## 2 Test Design
### Monitoring Scheme  
The tests aim to investigate the influence of different types of openings on the mainline shield tunnel. In the tests, the deformations of the main tunnel, dislocations between adjacent rings, stress states around the openings, and the external earth pressures on the main tunnel were monitored for different opening types and different overlying soil depths. Detailed monitoring schemes are as follows, and the overall monitoring scheme is shown in Figure 6.  
Figure 5: Three openings on the model tunnel.  
(1) Earth pressure. To investigate the external load on the mainline tunnel from the surrounding soil, three earth pressure sensors were arranged on the outer surface of the tunnel to monitor the vertical pressures at the vault and the invert, and the lateral earth pressure at the hance of the tunnel lining. The results of earth pressure can reflect the effect of soil arching and guide structural mechanics analysis. The monitoring section is set far away from the openings, as shown in Figure 6a, considered not to be affected by the openings.  
(2) Structural deformation. Laser displacement sensors are arranged inside the model tunnel. The vertical deformation of the tunnel can be obtained by monitoring the vertical displacement of the vault and the invert, while the horizontal deformation can be obtained by monitoring the horizontal displacement of the tunnel. Due to the limitation of the internal space of the model tunnel, the number of laser displacement sensors, and the monitoring channels, the vertical and horizontal deformations are monitored close to the small circular opening, whereas only the vertical deformation is monitored for the other three sections.  
Panasonic HG-1050 and HG-1100 laser sensors are employed. The HG-1050 laser sensor has a measuring range of 30 mm, a monitoring distance of 35-65 mm, and an accuracy of 30 \(\upmu\)m; the HG-1100 laser sensor has a measuring range of 70 mm, a monitoring distance of 65-135 mm, and accuracy of 70 \(\upmu\)m, meeting the requirements of test accuracy. The laser sensors need to be fixed inside the tunnel structure through combined alloy support. The layout of the laser sensors is shown in Figure 6b-d.  
(3) Dislocations between rings. Openings on the shield tunnel for cross passages will weaken the lining stiffness at the opening position, and then induce uneven deformation  
Figure 6: Schematic diagram of monitoring scheme: (**a**) layout of the openings, strain gauges, and earth pressure sensors; (**b**) layout of the laser sensors and support in the front view; (**c**) layout of the laser sensors and support in the side view for SC-Op; (**d**) layout of the laser sensors and support in the side view for other sections.  
between adjacent rings, causing dislocations. Up to now, there are few published appropriate approaches to monitor the dislocation between segments in the previous scaled shield tunnel model tests. In the tests, an indirect method was used to measure dislocations between adjacent rings. As shown in Figure 7, an aluminum sheet is arranged across the seam between two adjacent rings, as the two ends of the aluminum sheet are pasted on the adjacent segments, while the middle of the aluminum sheet can deform freely. When a dislocation occurs, the strain sensors attached to the aluminum sheet can monitor the length changes, so the corresponding dislocation amount can be calculated based on the geometric relationship. The tests monitored the dislocations close to the three openings, with the specific locations shown in Figure 6a.  
(4) Structural stress. Previous studies have demonstrated that the influence of the opening on the mainline tunnel stress is concentrated at the opening position and 1-2 rings nearby [12; 17]. Considering the symmetry, for each opening, three couples of strain gauges (one inside and one outside) are arranged longitudinally along the tunnel, to analyze the stress concentration degree and influence range, as shown in Figure 6a.