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

To address post-excavation deformation and failure in deep underground mine tunnels, this study investigates the auxiliary transport tunnel in the northern wing of the 3–1 coal seam at a coal mine in Inner Mongolia. Field structural sampling, laboratory testing, and in situ measurements were conducted to determine the rock mass structure, lithological parameters, and stress distribution in the tunnel roof and floor. A numerical model of the surrounding rock and an analytical model of tunnel floor displacement were developed to analyze the failure mechanism of the tunnel. A support scheme combining pile–foundation unit-type support with anchor bolts and cables is proposed to control tunnel deformation. The effectiveness of this synergistic support scheme was validated through numerical simulation. The research indicates that excavation redistributes the in-situ stress, generating zones of tensile stress and areas of high compressive stress concentration both around the tunnel and deep within the surrounding rock. This leads to the development of both tensile and compressive plastic zones within the surrounding rock, leading to deformation and failure of the rock-tunnel interface. The magnitude of roadway floor displacement is proportional to both the width of the plastic zones at the floor ends and the stress concentration factors on each side. The point of maximum floor displacement shifts toward the side with the wider plastic zone or the higher stress concentration factor. Under the combined support system, the maximum displacements of the roof and floor were reduced by 93% and 82%, respectively, while the maximum deformation values for the roadway sides decrease by 78% and 93%, respectively. The displacement patterns of the roof and floor were significantly improved, with the plastic strain zone in the surrounding rock completely severed and the maximum plastic strain value reduced by approximately 80%. Therefore, effective control of tunnel deformation relies on enhancing the integrity of the surrounding rock and reducing both stress concentration and plastic flow. The synergistic effect of the pile-foundation unit support with the anchor bolts and cables effectively mitigated post-excavation deformations, including floor heave, roof settlement, and sidewall convergence, demonstrating the effectiveness and feasibility of the proposed synergistic support scheme for mitigating tunnel deformation.