## 3 Monitoring and Analysis of Subsidence in Shield Tunneling Under-Passing the Bridge and Culvert
### Monitoring and Analysis of Ground Subsidence
#### 3.2.2 Monitoring and Analysis of Longitudinal Ground Subsidence  
The curve shown in Figure 12 demonstrates the surface subsidence at longitudinal measuring points under different soft-rock height ratios during the shield-tunneling process. Moreover, the measuring point has an obvious heave in the range of 15 to 20 m away from the heading face owing to the effect of tunneling pressure, which belongs to the negative stratum loss. At the site 5 m before the excavation face, the ground surface  
Figure 11: Transverse surface subsidence curve.  
deformation of the measuring point changes from heave to subsidence, presenting an accelerating trend. The ground surface deformation in this section is subsidence deformation resulting from the structural gap of the shield tunnel. Within 5 to 10 m behind the excavation face, the ground subsidence changes remarkably, with a maximum settlement value of 1.6 mm. This is a normal stratum loss. When the shield tunnel is excavated 20 m ahead of the measuring point, ground subsidence at the measuring point is no longer affected by shield excavation. Longitudinal subsidence is significantly affected by the soft-rock height ratio, and its ground subsidence increases with the increasing soft-rock height ratio. The longitudinal ground subsidence curves of the tunnel sections with varied soft-rock height ratios were compared, and the results showed that with an increase in the soft-rock height ratio, the surface settlement value of 5-10 m before the excavation became smaller. The subsidence rate of the ground behind the excavation face increased with the increasing soft-rock height ratio. In general, the degree and scope of influence of the shield tunnel on the longitudinal ground subsidence increased with the increasing soft-rock height ratio.  
The curve in Figure 13 is the final subsidence value of each longitudinal monitoring point in the left tunnel. The monitoring points within 4-12 are the points of the shield tunnel under-passing the bridge and the culvert area of the highway. The geological map below the curve shows that the excavation face where the monitoring points 1 to 8 were located is covered with soft rock. In the area, the maximum subsidence of 14.8 mm could be found at monitoring point 3. The subsidence value of monitoring point 7, which had similar geological conditions to monitoring point 3, was only 7.08 mm. This proves that the surface subsidence was well controlled after the stratum was strengthened based on the MJS method. The monitoring points 8-12 were in the upper-soft, lower-hard composite strata. As the soft-rock height ratio decreased along the tunnel driving direction, the subsidence value of the corresponding monitoring points also decreased. Because foundation reinforcement was not performed in the area after monitoring point 12, and the geological condition changed from hard rock to soft rock, the ground subsidence value increased rapidly and returned to the subsidence value before entering the reinforced area.  
Figure 12: Longitudinal surface subsidence curve.