## 5 Conclusions  
The pile end-bearing layer selected for this project was a strongly weathered sandstone, and both vertical static load testing of the monopile and pile stress testing were carried out. The results are shown below, combining the field tests and numerical simulation studies conducted on the pile side and pile end resistance.  
1. Under the same pile diameter and pile length, the single pile bearing capacity of the pipe pile by the inner digging method was 50% higher than that of the ordinary hammering method, and the enlarged part of the pile end had a significant effect on the improvement of the bearing capacity. Therefore, the bearing capacity of the structural foundation can be improved by using the inner digging method when the construction conditions allow.
2. The axial force of the middle-digging and bottom-expanding method pipe pile plays from top to bottom; when the top load is small, the axial force of the pile is distributed more evenly as the top load keeps increasing, and the axial force of the part of the pile near the surface increases obviously and decreases rapidly along the direction away from the surface, at this time the pile end force also keeps increasing, the pile side resistance and pile end resistance is an asynchronous process of play. The lateral resistance of the pile is more evenly distributed along the pile body.
3. When the top load of the medium-drilled pipe pile reaches the ultimate bearing capacity, the pile end-bearing capacity is strong, and the pile end resistance accounts for 40% to 50% of the pile top load. The pile will appear as an end-bearing friction pile or even as a friction end-bearing pile. In contrast, the higher pile end-bearing capacity helps to improve the pile side-bearing capacity, especially the lateral frictional resistance of the soil near the pile end. The piles have a high vertical load-carrying capacity.
4. The simulation results show that increasing the diameter of the enlarged head or enlarging the range of the enlarged head has a certain improvement effect on the bearing capacity and settlement of the pipe pile. The enlarged head can be increased from 1.2 times the pile diameter to 1.4 times or more by mechanical modification of the pile driver. The large diameter enlarged head is preferable for the actual construction of the pipe pile as far as the mechanical conditions allow.
5. When the pile is subjected to vertical load, it can be concluded that in different stages of the loading process, PHC pile inner diameter changes have different effects on the pile bearing capacity. When the pile side friction resistance is large in the early stage, the vertical bearing capacity of PHC pile with different wall thicknesses is not very different from other types of piles, and when the top pile load is large, the PHC inner diameter is too small to have a large effect on the pile bearing capacity, therefore, reducing the inner diameter of PHC pile within a certain range can improve the economy.  
**Author Contributions:** Conceptualization, Y.Z.; methodology, Y.Q.; software, Y.Q.; validation, Z.W.; formal analysis, Y.Q.; investigation, D.Y.; resources, J.S.; data curation, Y.Q.; writing--original draft preparation, Y.Q.; writing--review and editing, Y.Z.; visualization, Y.Q.; supervision, J.S.; project administration, J.S.; funding acquisition, J.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Written informed consent has been obtained from the patient(s) to publish this paper.  
The data used to support the findings of this study are available from the corresponding author upon request.  
The authors would like to express their sincere gratitude to Anhui Transport Consulting & Design Institute Co., Ltd. for their help in the course of the experiment.  
The authors declare no conflict of interest.