Soft rock tunnels in red beds frequently encounter anchor failure issues. Improving the mechanical properties of the red-bed soft rock-grout interface is highly significant for ensuring the stability of anchor support. This study employs laboratory direct shear tests in combination with digital image correlation (DIC) technology to investigate the mechanical properties (peak shear strength τ_p, residual shear strength τ_r, and shear stiffness k_s) of the grout-rock interface (GRI) among three typical soft rocks from red beds (red sandstone, mudstone, and grey sandstone) and two grout materials (ordinary Portland cement (OPC) and early high-strength cement (EHC)) under varying curing periods (6 h, 1 d, 2 d, and 7 d). The shear behavior and DIC-based failure mode are analyzed for red beds GRIs. The following key findings were observed: (1) As normal stress increases, both the shear strength (τ_p) and shear stiffness (k_s) of the GRI also increase, following the trend: red sandstone > mudstone > grey sandstone. The cohesion values of the red sandstone, mudstone, and green sandstone with the EHC grout-rock interface at 2 days are 2.4, 0.9, and 1.2 MPa, respectively. The corresponding internal friction angles are 57.6°, 38.0°, and 27.0°, respectively. (2) With an increase in curing age, both τ_p and k_s at the GRI increase non-linearly. EHC exhibited superior bonding performance compared to OPC, with τ_p reaching 7.9 MPa at 6 h and 90% of the 7-day τ_p being achieved at 2 d in red sandstone conditions. (3) The OPC-bonded specimens primarily exhibit grout failure near the GRI, whereas the EHC-bonded specimens show rock failure in red sandstone and mudstone conditions, and adhesive failure at the interface in grey sandstone. (4) An empirical model for the shear strength of the GRI in red bed soft rocks is proposed and validated, providing a framework for the rapid and reliable evaluation of anchorage strength in red bed soft rock tunnels.

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.