## 5 Field Application of Active Bearing of the FSP
### Application Effect Analysis of the FSP  
To verify the application effect of the active bearing of FSP in tunnel deformation control, deformation monitoring was carried out with the help of a total station, and the variation law of vault settlement with time under the support of FSP was analyzed, as well as the condition without FSP.  
After continuously monitoring the deformation of the arch frame with active bearing and the adjacent arch vault area without active bearing for two weeks, it was found that the amount and rate of the settlement of the arch vault were significantly reduced. The comparison of specific monitoring data can be seen in Figure 24.  
Figure 23: Site drawing of active loading of FSP on the upper bench of the first and second arches.  
Figure 21: Construction site of tunnel upper bench.  
Figure 22: Effect drawing of loading support installed on a tunnel arch.  
The excavation direction is from ZK280 + 575 to ZK280 + 590, and three sections were selected, among which ZK280 + 575 is the first section, which uses passive bearing and has the largest settlement value; ZK280 + 585 is the second section, which uses active bearing and has the smallest settlement value; ZK280 + 595 is the third section. The passive load was used, but it was affected by the active load effect of the previous section ZK280 + 585 and was close to the second section. The deformation value was less than 575, which is an intermediate value.  
Through the comparison of the settlement data of the arch vault, it can be found that after the active load is applied, the settlement and settlement value of the arch vault are significantly reduced within 1 to 3 days, and the settlement value is reduced by more than 50%. The main reason is that when the initial deformation of the surrounding rock just occurs, the FSP can provide more than 60% of the ultimate bearing capacity, and the deformation rate of the surrounding rock is limited obviously. With the continuous sinking of the arch vault, the lateral bearing stiffness of the steel pipe gradually decreases, and the vertical supporting capacity provided by the steel pipe gradually reaches the ultimate bearing capacity. Without other auxiliary measures, the deformation of the surrounding rock will not be effectively restrained until the surrounding rock pressure is slowly released through deformation and reduced to balance with the supporting force. When the middle bench is gradually excavated, the effect of the active bearing of the FSP on the upper bench is no longer obvious, but the overall settlement of the tunnel vault is smaller than that of the section without active bearing, with a reduction of about 2-5 cm, accounting for more than 20% of the total deformation during the excavation of the upper bench.  
Therefore, it can be seen that the active bearing of the FSP can reduce the deformation of the surrounding rock. However, due to the limited bearing capacity of the steel pipe itself, the optimal action time in deformation control is relatively short. If the closed loop of initial support can be completed as soon as possible, the effect of the active bearing of steel pipe to control surrounding rock deformation will be more obvious.