## 6 Conclusions  
As described in this paper, we carried out an experimental study on the displacement characteristics of the soil around the drill-expanded concrete pile. The VIC-3D non-contact full-field strain measurement system was used to record and dynamically observe the soil displacement characteristics around the pile under the load in real-time. The soil surface displacement field around the pile was obtained through digital image correlation technology, and then the displacement characteristics of the soil around the pile were analyzed. The following conclusions were drawn:  
1. There is a clear compression zone both in the undisturbed soil and sand under the expanded body, and the compression zone has a much higher soil displacement magnitude and density than other areas. In the undisturbed soil, there is an obvious separation area between the expanded body and the soil, which is not obvious in the sand, but there is a collapse area around the pile on the top of the sand.
2. In the undisturbed soil and sand tests, the displacement trend of the soil around the pile is basically the same. The closer the soil is to the expanded body, the greater the vertical displacement, and the variation of vertical displacement with the load is essentially the same as that of the pile. The horizontal soil displacement close to the expanded body first moves towards the pile body and then moves away from the pile body; the horizontal soil displacement far away from the expanded body moves away from the pile. The closer soil is to the pile bottom, the greater the vertical displacement under load. As the depth increases, the displacement gradually decreases.
3. Through numerical simulation, it can be found that the displacement of the soil around the pile is basically consistent with those of the model test, which shows that the model test results are reliable.
4. By using a half-face pile model test and digital image correlation technology, it is possible to measure the dynamic deformation process of the soil around the pile under load with greater accuracy, to quickly and effectively obtain the soil displacement field around the pile and to more intuitively display the dynamic changes of soil displacement around the pile with the load.
5. For clay, although the surface of the soil around the pile will not significantly collapse under the load, there is a clear separation zone between the soil and the top surface of the expanded body, and the lateral friction resistance of the soil in the  
Figure 18: Vertical displacement of different data points.  
separation zone will be reduced. Thereby, the bearing capacity will be affected; but for sand, the soil above the expanded body will move down with the pile under the load, forming a collapse on the surface. Those should be considered in actual engineering construction.  
Conceptualization, L.X. and Y.Q.; methodology, L.X. and L.N.; investigation, L.X. and H.D.; formal analysis, H.D. and D.S.; resources, L.X.; validation, L.X. and L.N.; writing--original draft preparation, L.X.; writing--review and editing, L.X. and Y.Q. 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 (No. 52008185 and No. 52078239).  
Not applicable.  
The data are contained within the article.  
The authors declare no conflict of interest.