## 4 Conclusions  
Both hydraulic pressure regulation and mechanical pressure regulation could achieve controllable proactive regulation effects on a steel sleeve's filler pressure. The detailed conclusion includes five parts as follows.  
1. By hydraulic pressure regulation, the linear and equally proportional variation in the filler pressure of the steel sleeve in response to external hydraulic pressure variation is observed. The pressure variation at the entrance point of the filler pressure monitoring cross section is 16-24% larger than that at the peripheral measuring points.
2. During mechanical pressure regulation, the pattern of its increase and decrease is obvious, the action mechanism is clear, and the pressure value is steady and controllable. Under the low internal pressure level (less than 0.15 MPa), the nonlinear variation in filler pressure is complicated by mechanical pressurization which could be divided into three stages. The filler pressure changed stably during mechanical pressurization and depressurization under a high internal pressure level (greater than 0.15 MPa).
3. The maximum stress of the new steel sleeve structure was 14.5 MPa under an elastic stress level. Variations in the jack reaction forces and cover plate displacement are stable, guaranteeing the safety of the steel sleeve structure.
4. Under the low internal pressure level (greater than 0.15 MPa), the operation of mechanical pressure regulation requires more prudential control. The slight slant of the cover plate should be dynamically revised by accurate adjustment of Jack pressure, and normal mechanical pressure regulation should implement after pressurization to 0.05 MPa via water injection. A simple and stable method involves regulating filler pressure using the hydraulic pressure regulation method. However, with the use of the mechanical pressure regulation method, faster regulation effects will be obtained with higher control precision. Two methods for regulating filler pressure should be selected according to practical operational capacity and pressure regulation requirements.  
Figure 11: Jack pressure-cover plate displacement curves.  
5. Under the proposed design scheme, the pressure in the steel sleeve is still slowly lost during pressure holding, and absolute sealing cannot be achieved in the whole pressure regulation process. The present solution is to compensate for the pressure according to the pressure monitoring result. The design scheme should be optimized from the view of seal ring material and seal structure design, and further related research work will be carried out. In addition, the current research is only on the principle and performance of the filler pressure regulation for the new steel sleeve structure, and the specific construction technology needs to be further studied in the engineering application.  
Conceptualization, J.Q. and Y.P.; Methodology, J.Q. and J.Z.; Validation, Y.P.; Writing--original draft preparation, J.Q. and Y.P.; Writing--review and editing, J.Z.; Funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript.  
This study was funded by the National Key Research and Development Program of China (Grant No. 2021 YFB2601100), the National Natural Science Foundation of China (Grant No. 2020J05145), and the Science and Technology R&D Program of China Construction Infrastructure Co., Ltd. (Grant No. CSCIC-2021-KT-02). The authors gratefully appreciate the support provided.  
Not applicable.  
The data presented in this study are available upon request from the corresponding author.  
The authors would like to acknowledge the administrative and technical support given by [PERSON] and [PERSON].  
The authors declare no conflict of interest.