## 3 Engineering Application
### Contact Pressure between Initial Support and Secondary Lining  
The vibrating wire pressure box is used to measure contact pressure between the primary support and the secondary lining. The instrument and on-site installation photos are shown in Figure 13. After long-term tracking measurement, the contact pressure time-history curves of the primary support and secondary lining of ZDK41+943, ZDK42+092, and ZDK42+330 sections are obtained, as shown in Figure 14.  
Figure 12: The design procedure of tunnel lining by water pressure reduction coefficient method.  
Figure 13: Field installation of contact pressure sensor: (**a**) earth pressure cell; (**b**) instrument installation.  
ZDK41+943 and ZDK42+330 sections are in class IV surrounding rock sections. After the second lining construction is completed, it does not bear excessive load immediately, the early contact pressure of each part of the lining is small, and the contact pressure at this stage shows great fluctuation. After reaching stability, the contact pressure at the arch crown and waist is large, basically 30-50 kPa, and the contact pressure at the arch shoulder is very small, all below 10 kPa. ZDK 42+092 section is located in class V surrounding rock section. Within a few days after constructing the secondary lining, the secondary lining bears a large load. Similarly, due to the influence of construction, it shows great fluctuation in the early stage and reaches a stable state after 1-2 months. After the contact pressure is stable, the maximum value appears in the right arch waist, reaching 131 kPa, followed by the arch crown and left arch waist, 94.1 kPa and 58.8 kPa, respectively.  
Through comparison, it is found that the secondary lining of grade IV surrounding rock section bears a small load and reaches stability after a long rising period. The secondary lining of the grade V surrounding rock section bears a large load, which rises very fast. It can be seen that in the class IV surrounding rock section, the initial support can play a better supporting role. The load is gradually transferred to the secondary lining with its slow deformation. It is worth noting that the secondary lining load obtained from field monitoring is less than the calculated value in the specification. Therefore, it is conservative to obtain the secondary lining load through the specification.  
Compared with the calculated surrounding rock load in Table 8, the on-site monitoring value of surrounding rock pressure shows that the surrounding rock load of surrounding rock IV is small, the vault is only about 28% of the calculated value of surrounding rock  
Figure 14: The contact pressure between surrounding rock and initial support: (**a**) the time-history curve of the contact pressure of the monitoring section ZDK41+943; (**b**) the time-history curve of the contact pressure of the monitoring section ZDK42+092; (**c**) the time-history curve of the contact pressure of the monitoring section ZDK42+330; (**d**) the contact pressure distribution along the section.  
load, and the left arch waist is the closest, accounting for about 78%. The surrounding rock load of class V surrounding rock is large, about 48% at the vault and about 219% at the waist of the right arch. The calculation assumptions of surrounding rock load are ideal conditions, but the surrounding rock does not meet the ideal assumptions of the theory. Therefore, there is a deviation between the calculated value of surrounding rock pressure and the measured value. On the other hand, the embedding method, location and quality of monitoring instruments may affect the measured value of surrounding rock pressure.