# Field Test Study on Vertical Bearing Characteristics of Coal Rock
## 5 Conclusions  
In this paper, three plate load tests and two bi-directional load tests were conducted, and the vertical bearing characteristics of the shallow foundation and the deep foundation on the coal rock were analyzed. The design parameters for practical use were obtained. Based on the test results and discussion, the following conclusions can be drawn.  
1. According to the results of the plate load test, the pressure-settlement curve of the coal rock is a steep-drop-type. The deformation modulus can be calculated from the Boussinesq solution. Under the working load, the deformation modulus is close to constant. Beyond the working load, the deformation modulus decreases with the increase in pressure, and the relationship between the two is an exponential function.
2. The bi-directional load test in which the load cell is placed at the bottom of the pile can be used to directly obtain the tip resistance-displacement curve and the side resistance-displacement curve, so as to obtain the load transfer function of the tip resistance and the side resistance.
3. The characteristics of the pile tip resistance of coal rock are: the tip resistance-displacement curve is steep-drop-type, and the distribution of the test points before failure is close to a straight line; the displacement value corresponding to the ultimate bearing capacity is about 1% of the pile diameter. The load transfer function of the unit tip resistance of coal rock is an ideal elastic-plastic curve. The deformation modulus can be calculated from the Mindlin solution. Under the working load, the deformation modulus is close to constant. Beyond the working load, the deformation modulus decreases with the increase in pressure.
4. The load transfer function of the unit side resistance of coal rock is a hyperbola.
5. According to the ideal elastic-plastic load transfer function of the unit tip resistance and the hyperbolic load transfer function of the unit side resistance, the equivalent pile top load-settlement curve is obtained, which is in good agreement with the results of the bi-directional load test method.  
The conclusions drawn from the study of the vertical bearing characteristics of the coal rock have certain engineering application value for the foundation design in similar projects. However, there are still some problems to be further studied: piles with different diameters can be tested to explore the size effect, which was not covered in this research, and other field test methods such as the Dynamic Cone Penetration Test can be used to establish the relationship between the field test parameters and the bearing capacity of the coal rock.  
Conceptualization, Y.M. and X.W.; data curation, Y.M.; writing--original draft preparation, Y.M.; writing--review and editing, Y.M. and X.W.; project administration, X.W.; funding acquisition, Y.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by General University Science Research Project of Jiangsu Province (22 KJD560002), and Science and Technology Project of Jiangsu Province Construction System (2020 ZD24). This work was also sponsored by High-end Research of Principal Investigator of Vocational College of Jiangsu Province (2022 GRGDYX007).  
**Acknowledgments:** The authors would like to express our gratitude to [PERSON] of Guizhou Sino Rock Engineering Testing Co., Ltd., Guiyang, China, and [PERSON] of Nanjing Dongda Bi-directional Pile Testing Co., Ltd., Nanjing, China, for their field-testing work. The authors would like to express appreciation to the editors and reviewers for their valuable comments and suggestions.  
**Conflicts of Interest:** The authors declare no conflict of interest.