## 4 Conclusions  
This study used discrete-element analysis to predict the excavation performance of a 7.73 m diameter EPB shield TBM under different operating conditions. The model of the machine comprises six parts--cutterhead, disc cutters, cutter bits, chamber, shield, and screw conveyor. It was simulated to advance through a ground model of size 1.5 m (width) \(\times\) 15.6 m (length) \(\times\) 14.6 m (height), comprising 400,000 spherical particles of diameter 100 mm, randomly generated and stacked by gravity, after placement of the TBM geometry. Together, the two models simulated excavation for 360 s at four cutterhead RPMs and three screw conveyor RPMs to determine the best operation conditions. The simulation showed that the lateral pressure acting on the side plate of the ground model increased almost linearly with the increasing depth of the plate. The ground also became less stable as the cutterhead RPM increased during excavation. The propagation of ground disturbance on the periphery of the cutterhead steadily increased as the cutterhead RPM increased. The particles' overall velocity in the chamber increased as the cutterhead RPM increased, with particles further from the chamber's center tending to move more quickly. The low flow of particles was expected in the center of the chamber. The number of particles in the chamber and the surface pressure on the chamber varied with the cutterhead RPM and screw conveyor RPM during the simulation. The ratio of compressive force and torque and the fluctuation of this ratio decreased as the cutterhead RPM rose. The overall compressive force acting on all disc cutters also increased linearly with the cutterhead RPM. The position of a disc cutter on the TBM \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Rank** & **Case Number** & **Rank** & **Case Number** & **Rank** & **Case Number** \\
**(From Lowest Standard** & **(Cutterhead** & **(From Lowest Standard** & **(Cutterhead** & **(Cutterhead** & **(Cutterhead** & **(Cutterhead** & **RPM/Screw RPM)** \\
**Numbers in Chamber)** & **RPM/Screw RPM)** & **Pressure in Chamber)** & **RPM/Screw RPM** \\ \hline \hline
1 & Case 8 & & & & \\ (2.0/9.0) & 1 & & & & \\ \hline
2 & Case 7 & & & & \\ (2.0/12.0) & 2 & & & & \\ \hline
3 & Case 10 & & & & \\ (3.0/12.0) & 3 & & & & \\ \hline
4 & Case 5 & & & & \\ (1.5/9.0) & & & & & \\ \hline
5 & Case 4 & & & & \\ (1.5/12.0) & 4 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Top five sets of optimal operating conditions sorted by three performance indicators. cutterhead had a strong influence, with higher compressive forces experienced by the cutters closer to the center. The impact wear on a disc cutter determined from the variation of normal accumulative contact energy depended on the cutterhead RPM and the cutter's position on the TBM. It clearly increased as the cutterhead RPM rose. Particularly, gauge disc cutters at the transition zone of the cutterhead showed more wear than those elsewhere, because of the large turning radius. The muck discharge rate was measured by a mass flow rate sensor. The average mass flow rate gradually increased as the cutterhead RPM increased. Additionally, the average driving power of the screw conveyor gradually increased as the cutterhead RPM increased. Overall, the cutterhead operating conditions affected the performance of the screw conveyor. Finally, comprehensive evaluation of TBM excavation under different simulation conditions led to the determination of the optimal operation conditions based on pressure balance and operational management of the TBM. Further work should focus on two aspects--soil conditioning and load (pressure) control; the excavated material properties generally change with soil conditioning. If the ground is modeled by considering the soil conditioning work and calibration work with relevant soil samples, it might improve the prediction of the excavation before the TBM starts tunneling. This study simulated excavation using only DEM, except for the overburden load control system.