Predicting Optimal Support Timing for Secondary Tunnel Lining Based on Creep Damage in Rock-Support Systems

The secondary lining serves as the safety reserve of the tunnel, and its reasonable timing provides a guarantee of safety for the tunnel during both the construction and operational phases. Taking Fengkeng Tunnel as an example, the paper proposes the viscoelastic-plastic creep equation of the peripheral rock-support structure system based on the Nishihara rheological model by using the non-associative flow rule based on the consideration of spatial effect. The optimal timing for providing support to the secondary lining was determined by taking the radius of the peripheral rock loosening area in the vicinity of the primary support spray-anchor reinforcement area as the node. The validity and reasonableness of the peripheral rock viscoelastic-plastic creep deformation equation and the timing of the secondary lining were verified by regression analysis of field monitoring data and numerical simulation of FLAC3D. The results show that: (1) The perimeter rock viscoelastic creep equation obtained by introducing the non-associative flow rule and the Nishihara rheological model can well reflect the characteristics of damage creep of the perimeter rock. (2) When adopting the CRD method, if the shear expansion angle ψ = 0°, the secondary lining should be completed within 25– 30 d of the palm face excavation advance or x = 75– 90 m, which is more reasonable; if the shear expansion angle ψ = 10°, the secondary lining should be completed within 15–20 d of the palm face excavation advance or x = 45 – 60 m, which is more reasonable. (3) Through the regression analysis of field data and numerical simulation of FLAC3D, the displacement and deformation of the surrounding rock coincide with each other, which verifies the validity and reasonableness of the theoretical derivation. The study on the timing of secondary lining support considering the creep characteristics of the rock-supporting structure system is of great theoretical and practical significance for the development and utilisation of tunnelling and underground spaces.