Reinforcement mechanism of 2G-NPR bolts for large deformation in soft rock tunnels based on FDEM

Rock bolts are a commonly used measure for controlling the stability of the surrounding rock in tunnels. The material-type constant-resistance large-deformation bolt (i.e., the 2G-NPR bolt), which features high resistance to tensile and shear failure, has been widely applied. However, it still faces limitations such as unclear reinforcement mechanisms, unclear interaction behaviors with the surrounding rock, and the lack of a suitable simulation and representation method. Based on the solid element modeling principle, a representation method for reinforcement by 2G-NPR bolts (including conventional ribbed bolts) using the combined finite-discrete element method (FDEM) is proposed. The effects of in-situ stress magnitude, bolt length, circumferential spacing, and longitudinal spacing on anchoring performance are investigated. The results indicate that: (1) The solid element modeling method and the two-dimensional equivalent method can effectively simulate prestressed anchoring and end-anchoring, and can also represent the influences of bolt diameter, bolt length, circumferential and longitudinal spacing, and installation timing; (2) Compared with Q235 ribbed bolts, 2G-NPR bolts exhibit greater resistance to shear failure and are applicable to conditions with a strength-to-stress ratio α ≥ 0.23, whereas Q235 ribbed bolts fail at α = 0.45; (3) The reinforcement mechanism, or the interaction mechanism with the surrounding rock, can be described as follows: the bolts arrest the propagation of conjugate shear fractures in the surrounding rock and provide arching points for the interlocking of fragmented rock blocks, thereby preventing deeper surrounding rock from squeezing into the tunnel space, with significant tangential stress concentration observed between the fragmented rock blocks; (4) Optimal ranges exist for bolt length and circumferential and longitudinal spacing. As bolt length increases and circumferential or longitudinal spacing decreases, the anchoring effect tends to stabilize. Under the conditions studied, when α = 0.23, the optimal values for 2G-NPR bolt length, circumferential spacing, and longitudinal spacing are 2.5 m, 1.0 m, and 1.0 m, respectively.