# Full-Scale Model Test of Subway Contact Channel under Mechanical Construction +
## 4 Strain Results and Analysis  
### Strain on the Segment of Cutting Rings  
Figure 12 shows the variation of the steel bar strain of each test condition. In this figure, the time of each working condition is set as a time factor. Additionally, the initial stage time is 0\(-\)18.5; the pre-shoring stage time is 18.5\(-\)30.5; the cutting stage is 30.5\(-\)42; the shoring stage is 42\(-\)55. It can be seen that the change laws of the stresses on the internal and external arcs of the cutting ring were basically the same. In the initial working condition, the steel bars at the top and bottom of the cutting ring were under pressure and the steel bars at the internal arc were under tension. The steel bar strain increased step by step with the loading condition. After the first nine stages of loading, the steel bar strains on the inner and external arcs were almost constant during the maintenance of the pressure of each hydraulic cylinder. After the 10 th stage of loading was completed, the compressive strain first increased and then decreased before it stabilized in the end. In the pre-support condition, after the top internal support was fitted to the segment, the load increased step by step until the loading process was completed, and the tensile and compressive strains first decreased and then increased. In the cutting condition, the cutter head started to drill as the top thrust of the shield machine increased, and the steel bar compressive strain at the top and bottom positions of the segment of the outer arc gradually decreased before it tended to stabilize. In the support-dismantling condition, the steel bar strain at the top and bottom positions increased as the top of the internal support was unloaded step by step. The top of the segment tended to converge inward again before it became stable in the end. The test was conducted at the contraflexure points of N4\(-\)9 and N4-10. Until the end of the whole test condition, the tensile strain of section N4\(-\)9 fluctuated within 100 \(\mu\) \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Working Condition** & **Convergence Deformation** & **Second Ring** & **Third Ring** & **Fourth Ring** & **Fifth Ring** & **Sixth Ring** \\ \hline \hline Initial condition & Top position & 16.36 & 16.27 & 16.33 & 16.18 & 16.61 \\  & Waist position & 15.25 & 15.15 & 16.79 & 15.64 & 21.66 \\ Dismantling condition & Top position & 18.42 & 17.48 & 17.85 & 16.76 & 14.36 \\  & Waist position & 20.06 & — & — & 20.61 & 29.63 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Overall convergence deformation of the five\(-\)ring segment under all working conditions (unit: mm). Figure 11: Overall convergence deformation of the five\(-\)ring segment under all working conditions. and only showed a slight increase under the support-dismantling condition. In general, the overall variation was consistent with the mechanical law of external tensile stress and internal compressive stress, though there was a slight difference in the steel bar strains of the internal and external arcs of each tested section due to the imbalance in the forces exerted by the support system with pre-support construction. Figure 13 shows the variation of the concrete segment strain at different locations of the cutting ring. The figure shows that the initial concrete strain of the segment at the top position (No. HN4\(-\)2) was within \(-10\)\(\mu\varepsilon\). Then, the concrete strain increased step by step with loading under the initial condition until the concrete tensile strain of the external arc stabilized at around 22\(-\)25\(\mu\varepsilon\) after the completion of the 10 th stage of loading. In the pre-support condition, the concrete strain was basically stable. In the cutting condition, the concrete strain on the top external arc gradually increased with the increase in the thrust of the shield machine and was finally maintained at around 30\(\mu\varepsilon\).