Analytical study and engineering application of NPR prestressed bolt is anchored by mechanical-type anchorage and resin

The NPR bolt exhibits excellent engineering properties, such as high tensile strength (950 MPa) and significant elongation (30 %). It can efficiently apply high prestress and endure substantial deformations in the surrounding rock. The anchor system of an NPR bolt consists of a mechanical-type anchorage and resin, both crucial for applying prestress and ensuring effective anchorage. This study delves into the action mechanism of the mechanical-type anchorage through mechanical analysis and the finite element method. Results indicate a linear increase in radial stress on the surface between the bolt and the wedge of the NPR bolt in the mechanical-type anchorage section, reaching approximately 13 MPa/mm. The interfacial shear stress on the surface of the NPR bolt increased in a vibrating-chord manner. It was zero at the initial point and peaked at the end of the bolt, which was approximately 110 MPa when the NPR bolt entered the plastic state. The maximum axial force of the bolt, corresponding to tensile load, occurs at the loading end, while the minimum, reaching zero, is at the tail. The decay rate of the axial force is positively correlated with the shear stress on the bolt's surface. The peak Mises stress appeared near the loading end and then gradually decayed, which is about 30 MPa larger than the tensile load strength. For the application of a 100 kN prestress to the NPR bolt, a pretension experiment was conducted, revealing that a pretension value of 140 kN ensures the desired prestress. The limit plate distance was 6 mm, and the wedge exposure was 2 mm after prestressed locking. To assess the resin anchorage performance of NPR bolts with spiral grooves and ribs, a resin anchorage experiment was conducted using C25 perforated concrete and CK2850 resin. Results indicate that use of two CK2850 resins ensured the anchorage stability, with the anchoring performance of NPR bolts with spiral ribs surpassing that of bolts with spiral grooves. Finally, the technique was applied to a shallow-buried tunnel with a long span. Monitoring the axial force of the NPR bolt and the surrounding rock response for approximately 140 days revealed the technology's effectiveness in applying prestress, maintaining the bolt's axial force stability above 100 kN. Arch settlement was controlled at approximately −4.46 mm, the horizontal convergence at about −3.56 mm, and ground surface settlement at about −8.41 mm.