## 3 PFC Numerical Simulation Test  
### PFC Model  
The model of the bird's nest anchor cable is established by PFC2D.The model is a cylinder with a base diameter of 250 mm and length of 1200 mm. It is composed of 115,245 particles, including 103,567 rock mass particles and 11,540 anchor agent particles. Figure 8: Shear stress distribution of the rock sample. Figure 9: Failure of the anchoring structure. (**a**) Cross section; (**b**) Longitudinal section. Moreover, the rock mass and anchorage agent are represented as gray and blue particles, respectively. The anchorage agent and rock mass are composed of particles with a radius of 0.5-0.85 mm. The cable is composed of 138 particles with a diameter of 17.8 mm and 3 particles with a diameter of 20 mm. According to the measuring point locations, three measuring circles are arranged in the anchorage agent to record the shear stress distribution, and four measuring circles are arranged on the anchorage agent interface to record the shear stress distribution of the anchorage agent interface (Figure 10). To highlight crack, tension and shear cracks are presented as green and red, respectively. The tensile strength and bond force of the rock mass anchored by the PFC are obtained by a uniaxial compression test and Brazilian splitting test and have a value of 3.5 MPa and 3.0 MPa, respectively. The physical and mechanical parameters of the resin anchorage agent and anchor cable are provided by the manufacturer and are equal to 3.0 MPa and 2.5 MPa, respectively. The parameters are constantly adjusted to calibrate the mechanical parameters of similar sample materials. Table 3 shows the parameters of similar materials used in PFC2D. agent depth, but the rock mass does not fail. After cracks develop near the bird's nest, they continue to extend inwards, and rock mass failure occurs. Stage II (debonding slip stage): loading occurs at steps 2600-10,000, the tension crack effect dominates, and shear crack number has a constant value. With additional debonding length, the tensile load of the anchor cable decreases. The relative slip of anchoring interface is no longer coordinated with the increase in load. The shear strength of the bird's nest anchor cable is composed of adhesion, mechanical interlocking, and friction in the axial direction at the coupling interface between the anchor cable and the anchorage agent. As the load increases, interfacial slip and shear dilation appear, and in turn, the interfacial radial force and friction force, as well as the peak shear strength, are elevated. A debonding region occurs at the load end and gradually expands to the other end. Stage III (bird's nest split rock failure stage): After step 10,000, the shear crack and tension crack number are basically a constant value, and a few cracks extend along the rock mass depth. The tensile load of the anchor cable declines significantly up to the residual tensile stress. The friction force and the extruding force of the bird's nest hinder the anchor cable from being pulled out. At this stage, the interface of the bird's nest generates radial cracks, and cracks rapidly expand the rock mass. The radial stress produced by mechanical biting force balances the annular tensile stress of the anchor agent, with only friction resistance remaining. The anchorage agent is sheared, and rock crumbs are drawn unceasingly with the pull-out of the anchor cable. Figure 11: Axial force distribution law of measuring point. Figure 12: Shear stress evolution process of measuring point. The failure mode is a mixed failure between debonding slip of the interface and shear failure of the rock mass (Figure 9). PFC2D can better reflect the internal load transfer behavior, failure mechanism, and failure process of the bird's nest anchor cable under tensile loads. Figure 13: Failure process of the anchoring structure during PFC simulation test.