Test and field application of fracture evolution of large-span tunnel under NPR bolt compensation support system

• Established load-bearing mechanical model for anchored structures under compensatory support. • Revealed mechanical response of compensatory and passive support systems under loading. • NPR bolts-based compensatory support successfully applied in shallow-buried large-span tunnels. Shallow-buried large-span tunnels may bend or collapse owing to loads, and their surface structures present considerable safety issues. At Huashanyilu station on Qingdao Metro Line 6 in China, theoretical studies and interior model tests were conducted to effectively increase the bearing capacity of the tunnel. The anchoring bearing mechanism of the high prestress compensating support system was revealed, and the system was built using a negative Poisson’s ratio (NPR) bolt at its core. We compared and analyzed the fracture evolution characteristics of the compensating and conventional support systems under various loads. The results showed that the compensating support system effectively increased the support strength and residual safety factor of the bearing arch, whereas the use of a high-prestress NPR anchor reduced the early deformation of the surrounding rock. The coupling failure modes of the arch tension extrusion failure and arch foot shear fracture occurred when the tunnel surrounding the rock was overloaded. The compensatory support system produces a bearing arch that is extremely resistant to external loads with minimal deformation of the tunnel surface and arch frame, excellent surrounding rock integrity, and a low stress rate. The radial and tangential peak stresses exceeded those of the passive support system, and the structural block fell when it became unstable. The maximum displacement of the arch stays constant at −5.7 mm after tunnel excavation. NPR bolts have remarkable applications in this field. The conclusions of this study have a significant impact on the regulation of the stability of the surrounding rock in large-span tunnels.