equations. Compared with existing methods, the proposed method has a significantly extended application range with improved accuracy, and the solution procedure demonstrates exceptional computational efficiency, with each case typically being solved in less than 1 s. The proposed methodology demonstrates outstanding qualitative and quantitative agreement with 3D FEM results across various fault types, as well as with the results obtained from the model test. Additionally, neglecting shear effects results in approximately 1.11 to 1.20 times higher bending moments and 1.13 to 1.19 times higher shear forces of the tunnel. Finally, a parametric analysis was conducted based on the proposed methodology to investigate the influence of critical parameters, such as fault displacement, buried depth, tunnel diameter, soil cohesion, and soil friction angle, on the tunnel response under fault dislocation. The results suggest that the tunnel responses positively correlate with the fault displacement, buried depth, and soil cohesion. Increasing the fault displacement amplifies the vertical shear force and bending moment asymmetry while increasing buried depth reduces this asymmetry. An increase in the tunnel diameter and soil friction angle is associated with a decrease in the peak axial force and an increase in the vertical shear force and bending moment. Additionally, variations in the friction angle do not exert a significant effect on the transverse shear force and bending moment.