## 5 Conclusions  
SEPT is an effective approach to study the reinforcement mechanism and bolting-support technology. To realize laboratory SEPT results reflecting site conditions, the influence of radial stiffness on reinforcement performance was studied. The following conclusions are summarized on the basis of laboratory testing and theoretical analysis.  
1. On the basis of thick wall cylinder theory, an equivalent radial stiffness method was developed to establish the relationship between the confining tube in the laboratory  
Figure 10: Failure modes of bolt specimens with different rib spacings.  
and the elastic properties of surrounding rock (Equations (1)-(3)). Calculation results showed that the PVC tube (\(E\) = 4.0 GPa and \(v\) = 0.35) corresponds to the radial stiffness of the weak surrounding rock (\(E_{r}\) = 1.0-3.5 GPa, \(v_{r}\) = 0.25-0.30); Al tube (\(E\) = 68.9 GPa and \(v\) = 0.33) corresponds to the medium-strong surrounding rock (\(E_{r}\) = 15.0-50 GPa, \(v_{r}\) = 0.20-0.25); and the steel tube (\(E\) = 206 GPa and \(v\) = 0.3) corresponds to the strong surrounding rock (\(E_{r}\) = 50.0-140.0 GPa, \(v_{r}\) = 0.15-0.20). This can be used to improve the accuracy of laboratory SEPT results towards the site outcome (Figure 2).
2. Two kinds of dextral-threaded bolt were studied using the developed method of this study. PVC, Al, and steel tubes were used as confinement in laboratory SEPT, and anchorage performance was obtained under different surrounding rock strength levels (Figure 7). The results show that axial load capacity increased with the increase in radial stiffness of the confining materials. Compared with PVC tube specimens, peak axial force was nearly double when using thick steel tubes (Table 2).
3. On the basis of theoretical analysis and curve fitting, the relationship between the average peak axial force and radial stiffness of the confining materials was proposed to be a negative exponential (Equation (4)) function. A validity study was conducted, and its result showed that the proposed relationship agreed well with the theory in the literature. The developed equation could be used to determine the performance of a specific bolt under different field geoconditions and provide accurate parameters for numerical modeling.
4. Post-testing specimens showed that the failure modes of the original and modified dextral bolts were parallel shear failure and expansive slippage failure, respectively (Figure 10). As anchorage performance is determined by the load-transfer mechanism in a bolting system, which is closely related to the failure model of rockbolting, it may be practical to design and realize different anchorage effects for different field geoconditions via controlling the technology of rockbolting failures, such as with rebar-profile modification.  
Conceptualization, J.H. and C.C.; methodology, C.C.; software, M.Z.; validation, M.Z., C.C., T.W., and S.M.; formal analysis, M.Z.; investigation, Z.B.; resources, J.H.; writing--original draft preparation, M.Z.; writing--review and editing, J.H., and C.C.; supervision, M.Z. and C.C.; project administration, J.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 52074145; and the Liaoning Revitalization Talents Program, grant number XLYC2002110.  
Not applicable.  
The data that support the findings of this study are available from the corresponding author upon reasonable request.  
The authors declare no conflict of interest.