Analysis and treatments of primary support failure in large-span tunnel under a soft-hard interbedded surrounding rock mass

The construction of large-span tunnels inevitably involves traversing uneven strata. Due to the influence of the flatness of the large-span tunnel structure and asymmetric surrounding rock loads, concrete cracking and excessive deformation of steel arches occur frequently. Taking a large-span highway tunnel with uneven rock strata as an example, the mechanical behavior, failure causes, failure mechanisms, and treatment techniques of the tunnel's initial support structure were studied. Firstly, field monitoring and systematic analysis of typical sections revealed that the displacement of tunnels on the soft rock side and the contact pressure between the surrounding rock and the support were significantly greater than on the hard rock side. Then, combined with the field test results, a similar model test was designed based on the tunnel structure performance testing platform developed by the research team to study the deformation and failure modes of conventional initial support systems for large-span tunnels under geological unbalanced pressure. Finally, an innovative technique is proposed to adjust external loads and optimize internal forces of the structure by locally adding long anchor rods or cables, and the treatment effects of various schemes are evaluated and compared. The results show that the initial support system exhibits a state of biased pressure in the presence of weak surrounding rock on one side. The maximum bending moment of the soft rock side spandrel bearing structure, the spandrel and vault on the soft rock side are the most vulnerable positions of the structure. By implementing local long anchor or bolts, the stress distribution within the initial support structure of a large-span tunnel can be effectively enhanced. This method offers significant practical value and flexibility, addressing the issue of initial support failure caused by a soft-hard interbedded surrounding rock mass in large-span tunnels.