Study on the mechanical characteristics and anchorage performance of the anchor cable with precast internal-anchor-head (PIAC)

In order to improve the anchoring force of traditional anchor cables while ensuring low cost, this paper proposed a new-type anchor cable structure with a precast internal-anchor-head (PIAH) at the anchor cable end. The reliability of the new structure was thoroughly validated through theoretical analysis, indoor pull-out experiments, and numerical simulations. Firstly, the mechanical pattern based on the characteristics and anchoring mechanism of the PIAH structure was analyzed, and the calculation methods under different failure modes were proposed. Secondly, a full-scale physical model experiment was conducted to study the effects of varying lengths PIAH on the tensile stress of the grout, stress distribution between grouting sections and the surrounding rock, shear stress distribution between PIAH and grout, and the yield conditions of the PIAH. Results show that after applying the PIAH, the ultimate bearing capacity increases from 796.4 to 921.2 kN, and the ultimate strain decreases from -1219.7 × 10^-6 to -309.5 × 10^-6, a reduction of 74.63%. It indicates that the strain state of the grout will be substantially improved after setting the PIAH in anchor cable, and the bond strength between the grout and the hole wall could be enhanced, result in effective increase of anchoring force. At the same time, longer PIAH are more favorable for controlling cable deformation. Furthermore, to further validate the experimental results, numerical models of multiple-types anchor cable structures were established based on finite difference numerical method. Simulation results show that, under the same conditions, the ultimate bearing force of the anchor cable increases as the length of PIAH increases, and the overall elastic modulus of the anchoring system could also be improved. Compared to the grout of the Traditional Prestressed Anchor Cable (TPAC), the concentration degree of axial stress of Anchor Cable with Precast Internal-anchor-head (PIAC) was only 28.57%, effectively reduced the degree of stress concentration. Additionally, the maximum effective length of PIAH is about 1.5 ~ 2 m under the test conditions of a 4 m PIAC and further length increases produce diminishing returns. Meanwhile, PIAC can be used not only in hard rock formations, but also in soft rock formations if improving the strength of the interface between the surrounding rock and the grout. The research results have valuable reference significance for the engineering application of PIAC.