## 3 Results and Discussion
### Results and Analysis of the Mechanical Pressure Regulation Test  
As shown in Figure 10, the filler pressure increased with mechanical pressurization under a low internal pressure level (less than 0.15 MPa), which could be divided into three stages. When the jack pressure was lower than 40 kN, the filler pressure increased slowly. However, it increased quickly for jack pressures in the range of 40 to 45 kN. Finally, the filler pressure increased stably with the jack pressure at a gradually decreasing growth rate. During the whole pressurization process, the filler pressure at all measuring points followed change curves. The filler pressure at the peripheral measuring points overlapped the change curves, whereas it grew faster at the central measuring point. Figure 9: Hydraulic pressure-filler pressure curves. (**a**) Pressurization by water injection; (**b**) Depressionization by water discharge. A pressurization test under a high internal pressure level was conducted when the filler pressure reached a high level (greater than 0.15 MPa) (Figure 10). In this case, the difference between the initial value of the filler pressure at the central measuring point (1-2) and that at the peripheral measuring points (2-2, 4-2, and 5-2) was further increased. The filler pressure at the measuring points of the two peripheral sides (4-2 and 5-2) was different from that at the peripheral distant measuring point (2-2). During the test, the filler pressure at each measuring point showed a linear growth relationship as the jack pressure increased continuously from the initial pressure of 150 kN. When the jack pressure increased by 1 kN, the filler pressure at the peripheral measuring point increased by 0.2 MPa, and that at the central measuring point increased by 0.25 KPa. As shown in the unloading area of Figure 10, the filler pressure at each measuring point declined with the reduction in jack pressure, and the deceleration rate was inversely proportional to the jack pressure. The filler pressure at the central measuring point declined the fastest, followed by those at the two peripheral sides and the peripheral distant measuring point. Thus, the filler pressure difference between different measuring points was gradually narrowed during the depressurization process. Compared with the hydraulic pressure regulation process, the mechanical pressure regulation process was more complicated, especially the mechanical pressurization method using a low internal pressure level. During the mechanical pressure regulation, a proactive pressure was applied to the filler inside the steel sleeve by multiple jacks, but the filler was an inhomogeneous semifibtal mixture composed of sandy soil, water, and air. Under a low jack pressure, the filler was relatively loose and nonuniform, and the compressive deformations at the points of action of different jacks differed considerably. As a result, the cover plate was more prone to tilt and further contacted the inner wall of the steel sleeve. In this case, the partial jack pressure was directly transferred to the steel sleeve and effectively transferred to the filler. Thus, the filler pressure grew slowly during the test when the jack pressure was lower than 40 kN. Subsequently, as the jack pressure was increased, the pressure and displacement of each jack were dynamically adjusted, the deviation of the cover plate was corrected, and the partial pressure directly acting upon the sleeve was redistributed to the filler, which resulted in a sudden increase in filler pressure. The filler was gradually compacted after compression, exhibiting more stable and homogeneous mechanical properties, which facilitated the stable control of the pressure and displacement of each jack. Finally, the cover plate steadily applied pressure inside the sleeve, and the filler pressure stably changed with the jack pressure. Figure 10: Jack pressure-filler pressure curves. Under a low internal pressure level, the filler pressure was elevated with mechanical pressurization, which was manifested in three stages (Figure 10).