Mudstone exhibits strong hydrophilicity, leading to volumetric expansion upon exposure to water, which adversely affects the construction and operation of tunnel engineering projects. To investigate the impact of rock expansion forces on the mechanical characteristics of secondary lining in tunnels, the Heijing Tunnel in Chuxiong Prefecture, Yunnan Province, is utilized as a case study. Based on the theory of stratum structure method, a comprehensive analysis was conducted on the mechanical characteristics and safety of secondary lining under both the influence and absence of rock expansion forces. Additionally, an economically viable design scheme for anti-expansion secondary lining is proposed. The research results indicate that: Variations in the rise-to-span ratio of the invert primarily affect the secondary lining in the region below the tunnel's springline (including the side walls, arch foot, and invert). An increase in the rise-to-span ratio can optimize the stress state of the structure, thereby enhancing its safety. Under the influence of expansion forces, the safety of the secondary lining will diminish, and the most critical structural positions will shift in response to variations in the rise-to-span ratio of the arch. When the expansion force is within 100 kPa, the structural safety and crack requirements can be met by increasing the reinforcement ratio. However, when the expansion force exceeds 100 kPa, adopting a rise-to-span ratio of 0.12 significantly mitigates the adverse effects of the expansion force on the structure, allowing for a reduction in the reinforcement ratio and a corresponding decrease in costs.
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