## 3 Discussion of Analysis Results
### Chamber Pressure, Discharge, and Surface Settlement  
Figure 12 illustrates the variations in the monitored chamber pressure during the EPB shield advance. Each case shows a decreasing trend from the large initial chamber pressure, because the chamber is filled with prestressed balls. Figure 13 compares the average chamber pressures after the first 100 mm of advance, when the initial variations are stabilized. Contrary to the torque and thrust force results, the chamber pressure is maintained within the set operational range only under automatic control of the penetration rate and rotational speed of the screw conveyor (case 5). These results show that when tunnelling with insufficient chamber pressure (case 2), tunnel face instability and large surface settlement can be expected. Conversely, when the chamber pressure is large compared with the estimated pressure (cases 1, 3-5), heaving of the surface will occur.  
Figure 11: Comparison of average torque and thrust force during advance of the EPB shield after its first 100 mm advance: (**a**) average torque; (**b**) average thrust force.  
Figure 10: Measured torque and thrust force data during EPB shield tunnelling simulation: (**a**) torque; (**b**) thrust force.  
During simulations, particles inside the chamber are successfully discharged by the rotation of the screw conveyor. Figure 14 compares the discharged volume of particles and the discharge rate with time. As the screw conveyor is unfilled at the beginning of the analysis, there is a time delay in discharge data acquisition. The figure shows that the discharged volume and discharge rate are closely related to the rotational speed of the screw conveyor, with the latter being exactly proportional to the conveyor's rotation. That is, discharge rates of 0.004, 0.011, 0.02, and 0.04 m\({}^{3}\)/s occur at rotational speeds of 2, 5, 10, and 20 rpm, respectively. When the operational conditions are automatically adjusted, the discharge rate varies twice with changes in rotational speed of the screw conveyor (case 5). Figure 15 compares the total discharged volume after the TBM has advanced 300 mm. Although direct comparison of the discharged volume is impossible because of differences in the analysis time among the cases, it can be inferred that a large discharge will destabilize the tunnel face and cause settlement (case 2) and that a small discharge will induce surface heaving (case 3).  
Figure 12: Measured chamber pressure during EPB shield tunnelling simulation.  
Figure 13: Average chamber pressure during the EPB shield advance after the first 100 mm.  
Figure 16a,b give the transverse and longitudinal surface settlement trough after the completion of tunnelling. As expected from the chamber pressure data described above, heaving occurs due to excessive chamber pressure in cases 1, 3, and 4. In contrast, relatively large settlement with insufficient chamber pressure is observed in case 2. However, the settlement in case 5 is successfully suppressed compared with that in case 2. As noted above, these results can also be deduced from the discharged volumes of particles in Figure 15. Even though the settlement results are not exact, the numerical results confirm the significance of maintaining appropriate chamber pressure and discharge in controlling surface settlement. That is, both are key factors to guarantee safe tunnelling with an EPB shield.  
Figure 14: Measured discharged volume and discharge rate data during EPB shield tunnelling simulation.  
Figure 15: Comparison of discharged volume after the EPB shield has advanced 300 mm.