## 7 Conclusions  
According to the basic principle of similarity model tests, a large-diameter composite shaft lining model test was reasonably designed. After practical engineering application and analysis of field measurements, the following main conclusions were obtained:  
1. Under the hoop constraints of the inner and outer steel plate, the ultimate strain of the specimens of the dual-steel-plate-confined high-performance composite shaft lining structure reached \(-3106\)\(\mu\varepsilon\), \(-3070\)\(\mu\varepsilon\), \(-2820\)\(\mu\varepsilon\), and \(-3680\)\(\mu\varepsilon\), respectively, demonstrating that the brittleness of the concrete was improved considerably.
2. The model test results showed that the compressive strength of the concrete in the dual-steel-plate-confined high-performance composite concrete shaft lining structure for large-diameter drill shafts increased by a factor of 1.97-2.52, resulting in a considerable increase in its bearing capacity.
3. The model test results showed that the maximum thickness of the dual-steel-plate-confined high-performance composite concrete shaft lining structure could be 800 mm to satisfy the safety requirements of the control layer of the intake shaft in the west zone of Zhangji coal mine. In the end, the project was recognized as a new record for constructing a large-diameter shaft using the drilling method.
4. The field measurement data showed that the hoop strain of the concrete layer in this new type of shaft lining structure was \(-487\)\(\mu\varepsilon\), far smaller than the ultimate strain of C65 concrete. This result demonstrates that this shaft lining structure is safe and reliable under typical working loads.  
The main purpose of the study was to solve the engineering technical problem of a shaft lining with large diameter using the drilling method. Next, we will systematically study the influence of factors such as the diameter and thickness of the shaft lining, the thickness of the outer and inner steel plate, strength of concrete, and size of the anchor by combining numerical calculations and model testing, so as to find the optimal design parameters. In future research, optical fiber sensors and other advanced test elements will be used to monitor the strain of the shaft lining model. Then, the stress and deformation characteristics of the cross section of the shaft lining model will be analyzed systematically, and the mechanical mechanism will finally be revealed completely.  
Structural model testing is a research method used to solve support problems in underground engineering. Usually, the geometric similarity ratio is 5-20. The larger the similarity ratio, the larger the size effect error. However, when the geometric similarity is small and the model specimen is large, the error is usually within the allowable range for engineering. The geometric similarity ratio of the shaft lining model test was 10.7, which belonged to a larger structure model, so the test error was small. In future research, experimental research with different similarity ratios should be carried out to obtain the size effect rule.  
Conceptualization, Z.Y. and P.Z.; methodology, Z.Y.; software, W.X.; validation, Z.Y., H.C. and W.X.; formal analysis, X.L.; investigation, P.Z.; resources, H.C.; data curation, W.X.; writing--original draft preparation, P.Z.; writing--review and editing, Z.Y.; visualization, X.L.; supervision, Z.Y.; project administration, W.X.; funding acquisition, H.C. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Anhui University discipline professional talented person (No. gxbjZD09), the Science Research Foundation for Young Teachers in Anhui University of Science and Technology (QN201710), and Anhui Provincial College of Natural Science Research Key Project (KJ2018A0098).  
The authors declare no conflict of interest.