## 3 Test Results
### Formation Stress Gradient Rule  
(1) Gradient rule of formation stress under no train vibration load  
The study of the gradient law of the internal stress in the tunnel face caused by cutter head thrust during the shield driving process can provide an important basis for understanding the correlation between shield driving and stress formation. Figure 21 shows the gradient rule of stress on the face of the tunnel without a train vibration load under different processes. With the continuous advancement of the horizontal servo actuator, the pressure plate of the force transmission rod gradually moves towards the tunnel excavation direction face, and the soil surface of the tunnel face is gradually compressed; the influence range of the pressure inside the formation becomes more and more obvious. When the transmission rod is moved to 19 mm, the soil mass in the initial formation is relatively loose, which is mainly due to the compression and deformation of the loose soil, and the internal stress is small. With the process of the transmission rod moving in, the internal stress increases to 38 mm, and the maximum stress is mainly concentrated in front of the tunnel face. The maximum stress range extends to about 1 times the diameter of the tunnel at the top of the tunnel face, and when the process increases to 76 mm and 95 mm, the stress range extends to 2 times the diameter of the tunnel along the driving direction and 1.2 times the diameter in the vertical direction.  
Figure 21: The stress gradient of the tunnel face without train vibration load in different processes: (**a**) 0 mm; (**b**) 19 mm; (**c**) 38 mm; (**d**) 57 mm; (**e**) 76 mm; (**f**) 95 mm.  
(2) Gradient rule of formation stress under train vibration load  
As can be seen from Figure 22, with the continuous advance of the horizontal servo actuator, the pressure plate of the force transmission rod gradually advances towards the tunnel excavation direction face, and the soil mass on the tunnel face is gradually compressed; the influence range of the pressure inside the formation becomes more and more obvious. Meanwhile, the stress phenomenon caused by the train load also appears in the upper stratum. When the rod is moved to 28 mm, due to the loose soil in the initial formation, the internal stress is small, but the train load causes a significant wavy stress range in the upper formation, which increases to 56 mm with the moving process of the rod, and the maximum stress appears in the range of one times the diameter in the horizontal direction and two times the diameter in the vertical direction in front of the tunnel face. When the process is further increased to 84 mm, the stress only increases along the horizontal direction, and the stress caused by vibration load becomes weak. When the process is increased to 112 mm and 140 mm, the stress range increases along the horizontal and vertical directions, and the stress caused by train vibration load also increases at 140 mm.  
Figure 22: The stress gradient of the tunnel face under train vibration load in different processes: (**a**) 0 mm; (**b**) 28 mm; (**c**) 56 mm; (**d**) 84 mm; (**e**) 112 mm; (**f**) 140 mm.