# Criteria for Cutting Head Clogging Occurrence during Slurry Shield Tunneling
## 5 Discussions  
(1) Parameters influencing total thrust and cutting head torque of a machine  
The necessary total thrust and cutting head torque of a shield machine is bound up with its dimension, shape, structure, as well as the tunnel burial depth and the soils (soft, hard, or both) within the tunnel alignment. For a tunneling machine, its advance rate and cutting head rotation speed also contribute to the actual total thrust and cutting head torque on the machine. A higher advance rate and slower cutting head rotation speed mean a higher penetration depth that requires higher cutting head torque and bigger total thrust [16]. For a tunneling machine, the soils that will be excavated and the tunnel alignment are predetermined, and the only choice to change the total thrust and the cutting head torque is to alter the penetration depth per revolution by changing the advance rate and the cutting head rotation speed. Moreover, when analyzing variations of the total thrust and the cutting head torque within a small tunneling range, considering the ground conditions, i.e., the soils and the tunnel burial depth to be constant in uttermost cases is reasonable. Therefore, when referring to alternations of the total thrust and the cutting head torque of a tunneling machine, the parameter of penetration depth per revolution does play a vital role and must be considered. (2) Performance of machines due to cutting head clogging  
As mentioned above, the total thrust and the cutting head torque of a machine have something to do with the machine structure, particularly the cutting head structure. With the excavated soils flowing into the excavation chamber through the cutting head opening, the shield machine advances with driving parameters fluctuating within a normal range. For a machine with cutting head clogging, the opening area ratio of the cutting head declines, even to zero, causing difficulty in entrance into the excavation chamber of the excavated soils and causing the rapid increase in the total thrust and the cutting head torque of the machine. The scenarios were found at the Beijing south-to-north water diversion auxiliary project and presented in Figure 4.  
When the cutting head opening is partially but not totally blocked, the excavated soil can still enter into the excavation chamber but at a slower speed. In this case, the total thrust and the cutting head torque might not experience rapid increases but retain normal fluctuations. The most influential is the decrease in penetration depth per revolution, which is the quotient of the advance rate and the cutting head rotation speed. This scenario happened at the Wuhan Metro Line 8 Yangtze River Crossing tunnel construction and is presented in Figures 16 and 17. That is to say that the total thrust and the cutting head torque of a tunneling machine perhaps do not rise rapidly and remain basically unchanged if its cutting head opening is only partially blocked. The results are heavily dependent  
Figure 18: Changes of the two composite parameters at the Wuhan Metro Line 8 Yangtze River Crossing tunnel construction. on the resulting penetration depth per revolution. The parameter of penetration depth considered, two composite parameters (total thrust/penetration depth and cutting head torque/penetration depth) are used to characterize the load development on the tunneling machines. The re-plotted curves are listed in Figures 7, 12 and 18. From these curves, the occurrence of cutting head clogging can be more easily identified, and the effect on account of the countermeasures taken to cope with the cutting head clogging is also more clearly displayed. Most importantly, the curves concerning the total thrust and those of the cutting head torque are more alike. A single curve is sufficient to determine whether cutting head clogging occurs or not.