Application of excavation compensation method for constructing shallowly-buried super-large span subway tunnel

Stress loss at the excavation face, stemming from underground chamber excavation, is the primary culprit behind geotechnical accidents. Implementing timely stress compensation is vital to control potential instability or failure in the surrounding rock effectively. Shallow-buried, large-span subway tunnels, characterized by complex geological conditions, significant construction challenges, and extensive engineering risks, often exceed traditional methods' capacities, resulting in excessive deformation of initial support or localized stress surpassing the overload limit during construction. This study delves into the excavation unloading effect, namely radial stress loss and tangential structural damage, offering a comprehensive understanding of rock excavation's mechanical behavior. It introduces an excavation compensation method, formulated from a mechanical perspective, specifically tailored for controlling shallow-buried, large-span tunnels. An excavation equivalent stress compensation experiment was designed, validating the substantial influence of compensation stress in controlling rock cracking, mitigating tensile stress areas in the arch, augmenting peak bearing capacity, and enhancing plastic deformation capacity. The final part of the study conducts a comparative analysis of the tensile mechanical properties and shear resistance of Negative Poisson's Ratio (NPR) anchor steel. The results reveal that NPR anchor bolt exhibits high strength, constant resistance, and substantial elongation. When compared with ordinary anchor bolt, its tensile strength and elongation surpass the latter by more than double, and its shear energy absorption is 4.37 times that of traditional Poisson's Ratio (PR) material anchors. NPR bolt demonstrates uniform deformation during tension and shearing processes, without significant necking, and prestressing can be applied to 70%− 90% of the yield strength, thus providing a material foundation for radial stress compensation and tangential structural compensation. This technology has been successfully applied to shallow-buried, large-span subway tunnels, effectively controlling the deformation and settlement of large-span tunnels and thereby demonstrating the applicability of the proposed excavation compensation methods and techniques.