Study on the Effect and Mechanism of Combined Support Using Foot-locking Bolts and Controlled Grouting in Tunnels within Fault Fracture Zones

To address the difficult problem of controlling large deformations in weak tunnel surrounding rocks caused by fault fracture zones, a novel combined support technology integrating new-type foot-locking bolts with controlled grouting is proposed, based on the high-risk section of the Yangzong Tunnel. This technology utilizes new-type bolts equipped with expansion-type mechanical anchor heads, coupled with a membrane bag sealing and segmented grouting process, to inject P.O 42.5 cement slurry mixed with chemical control agents into the stratum to control deformation. Through field tests, the supporting effects of the new-type foot-locking bolts under different parameter combinations of diameter, inclination, and length were systematically evaluated, and combined with numerical simulations, the displacement constraint mechanism and slurry diffusion laws of the combined support system were thoroughly revealed. The results indicate that the combined support significantly mitigates stress concentration at the key parts of the primary support; the stresses at the left arch feet of the 1# and 5# support arches decreased by 31.21% and 22.84%, respectively, while the stress at the right arch foot of the 3# support arch decreased by 37.35%. Simultaneously, the inward extrusion convergence of the surrounding rock was effectively restrained, with a maximum horizontal displacement reduction of 18.79%. Numerical simulations demonstrate that the effective slurry diffusion radius (solid particle volume fraction δ > 0.025) reaches 2.8 m, and the porosity of the surrounding rock at the grouting pipe orifice decreases from 0.40 to approximately 0.35, rendering the soil mass more compact. Comprehensive research confirms that this combined support technology can effectively control stress release and abrupt deformation of surrounding rock in large deformation zones, thereby enhancing the long-term stability of the support system.