A new method of controlling excavation face balance by using cutter torque under the condition of a small head aperture ratio

Abstract

When the penetration and head aperture ratio of a cutter head are small, much earth will be clamped between the cutter head and the excavation face. This clamped earth directly balances most of the pressure from the excavation face, instead of the slurry pressure in the head chamber. In this case, some operators of shield machines and large pipe jacking machines in China have begun to use a new method, through which the earth pressure balance state can be controlled by the cutter head torque, instead of the slurry pressure in the head chamber. In this study, we investigate the formation process and stress state of the clamped earth between the cutter head and the excavation face. Contrary to common explanations, our research shows that the cutter head torque is not caused by the “earth–steel” frictional force between the cutter head plate and excavation face. The average frictional force between them can be close to zero in some situations. The cutter head torque is usually caused by an “earth– earth” friction between the clamped earth and excavation face. We also treat the clamped earth as plastic soil, which flows from the excavation face, via the section filled by the clamped earth, to the head chamber. In this way, the relationship among the cutter head torque, earth pressure, and excavated velocity is determined. Results indicate that controlling the cutter head torque is equivalent to controlling the state of the clamped earth. This method can avoid the uncertainty and fluctuation of friction on the shield shell and shield correction.