Study on load transfer mechanism of underreamed anchor system with expandable reinforced cage

Compared with normal anchors, traditional underreamed anchors can provide higher pullout capacity, but the insufficient reinforcement of grout in the expanded section leads to poor integrity and limited improvement of pullout resistance. By installing the reinforced cage in the expanded section, the newly developed reinforced-cage underreamed anchors enhance the overall mechanical performance and anchoring effect. To further explore the bearing mechanism of such anchors, laboratory model tests and numerical simulations were conducted to investigate the load transfer mechanism of the first-interface failure under pullout load, and to analyze the influence of grouting strength and transverse brace spacing on the pullout capacity of anchor rods. The results indicate that: (1) The first-interface ultimate pullout capacity of pressure-type reinforced-cage underreamed anchors increases by 75% compared with that of traditional underreamed anchors, while tension-type anchors show a 150% improvement. (2) The load of pressure-type anchors is first transmitted through the anchor bar bonding section and high-strength nut, and then directly to the lower transverse brace, while the tension-type anchors act on each component of the steel reinforcement cage step by step along the axial rod, and both types eventually fail due to the bearing failure of the lower grout. (3) The pullout capacity of anchors increases with grout strength. When the grout strength increases from C20 to C40, the pullout capacity of pressure-type and tension-type anchors increases by 6.3% and 9.7% respectively. When the transverse brace spacing is reduced by half, pressure-type anchors show an increase in bearing capacity, while tension-type anchors experience an 8.8% reduction.