Research on grouting reinforcement and seismic response of a grid-type tunnel anchorage in soft rock conditions

Under soft rock conditions, conventional tunnel-type anchorage (TTA) relies strongly on the clamping action of the surrounding rock, which may limit their load-transfer efficiency and seismic performance under unfavorable geological conditions. To improve the interaction between the anchorage body and surrounding rock, a grid-type tunnel anchorage (GTA) was proposed by introducing regularly distributed grid holes on the surface of the anchorage body to enhance mechanical interlocking and load diffusion. A theoretical model based on a surrounding rock–grouting reinforcement layer–anchorage body three-medium system was first established to investigate seismic-wave propagation and energy transmission characteristics. Subsequently, a three-dimensional dynamic numerical model was developed using FLAC3D to evaluate the effects of grouting reinforcement thickness, grouting configuration, grouting reinforcement level, and the thickness of the underlying soft rock layer on construction-induced settlement, displacement response, stress response, and energy transmission. The results indicate that construction-induced settlement is governed primarily by grouting reinforcement rather than anchorage configuration, whereas the advantages of the GTA become more pronounced under seismic loading. Compared with the conventional TTA, the GTA exhibits a lower displacement response, a more uniform stress distribution, and reduced sensitivity to variations in geological conditions. Within the investigated range, a grouting reinforcement thickness of 2–3 m provides effective deformation control, whereas further increasing the grouting reinforcement extent yields limited additional benefits. Full-ring grouting produces a more uniform stress distribution, and even the lowest grouting reinforcement level considered in this study can effectively improve the stability of soft rock. The energy analysis further demonstrates that the combination of the GTA and continuous grouting reinforcement reduces seismic energy transmission into the anchorage system. These findings provide insights into seismic design and reinforcement strategies for tunnel-type anchorages under soft rock conditions.