Research on deformation mechanisms in deep excavation tunnels and the application of the pile foundation-unit-type support

To address the issue of tunnel deformation and failure following excavation in deep underground mining tunnels, this study focuses on the auxiliary transport tunnel in the northern wing of the 3 − 1 coal seam at a coal mine in Inner Mongolia. Through structural sampling, laboratory processing experiments, and on-site testing, the rock mass structure, lithological parameters, and stress distribution of the tunnel roof and floor were determined. A numerical model of the deep underground tunnel rock mass and a mechanical theoretical model of the tunnel floor displacement were established to analyze the failure mechanism of the tunnel. A support scheme combining the pile foundation-unit-type support with anchor bolts and cables was proposed to prevent tunnel deformation. The effectiveness of this synergistic support scheme was validated through numerical simulation. Research indicates that tunnel excavation causes redistribution of in-situ stress, forming zones of tensile stress and high-value compressive stress concentration around the tunnel and deep within the surrounding rock. This results in the formation of plastic zones of “tension” and “compression” within the rock mass, leading to deformation and failure of the rock-tunnel interface. The magnitude of roadway floor displacement increases with the width of the plastic deformation zones at both ends of the floor and the stress concentration factor on either side. The location of maximum floor displacement occurs near the side with either a wider plastic deformation zone or a higher stress concentration factor. Under cooperative support, the maximum deformation values for the roadway roof and floor decrease by 93% and 82%, respectively, while the maximum deformation values for the roadway sides decrease by 78% and 93%, respectively. The displacement curve patterns of the tunnel roof and floor changed, with the plastic strain zone in the surrounding rock completely severed and the maximum plastic strain value reduced by approximately 80%.Therefore, enhancing the integrity of the surrounding rock and reducing stress concentration and plastic flow within the rock mass are key factors in controlling tunnel rock mass deformation. The combined action of the pile foundation-unit-type and anchor bolts/cables effectively suppressed post-excavation deformations such as floor displacement, roof settlement, and sidewall inward compression, demonstrating the feasibility of the synergistic support scheme in preventing tunnel deformation.