## 3 Results and Discussion  
### Steel Sleeve Pressure-Holding Sealing Performance  
Figure 4 shows that the internal hydraulic pressure increases stably and rapidly during the whole pressurization process. Pressurization via water injection was initiated, the internal hydraulic pressure increased gradually, and the rate of increase gradually accelerated and stabilized at -0.6 kPa/s when the hydraulic pressure reached 0.075 MPa. The different rates of increase probably occurred because the cover plate slid slightly upward under hydraulic action and the jack at the top gradually pressed closer to the reaction force frame in the initial pressurization stage by water injection. In addition, the hydraulic pressure in the steel sleeve showed a slow declining trend in the pressure-holding test.  
The internal pressure was generally lower than 0.2 MPa in the steel sleeve receiving process. Hence, the test data in the pressure-holding stage under different hydraulic pressures were selected for analysis. The steel sleeve hydraulic pressure loss time curves (Figure 6) were plotted to identify the hydraulic loss within 3 min. As a result, the hydraulic pressure in the steel sleeve was subjected to loss in the pressure-holding stage. The hydraulic pressure decreased rapidly within the first 10 s of pressure holding, accounting for ~40% of the total hydraulic pressure loss. Subsequently, the hydraulic pressure loss rate quickly decreased and stabilized at approximately 1 min, while the internal hydraulic pressure continuously decreased. The greater initial hydraulic pressure resulted in a higher hydraulic pressure loss rate, while the absolute hydraulic pressure loss under each initial pressure level was always lower than the corresponding magnitude of the pressure value.  
The hydraulic pressure loss curve inside the steel sleeve within 12 min under an initial hydraulic pressure of 0.205 MPa is shown in Figure 7. As a result, after 3 min of pressure holding, the hydraulic pressure loss shows a linear growth trend, and the pressure loss rate was approximately 0.16 kPa per min, which accounted for only 0.08% of the initial pressure. Therefore, the filler pressure loss inside was stable and sustained at a low speed, and it could be naturally eliminated with normal filler pressure regulation during shield receiving. This would generate no adverse effects on the internal filler pressure regulation.  
Figure 6: Steel sleeve hydraulic pressure loss time curves (pressure holding time: 3 min).  
Figure 8 shows the relationships between the internal hydraulic pressure and the stress on the steel sleeve body in the pressurization stage. The results show that the circumferential stress was always eight times the longitudinal stress. Except for the A-1 monitoring section at the bottom, the longitudinal stress was similar to the circumferential stress at other positions. Moreover, the pressure in section A-1 was much smaller than that in the other sections, which was probably because the internal pressure induced by the hydraulic pressure at this position was borne by the nearby sealing steel plate at the bottom. During the test, when the hydraulic pressure in the steel sleeve reached its maximum value (0.205 MPa), the maximum tensile stress and maximum compressive stress were 14.5 MPa and 2.5 MPa, respectively. The maximum stress is only 6% of yield strength suggesting a low elastic stress level. The design of the shield-receiving steel sleeve in practical tunneling is conservative, and the thickness of the steel sleeve body should be decreased according to the test result of stress and expectant safety.  
Figure 8: Hydraulic pressure stress on the sleeve body curves. (**a**) Circumferential stress; (**b**) Longitudinal stress.  
Figure 7: Steel sleeve hydraulic pressure loss time curve (pressure holding time: 12 min).