## 6 Conclusions  
In this study, a group of in-situ tests including the SPT, CPT and SLT were carried out to investigate and evaluate the UBC of a single PHC pipe pile embedded in saturated coarse sand and medium sand. Based on the analysis of the test results, the main findings specific to the soil layers and in-situ tests for PHC piles can be summarized as follows:  
1. For in-situ SPTs, three calculation methods, including the JGJ94-2008 method, Meyerhof method, and Schmertmann method, were adopted to calculate the UBC of a single PHC pipe pile based on in-situ SPTs. The calculated results of the three methods were similar, while the calculated UBC values were approximately 1.17-2.35 times and 1.15-3.24 times as much as the recommended values in the survey report for long piles with a length of 21 m and short piles with a length of 12.5 m, respectively.
2. The CPT with a double bridge probe was conducted to obtain the cone-tip resistance and sleeve friction close to SPT test points. The results were similar to the results calculated with in-situ SPTs, and the average value was 1.57 times and 1.81 times bigger than the value recommended in the survey report for long PHC pipe piles embedded in medium sand and short PHC pipe piles embedded in coarse sand, respectively.
3. The SLTs were conducted for three types of PHC pipe piles. It shows that the UBC based on SLTs was similar to the results acquired with SPTs and CPTs. However, it was 2.01 times and 2.24 times as much as the values recommended in the survey report for long and short PHC piles, respectively.
4. The conservative values recommended in the survey report were also calculated by the JGJ94 method based SPTs in the survey. The SPT blow counts (SPT-N) based on in-situ SPTs increased dramatically compared to the blow-count values in the survey report, especially in saturated sand layers at the position of 10 m below the ground. It might be caused by bentonite mud technology, which plays a significant role in preventing the drilling wall from collapsing.  
Figure 12: Penetration depth versus the blow count (N) of the standard penetration test (SPT) (SPT-N).  
Several evaluation methods were adopted and carried out in this study to study the UBC of a single PHC pile embedded in saturated sand layers. The application of bentonite mud technology in SPTs in a saturated sandy layer with cohesionless characteristics can provide more reliable parameters for evaluation of the UBC of a pipe foundation in the survey phase. CPTs can provide reliable results regardless of soil characteristics and groundwater as long as the soil layer could be penetrated. SLTs are necessary to accurately determine the UBC of pile foundation in complex stratum.  
Conceptualization, Y.W.; methodology, Y.W.; software, Y.W., J.L. ([PERSON]); validation, J.L. ([PERSON]), formal analysis, Y.W.; investigation, Y.W., J.L. ([PERSON]), Z.K.; resources, D.W., J.L. ([PERSON]); writing-original draft preparation, Y.W., J.L. ([PERSON]), Z.K., T.W.; supervision, D.W., Y.J.; project administration, D.W.; Founding acquisition, Y.W., Y.J. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors would like to express their special thanks to AVIC Institute of Geotechnical Engineering Co., LTD. for allowing the field test and these data obtained on the site to be published. The research work described herein was also founded by the China Postdoctoral Science Foundation (Grant Nos. 2020M670604), the National Key Research and Development Program of China (Grant Nos. 2017 YFC0805008), and the National Nature Science Foundation of China (NSFC) (Grant Nos. 41790434).And the APC was funded by (2020M670604). The financial supports are gratefully acknowledged.  
The authors declare no conflict of interest.