Mechanism analysis and treatment effect of compound large deformation in deep coal-bearing strata tunnel

Compound large deformation in tunnel has been a challenging issue influenced by complex geological conditions such as weak surrounding rock, high ground stress and groundwater. This paper summarizes the characteristics of compound large deformation in Gaopo tunnel, which passes through a weak coal-bearing stratum. Mechanical and swelling tests were conducted, alongside in-situ stress tests, to analyze the potential for squeezing and swelling deformations. A numerical simulation using an elastoplastic damage model, coupled with a method that calculates the humidity field based on the temperature field, was carried out to quantitatively analyze the mechanisms of compound large deformation. It was found that ground stress emerges as the primary determinant of the compound large deformation in Gaopo tunnel, followed by weakened surrounding rock and the humidity field. To address these problems, a new support scheme primarily based on double primary support was proposed. Field monitoring was conducted to evaluate the mechanical and deformation characteristics of support structure during the construction process. The results indicate that the new support scheme effectively controlled the compound large deformation, and the timing of the second primary support installation was found to be satisfactory. The treatment experience provides valuable insights for support design in tunnels experiencing compound large deformation issues.