Long-Term Stability Analysis and Structural Safety Assessment of Tunnel Surrounding Rock in Carbonaceous Slate Formation

Carbonaceous slate, widely distributed in geological formations, exhibits pronounced creep characteristics under long-term loading. In the context of the engineering background of the Baoligang Tunnel on the Ehan Expressway, this study focused on the long-term stability analysis and the structural safety assessment of the tunnel surrounding rock in carbonaceous slate formation. The results indicated that carbonaceous slate demonstrates noticeable strain softening in both uniaxial and triaxial tests, with failure occurring along bedding planes through splitting. Under creep conditions, carbonaceous slate's mechanical properties undergo significant changes, with a 9.2% and 23.8% reduction in failure and residual stress, respectively. The total strain at failure increased by 17.1%, and post-peak strain softening became more pronounced, although the failure mode remained the same. The advanced small pilot tunnel proved effective in controlling the long-term stability of surrounding rock in the Baoligang Tunnel, particularly with significant benefits observed after the secondary lining application. Under the influence of creep, the safety factor of the initial support vault in the tunnel was below the minimum threshold of 3.6 at 10 days, indicating a hazardous condition at that specific location. After the secondary lining, the safety factors at different positions were considerably higher than the minimum safety threshold of 2.4, thus ensuring comprehensive safety.