Study on Construction Mechanical Characteristics and Offset Optimization of Double Side Drift Method for Large-Span Tunnels in Argillaceous Soft Rock

This study focuses on a large-span highway tunnel in argillaceous soft rock. Numerical simulations were conducted to investigate the mechanical characteristics of the tunnel, constructed using the Double Side Drift Method (DSDM), and the effects of the offset distance between drift faces. Subsequently, field monitoring was performed to analyze the deformation patterns of the primary support at typical cross-sections. The results indicate the following: (1) During DSDM construction in argillaceous soft rock, the crown settlement of the left drift is the largest, while that of the central drift is the smallest. The left and right drifts converge inward, whereas the central drift expands outward, resulting in overall inward convergence of the tunnel section, with the left drift exhibiting a larger convergence. The crown settlement and horizontal convergence induced by excavation of the upper benches of each drift are greater than those caused by the lower benches. (2) The stresses in the primary support increase rapidly after excavation of each segment and then tend to stabilize. The maximum tensile stress occurs at the left haunch, reaching 0.41 MPa, while the maximum compressive stress occurs at the left arch waist, reaching 14.56 MPa. After the tunnel excavation is completed and the section is enclosed, the stress on the left side is significantly higher than that on the right, indicating an eccentric stress state. The plastic zones in the surrounding rock exhibit a butterfly-shaped distribution, mainly concentrated at the haunches and arch springings on both sides. (3) As the offset distance decreases, the deformation of the primary support reduces, whereas the stress and the area of the surrounding rock plastic zones increase. When the offset distance is less than 15 m, both the stress in the primary support and the plastic zone area increase sharply, suggesting that the drift face offset distance should not be less than 15 m. (4) Field monitoring shows that the maximum cumulative crown settlement of the primary support reaches 30.2 mm, and the cumulative horizontal convergence of the section is 35.6 mm, both of which are below the reserved deformation allowance.