# Full-Scale Model Test of Subway Contact Channel under Mechanical Construction +
## 4 Strain Results and Analysis
### Segment Strain of the Semi-Cutting Ring  
Figure 14 shows the variation of the steel bar strain at different locations of the semi-cutting ring. Figure 14a shows the consistency of the overall changes in the strain of the steel bar at the top position of the semi-cutting ring. In the initial working condition, the external arc steel bar at the top position of the segment gradually showed a compression trend, whereas the internal arc steel bar was still under tension conditions. In the pre-support condition, the strain on the tensile and compressive sides showed a decreasing trend. In the cutting condition, the tensile and compressive strains decreased first and then increased. At this time, the steel bar strain of the external arc gradually became tensile.  
Figure 14b shows that in the initial working condition, the steel bar strain of the external arc was gradually under tension conditions and it tended to be stressed by tensile to compressive conditions, but the compression was not obvious. In the pre-support condition, the strain of the internal and external arcs gradually increased because of the back support. In the cutting condition, the strain value did not change significantly. In  
Figure 14: Variation of the steel bar strain at different positions.  
Figure 13: Variation of the concrete segment strain at the top and bottom positions.  
the support-dismantling condition, the strains in the internal and external arcs varied considerably but then stabilized gradually. As the internal support system was not yet unloaded after the cutting condition, the system still provided support to the segment. Thus, the steel-ring secondary lining composite segment was stressed significantly. Therefore, when the internal support is unloaded by grading, it is necessary to wait for each unloading to be stable before proceeding to the next level of operation, to which attention should be paid in the construction operation.  
Figure 15 shows the variation of the concrete strain at different locations of the semi-cutting ring. In Figure 15a, the trend of the concrete strain variation at the top of the steel-ring secondary lining composite segment was relatively consistent. In the initial condition, the concrete segment strain at the top position increased step by step with loading. In the pre-support condition, the strain value remained stable during the process and then increased continuously in a negative direction after the condition was completed. In the cutting condition, the strain was basically stable. In the support-dismantling condition, the strain value increased positively and then gradually stabilized with the load.  
As can be seen from Figure 15b, the initial value of the concrete segment strain ranged from \(-20\) to \(0\)\(\mu\epsilon\) and then increased step by step with loading. In the pre-support condition, the concrete segment strain at the waist position increased first and then decreased with slight compression, and it was mainly under tension conditions in the later stage. In the cutting condition, the concrete segment strain basically became stable. In the support-dismantling condition, the strain at the waist position significantly increased before it gradually stabilized in the end.