# Enlarged Head Pressure-Dispersed Anchor Cable for Foundation Pit Engineering Purposes
## 2 Analysis of the Force Mechanism
### Internal Force Analysis of the Anchor Cable  
Due to the particularity of the geometric size of the enlarged head pressure-dispersed anchor cable and the dispersion of the transmission path, the shear stress around the anchor cable exhibits different distribution forms with the increase in the pull-out resistance. The discussion is divided into three stages according to the displacement of the anchor cable and the distribution of shear stress. 1. Static earth pressure stage: In the initial stage of axial force growth at the anchor head and the anchor cable, there is only a relative displacement trend between the anchor solid and the soil, i.e., only the static friction force around anchor solid. It is pointed out in the relevant literature [24; 25] that the generation of passive earth pressure requires a certain amount of displacement, while the displacement of the anchor solids at this stage is very small. Therefore, the extrusion of enlarged head anchor solids by the soil in front of the end is the static earth pressure. Due to the small displacement of the anchor cable in this stage, the pull-out bearing capacity is mainly provided by the side friction of the ordinary anchorage section and the enlarged head anchorage section. 2. Transition stage: As the load continues to increase, the interface will plastically yield when the stress state at the interface between the anchor solid and the soil interface reaches the shear strength. We continue to increase the tension of the anchor cable. Due to the insufficient shear strength of the soil mass, the soil mass around the anchor solid is damaged and connected. The enlarged head anchorage section and ordinary anchorage section have obvious displacement. The pressure of the soil mass on the end face of the enlarged head gradually increases, and the soil mass in front of the enlarged head end forms a plastic zone in the local set. A further increase in the axial tension of the anchor cable leads to the compaction in the soil at the front of the variable section, gradually forming a plastic sphere space. At this moment, the  
Figure 2: Pile–anchor supporting structure. end pressure resistance becomes an important part of the bearing capacity, and the displacement growth rate in this stage is larger than that in the previous stage. Since the compression of the soil mass at the front of the enlarged head anchor cable is a progressive developmental process, the increase in the pull-out force at this stage is nonlinear. The displacement of the variable sections results in the extrusion of the front-end soil mass, which directly increases the end resistance. The soil mass at the front of the variable section is compressed all around, resulting in the compression of the soil mass at the front of a variable section and confined by the further soil mass and anchor solid. 3. Plastic development stage of soils in front of the end: With an increase in the tension, an obvious \"end-pressing inflection point\" appears on the tension F (the sum of axial force on the anchor head and anchor cable)-displacement S (the front end of the anchor solid) curve. Subsequently, the soil mass in the plastic zone becomes further compressed by the enlarged head and the surrounding soil mass, as shown in Figure 3. When the anchor cable is subject to tension F and produces displacement or deformation, the variable section of the enlarged head anchor cable will produce end resistance \(\sigma_{1}\), and shear stress \(\tau_{\text{f}}\) will be generated between the soil and the anchor cable. The soil mass in the plastic zone is further squeezed by the enlarged head and the surrounding soil mass and further enhances the pressure \(\sigma_{2}\) of the soil mass on the enlarged head. The sliding surface of the soil mass in the front of the enlarged head no longer occurs along the grout-soil interface but moves outwards. Soil in the plastic zone undergoes a plastic expansion and internal force rebalancing at the same time.