## 1 Introduction
### Structural Design of Three-Dimensional Borehole Full-Stress Monitor  
As an innovative anchorage structure, the enlarged head bolt has realized the technical breakthrough of the traditional bolt through its unique force transmission mechanism, which is defined as the \"bearing-friction\" composite bolt by the academic community [1]. The bearing capacity of the structural system is characterized by the synergistic effect of dual mechanical mechanisms: one is the end bearing resistance provided by the soil at the enlarged head section and the other is the bond friction resistance generated by the contact interface between the anchorage body and the stratum. The mechanical model is shown in Figure 1, where L1 is the anchorage section, L2 is the enlarged head section, and L3 is the expansive body section. Based on a large number of experimental studies and engineering practices, it is shown that the soil end bearing resistance plays a dominant role in the bearing capacity of the composite anchor. Therefore, how to effectively stimulate and make full use of the soil end bearing resistance at the variable cross-section of the enlarged head becomes the key technology to improve the uplift bearing performance of this kind of anchor. This mechanical property not only reflects the essential difference between the enlarged head bolt and the traditional bolt but also provides an important research basis for further optimizing the design theory of the umbrella-shaped enlarged-head hollow grouting bolt with expansion pipe. When discussing the calculation of the end pressure of the enlarged head bolt, the primary task is to deeply analyze its potential failure mode. According to the existing research [30; 31], the failure modes of the enlarged head bolt can be summarized into three categories: one is the failure of the rod body itself, which is manifested as the fracture of the steel strand or the yield of the steel bar; the second is the failure of the gripping force between the grouting body and the rod body in the enlarged head section, resulting in the rod body being pulled out from the enlarged head; the third is to expand the damage of the soil around the head end. In view of the fact that the first two failure modes can be relatively easily controlled by optimizing the structural parameters of the anchor, this study will focus on the third failure mode, that is, the failure of the soil around the enlarged head. Based on the mechanical model of a hollow grouting bolt with umbrella-shaped expansion head with expansion pipe shown in Figure 1, the mechanical behavior process of the model is systematically studied, as shown in Figure 2. The mechanical process can be divided into three stages: static earth pressure stage, transition stage, and plastic zone compaction-expansion stage. In the initial stage, that is, the static earth pressure stage (as shown in Figure 2a), when the external tension applied to the bolt is small, the mechanical response of the bolt system is mainly characterized by the following characteristics: the side wall surface of the ordinary anchoring section and the umbrella-shaped expansion head section jointly bear the friction resistance generated by the soil, and the end face of the umbrella-shaped expansion head section is subjected to the static earth pressure. At this stage, the overall displacement magnitude of the bolt is small, and the overall stress deformation depends on the frictional resistance between the anchorage section and the surrounding soil. The second stage is the transitional stage, and its mechanical behavior shows significant progressive characteristics. When the side wall resistance of the umbrella-shaped expansion head reaches the peak of the static friction resistance, with a continuous increase in the external load of the anchor rod, the umbrella-shaped expansion head begins to move significantly. In this process, the pressure on the front end of the enlarged head continues  
Figure 1: Mechanical model of umbrella-shaped expansion head expansion bolt.