Softening Effect and Control Measures of Surrounding Rock in Deep Buried Saturated Loess Tunnel

Under the action of groundwater, the surrounding rock of a loess tunnel will undergo varying degrees of softening, greatly weakening its strength, which brings many difficulties to the design and construction of the tunnel. Based on a loess tunnel in Longdong, different degrees of softening were set up, and finite element numerical simulation software was used to calculate and analyze the deformation of the tunnel surrounding rock and the stress changes of lining structure under different degrees of rock softening. At the same time, a comparative analysis was conducted on the on-site reinforcement measures after the softening of the surrounding rock and the lining structure of the enlarged arch foot. The results show that after the strong softening of the deeply buried loess tunnel, the settlement of the arch top increased by 271%, the uplift of the arch bottom increased by 283%, the horizontal convergence increased by 205%, and the maximum principal stress of the initial support increased by 107%. The most unfavorable stress position appeared at the arch shoulder. After adopting two reinforcement measures and expanding the arch foot, the vertical displacement of the surrounding rock decreases by 5.1% to 77.81%, and the maximum principal stress of the initial support decreases by 17.7% to 46.5%. From the reinforcement effect, it can be seen that the locking foot steel pipe pile reinforcement only improves the deformation of the arch foot surrounding rock and the stress on the lining, but the effect is not significant. Using enlarged arch feet in the lining structure can effectively control the deformation and stress of the lining structure, and the construction difficulty is small and the cost is low. The curtain grouting reinforcement has the best effect on improving the deformation and supporting structure stress of saturated loess tunnel surrounding rock after softening.