# Research on the Self-Drilling Anchor Pull-Out Test Model and the Stability of an Anchored Slope
## 2 Experimental Analysis of the Key Performance of Hollow Self-Drilling Anchor Bolts  
### Experimental Design  
This study is based on the hollow self-drilling anchor used in the project, which is R38S (produced by Hengnuo Anchoring Technology Co., LTD., Luoyang, China), as shown in Figure 1. The shaft is made of a thick-walled seamless steel pipe, with standard threaded connections along its entire length. The yield force of this type of anchor is 450 kN, the tensile force is 550 kN, the cross-sectional area is 631 mm\({}^{2}\), and the elongation is 8%. The drill bit is a full-steel ball-tooth drill bit, which is suitable for fractured formations, soft sandstone, limestone, gravel, and soft rock layers. The coupling sleeve has the same strength as the hollow shaft. The cement used is 42.5 grade composite silicate cement, and the grouting pressure is selected as 0.8 MPa with a water-cement ratio of 0.8, as per the design specifications.  
Figure 1: Structural diagram of a self-drilling anchor bolt.  
The experimental design follows the requirements of the Technical Code for Ground Anchorages and Soil Nails (CECS 22:2015) and the Technical Code for Retaining and Protection of Building Excavations (JGJ 120-2012) [11]. The study focuses on verifying the anchoring mechanism and load-bearing characteristics of self-drilling anchor bolts in complex rock formations. The drilling process of self-drilling anchor bolts is illustrated in Figure 2.  
During the experimental loading process, a QF-100-100 type hollow hydraulic jack (produced by Dezhou Huali Hydraulic Press Co., LTD., Dezhou, China) was used, as shown in Figure 3. Additionally, high-precision load sensors were installed throughout the system to ensure the accuracy of load measurements. To comprehensively monitor the displacement variations during the experiment, a total of four displacement meters were installed, with an accuracy of up to 0.01 mm. The design of the entire loading system followed the requirements outlined in Appendix D of the \"Technical Code for Building Foundation Pit Support\" (JGJ 120--2012), adopting a single-cycle incremental loading mode. In this mode, the load steps were set as 0.1 P, 0.5 P, 0.7 P, 0.8 P, and 0.9 P, progressing until the load corresponding to the estimated ultimate bond strength, denoted as P, was reached. The value of P was selected based on the engineering design documents. To ensure the stability and accuracy of the data, the duration of each load step was no less than 5 min, and the initial displacement was recorded prior to the start of loading. During the loading process, data were collected every 30 min to monitor displacement changes. The loading system continued to monitor the displacement until the convergence criteria were met, i.e., the displacement increment within two consecutive 1-h intervals did not exceed 0.1 mm. Only then would the next load step be applied, ensuring the rigor of the experiment and the reliability of the results.  
Three failure criteria were established as the basis for determining failure and stopping the test:  
1. Displacement divergence criterion: The displacement rate continuously increases at a given load level and exceeds 0.5 mm within one hour;
2. Load-bearing failure criterion: A significant turning point appears in the load-displacement curve, and the residual deformation reaches 80% of the peak displacement;
3. Rod body fracture criterion: The anchor bolt exhibits necking or fracture.  
Figure 2: Drilling process of a hollow self-drilling anchor bolt.  
Once any of these criteria are met, the test is immediately terminated. The load level preceding failure is recorded as the ultimate uplift bearing capacity.