Abstract: Weak and fractured rock masses with high in-situ stress are prone to large deformation when tunnels pass through. To address this issue, a gradual-pressure-relief control concept, centering on the convergence-confinement method, is proposed. A resistance-reducing energy-dissipating support structure with a "stiff-gradual relief-stiff" mechanical characteristic is designed. A comprehensive study on the large deformation disaster of the Qiaojia tunnel under gently inclined and asymmetrical pressure is conducted using multiple research methods, including laboratory tests, field monitoring, and in-situ tests. Conclusions are drawn as follows: (1) The gradual-pressure-relief control concept not only endows the support structure with high ductility, but also ensures consistently low radial unbalanced forces within the surrounding rock during deformation. This effectively slows down the deformation rate, reduces disturbances to the deeper surrounding rock caused by excavation and support operations, and enhances overall stability. (2) The resistance-reduction and energy-dissipation components absorbs external energy through the bending yield of corrugated steel plates. Uniaxial compression tests demonstrate that the structure possesses excellent deformation capacity and gradual-relief mechanical performance. By adjusting the thickness and spacing of the zigzag-shaped steel plates, the support system can meet varying requirements for tunnel support resistance. (3) After implementing the resistance-reducing and energy-dissipating support system, the difference deformation of left and right arch shoulders in the tunnel is reduced by 66.7%, the peak surrounding rock pressures at the left arch shoulder, crown, and right arch shoulder decrease by 28.8%, 13.8%, and 54.5%, respectively, compared to rigid support. Additionally, the difference in peak pressures between the left and right arch shoulders is reduced by 72.6%. These demonstrate that the large deformation of asymmetrical-pressurized rock has been effectively controlled.

Key words: tunnel engineering, gradual-pressure-relief control, resistance-reducing energy-dissipating support, asymmetrical-pressurized strata