## 3 Large Rock Particles and Flow Rate of the Carrier Slurry  
A segmental lining consisting of segment rings was employed in the construction. In terms of the ring number, the segment rings 1-740 were erected in the pebble layer, rings 741-1379 were located in the rock ground, and the remaining rings 1380-1416 were located in the sandy clay with gravel. Transporting the hard large particles caused severe damage and wear to the pipes and elbows/bends. The damage and wear were initially closely related to the physical and mechanical properties of the particles, such as their shape, size, and hardness, and were secondarily closely related to the in-pipeline movement of the particles. The first influencing factor was mainly determined by the excavated ground. Regarding the second factor, both the size and weight of the particles and the speed of the carrier fluid contributed to the movement of the particles in pipelines. Separated rock particles found at the separation plant at construction segment rings 9, 199, 499, 603, 649, and 729 are presented in Figure 4. The presented particles are generally rounded or subrounded, and the size of the large particles ranges from 3 to 7 cm. Separated rock particles found at rings 839, 1019, 1056, 1120, 1211, and 1216 are shown in Figure 5. As a result of rock cutting by disc rolling cutters, the large particles have sharp edges and a size of 2-5 cm.  
Figure 4: Separated rock particles when shield tunneling in the pebble layer. (The Chinese characters on the card are the date, ring number, time and geological type of the rock particles being recorded).  
The movement of the rock particles in pipelines is heavily dependent on the speed of the carrier fluid, whose average is the result of the flow rate divided by the internal cross-sectional area of the pipeline. The flow rate of the slurry discharge pipeline was adjusted to help maintain the stability of the cutting face in the varied ground during tunneling. As presented in Figure 6, a flow rate ranging from 7 to 10 m\({}^{3}\)/min was employed in the pebble layer, and the flow rate was decreased to 4-7 m\({}^{3}\)/min in the rock ground. Therefore, the speed of the rock fragments when tunneling in the rock ground was generally less than that of the rock particles in the pebble layer.  
Figure 5: Separated particles when shield tunneling in the rock ground. (The Chinese characters on the card are the date, ring number, time and geological type of the rock particles being recorded).  
Figure 6: Flow rates of the slurry discharge pipeline at different segment rings.  
To better form a filter cake and maintain the stability of the cutting face, a bentonite suspension was fed as the support media and accordingly played the role of a carrier to transport the excavated rock particles. The physical and mechanical properties of the carrier fluid were adjusted to adapt to the varied ground during tunneling. The carriers did influence the movement of the carried particles and the wear on pipelines associated with the particle movement. The slurry viscosity in terms of secondary and slurry specific gravity used in each segment ring is presented in Figure 7. Figure 7 shows that the specific gravity becomes significantly larger near rings 200 and 800. This is caused by the shield crossing a clay interlayer near ring 200 and reaching the fault fracture zone near ring 800, where the content of microscopic clay particles in the stratum increases. The slurry viscosity is measured by a Soviet funnel, and its magnitude depends on the amount of bentonite added and the microscopic particle content in the stratum and is therefore in dynamic fluctuation.