Optimization of design parameters for support scheme of a high compressible layer in a diversion tunnel

Soft rock tunnel surrounding rock deformation exhibits significant time-dependent characteristics, potentially causing cracking and damage of tunnel linings during operation. This study focuses on a highly deformable mudstone section in a water diversion tunnel in Xinjiang, and proposes a support scheme with a high compressible layer between initial support and secondary lining to ensure the long-term safety of the tunnel. The existing compressible layer support scheme was initially subjected to on-site monitoring and structural forces analysis. Subsequently, numerical simulation methods were used to optimize the compressible layer support parameters. Finally, the optimized and original schemes were compared to analyse their respective support effects. (1) Monitoring of the existing support scheme reveals that, with the installation of a 5 cm polyethylene compressible layer at a density of 90–100 kg/m³, the surrounding rock pressure reaches 0.36 MPa, indicating the compression phase of the compressible layer. The non-uniformity of lining force is evident, suggesting potential for optimizing the compressible layer material and thickness. (2) Optimization of compressible layer support parameters indicates that if the stress of buffer layer platform is too high, it cannot fully absorb energy, and if too low, it cannot effectively limit surrounding rock deformation. Both scenarios result in insufficient energy absorption and low lining safety. Increasing the compressible layer thickness gradually reduces lining damage degree, but the reduction rate diminishes over time. For this project, the optimal compressible layer support is achieved with a platform stress of 0.5 MPa, a compression ratio of ≥0.6, and a thickness of 10 cm. (3) Comparative analysis indicates that the optimized compressible layer support reduces the maximum principal stress on the secondary lining by 20%–30% compared to the original scheme, alleviating stress concentration in the lining and ensuring the long-term stability of the tunnel support structure.