Three-dimensional Hybrid Finite Element-Material Point Method for analyzing reinforcement and stability of tunnel face with fiberglass anchor bolts

Tunnel face instability and excessive deformation are common challenges during excavation in poor geological strata without pre-reinforcement. Fiber-reinforced polymer (FRP) anchor bolts offer an effective solution for stabilizing tunnel faces under such circumstances. This study addresses the issue of tunnel face instability and large deformation in geologically unfavorable segments using the recently developed Material Point Method (MPM), and highlights the effects of pre-reinforcement by integrating FRP anchor bolts using bar elements within the MPM framework. To achieve this, a three-dimensional hybrid finite element-material point method (HFEMPM) in-house code is developed to simulate the coupled deformation of FRP anchor bolts and the surrounding stratum mass at the tunnel face. The coupling algorithm and numerical implementation of the proposed 3D HFEMPM are comprehensively outlined. The validity of this method is confirmed through comparisons with centrifuge tests on reinforced soil slopes and scaled tunnel excavation tests involving FRP bolt reinforcement. Furthermore, the influences of FRP anchor bolt density, length, and diameter on the tunnel face stability and associated failure/deformation mechanisms are explored using the proposed 3D HFEMPM. Numerical results demonstrate that increasing the anchor bolt diameter and reinforcement density significantly enhances the tunnel face stability. Additionally, the optimal reinforcement length of FRP anchor bolts is concentrated within a localized region in front of the tunnel face. When the bolt diameter and density exceed certain thresholds, the surrounding stratum tends to stabilize. The proposed 3D HFEMPM offers a theoretical foundation for understanding tunnel face failure/deformation mechanisms in poor geological conditions and provides guidance for selecting effective pre-reinforcement strategies.