Three-Dimensional Parametric Analyses of Cross-Passages in Shallow Tunnels Within Noncohesive Soils.

Cross-passages are small-diameter tunnels between running tunnels or between a tunnel and a shaft, essential for safety, maintenance, emergency evacuation, and ventilation. The geotechnical design of these cross-passages is complex and not well understood. This paper presents results from a parametric study carried out in a three-dimensional finite element framework using an advanced nonlinear elasto-plastic constitutive model to represent the surrounding soil. Key parameters varied include the aspect ratio of the cross-passage to the parent tunnel, soil strength, and intersection angle. Findings indicate a significant increase in hoop compression at the springlines and longitudinal tension at the crown and invert of the cross-passage. The aspect ratio significantly affects stress distribution, with maximum stress arching occurring at an aspect ratio of 0.5. With the increase in the opening size, stress redistribution is reduced, and opening deformation is increased. The ideal intersection angle between a cross-passage and parent tunnel is found to be 90°, with angles below 70° leading to a large increase in stress concentrations. Extent of stress redistribution due to cross-passage is found to range from 1.5 to 1.2 times its diameter for large openings in loose soils and smaller openings in dense soils, respectively.