## 3 Engineering Application
### Stress of Circumferential Main Reinforcement of Secondary Lining  
The vibrating wire reinforcement stress meter is used for the stress measurement of circumferential main reinforcement of secondary lining. The instrument and on-site installation photos are shown in Figure 15. After long-term tracking measurement, the stress curves of secondary lining reinforcement at sections ZDK41+943, ZDK42+092, and ZDK42+330 are obtained, as shown in Figures 16-19.  
Figure 16: The time-history curve of reinforcement stress in the ZDK41+943 section: (**a**) reinforcement meter near the initial support side; (**b**) reinforcement meter away from the initial support side.  
Figure 15: Field installation of reinforcement stress sensor: (**a**) reinforcement stress sensor; (**b**) instrument installation.  
The curve shape of ZDK41+943 and ZDK42+330 sections of grade IV surrounding rock section is relatively similar. Among them, the stress of secondary lining reinforcement  
Figure 19: The reinforcement stress along the section: (**a**) reinforcement meter near the initial support side; (**b**) reinforcement meter away from the initial support side.  
Figure 17: The time-history curve of reinforcement stress in the ZDK42+092 section: (**a**) reinforcement meter near the initial support side; (**b**) reinforcement meter away from the initial support side.  
of the ZDK41+943 section is only about 5 MPa for a long time. Then, after a period of rising volatility, it gradually reached stability. The stress time history curve of secondary lining reinforcement at section ZDK42+330 changes evenly. After the two-section curves are stable, the reinforcement stress at the spandrel and waist is large, basically 30-60 MPa. The reinforcement stress at the vault is small, lower than 15 MPa. The section curve of ZDK42+092 of grade V surrounding rock section shows a stepped rise, stable after two rises. After the curve is stable, the stress value of the arch shoulder and arch waist is large, and the arch crown is small.  
The maximum stress of the reinforcement near the initial support side is at the left arch waist. The maximum value of the reinforcement meter on the side away from the initial support appears at the left spandrel, 227 MPa and 187 MPa, respectively. The stress of secondary lining reinforcement is consistent with the contact stress law. The secondary lining reinforcement in grade IV surrounding rock section only bears the internal force less than 60 MPa, far from reaching the yield stress of 400 MPa, and has a relatively slow rising period. The secondary lining reinforcement in grade V surrounding rock section has large stress, the maximum value reaches 227 MPa, and the stress value increases very fast. The field test of lining structure stress found that the stratum conditions greatly impact the stress of the lining structure. The stress of secondary lining in class V surrounding the rock section is much greater than in class IV. However, there is still a large strength reserve after the stress of secondary lining reaches stability. Because the axial force and bending moment are similar to that of plain concrete when the Load-structure method is used for secondary lining design, the stress of reinforcement is not studied. In this section, the stress monitoring of reinforcement mainly illustrates the safety of secondary lining reinforcement and verifies the rationality of the water pressure reduction coefficient by comparing the difference between the actual stress of reinforcement and its yield strength.