Experimental investigation of mechanical characteristics for linings of twins tunnels with asymmetric cross-section

A large-scale 3D combined geomechanical model test is conducted in this study to investigate the mechanical characteristics for linings of asymmetrically closely-spaced twin-tunnels constructed in sandy ground. A series of experimental results (i.e., excavation and overloading) effectively reveal the mechanical features of the linings and the stress release and displacement characteristics of the sandy ground for asymmetrical twin-tunnels during the excavation and overloading under the condition of great buried depth (vertical stress) and high stress coefficient (the ratio of lateral to vertical stress). Afterward, numerical analyses are conducted by a finite difference program FLAC3D to verify experimental results. The influences of construction sequence, stress coefficient, clear distance and lining stiffness on mechanical behaviors of linings and stratum are discussed in some details. The experimental results show that during the excavation of the asymmetrically closely-spaced twin-tunnels, the linings are in a state of compression. The lining of the smaller diameter tunnel which excavated later is susceptible to bias pressure. The bending moment located on the lateral side of the larger diameter tunnel close to the smaller diameter tunnel decrease when the smaller diameter tunnel is excavated. The change of stress release direction in the middle soil pillar is the main reason that results in the change of stress state in linings. The overloading process only causes the bending moments increasing continuously, but not affect the basic distribution patterns in linings. Numerical results indicate that if the smaller diameter tunnel is excavated first, the continuous tunneling of the larger diameter tunnel will cause sharp changes in lining moment of smaller diameter tunnel, but it will not cause serious adverse effects on the existing tunnel lining (larger diameter tunnel) if the larger diameter tunnel is excavated first.