Three-dimensional numerical investigation on spatial interaction and ground deformation of twin tunnels based on the convergence-confinement method

Twin-tunnel excavation inevitably induces overlapping unloading effects, causing complex spatial interactions and ground deformations. To quantitatively elucidate these three-dimensional (3D) mechanical mechanisms, a sophisticated elastoplastic numerical model incorporating the convergence-confinement method was established. The structural and ground responses under varying clear spacings ( L ), cover depths ( H ), and stress release rates (λ) were systematically investigated. The results reveal that the intermediate soil pillar acts as the mechanical core governing the spatial interaction, inducing a "butterfly-shaped" shear stress concentration and a "vertical compression and horizontal elongation" lining deformation mode. A critical clear spacing of L = 20 m (approximately 2.2 times the tunnel diameter) is identified, beyond which the stress superposition effect is minimized, and the surface settlement trough evolves from a deep single "V" shape to a wide "double-V" shape. Moreover, the cover depth dictates the surface deformation mechanism, transitioning from a "W"-shaped heave under shallow cover to a deep "V"-shaped settlement under deep cover. Furthermore, as the stress release rate increases from 30% to 70%, the severe stress relaxation of the intermediate soil pillar causes the lining deformation mode to shift from "outward expansion" to "overall translation", significantly reducing lining stress but increasing the maximum surface settlement from 6.28 mm to 16.64 mm. These findings provide practical engineering guidelines: early support is recommended for settlement-sensitive shallow tunnels, whereas moderately delayed support is preferable for deep tunnels to utilize the self-supporting capacity of the surrounding soil.