Three-dimensional simulation analysis of the impact of excavation on non-level crossing tunnels.

Abstract

The rapid growth of underground infrastructure in densely populated urban areas has increased the need for reliable design guidelines concerning the interaction of non-level crossing tunnels. The proximity of these tunnels often leads to complex soil-structure interactions, where excavation-induced stresses and deformations can significantly affect the safety and serviceability of existing tunnels. Despite the significance of this issue, few studies have comprehensively examined the combined impact of intersection angle, vertical spacing, and excavation sequence on the mechanical behavior of tunnels. This research aims to address this gap by performing a systematic three-dimensional finite element analysis using Plaxis 3D, focusing on the structural response of the lining and deformation patterns of the surrounding soil. A parametric study was conducted by varying three critical parameters: the intersection angle of the crossing tunnels, the vertical distance between tunnel axes, and the excavation sequence. The results included vertical displacements of the existing tunnel invert and crown, axial forces, bending and torsional moments in the lining, and soil failure indicators such as plastic point development and shear strain contours. Findings revealed that the intersection angle plays a dominant role, with mid-range angles (around 45°) producing the lowest settlements (up to 60% less than at acute angles) and minimizing lining forces. In contrast, near-orthogonal crossings (75°-90°) resulted in the most unfavorable conditions. Vertical spacing was also crucial: an intermediate spacing of 12.5 m resulted in nearly 30% lower settlements compared to 10 m, whereas a larger spacing (15 m) led to higher displacements due to stress redistribution. Additionally, the excavation sequence significantly influenced deformation patterns: shallow-first excavation caused more localized yet higher settlements, whereas deep-first excavation reduced peak displacements by roughly 10-12% but spread them over a broader zone. Overall, these findings emphasize the importance of carefully considering geometric and construction parameters in tunnel design. The results suggest that adopting intermediate intersection angles and moderate vertical spacing offer the best conditions. At the same time, the choice of excavation sequence should strike a balance between peak settlement and the extent of ground disturbance. These insights contribute to the development of safer and more effective design strategies for non-level crossing tunnels in complex urban environments.