Characteristics of conditioned sand for EPB shield and its influence on cutterhead torque.

The cutterhead of an earth pressure balance (EPB) shield usually relies on a large torque when tunneling through a water-rich and high-permeability sand layer. Sandy soils may lead to severe wear of the cutter tools and make the screw conveyor vulnerable to spewing. In this study, we investigate the characteristics of the soil conditioned by means of foam, bentonite slurry, and carboxymethyl cellulose solution. The mechanical and friction properties of the conditioned soil were tested by the triaxial shear, Laboratoires Central des Ponts et Chaussees (LCPC), and sliding tests, under considering the influence of soil consolidation time. The test results showed that the conditioned sand presents dilatancy, and its cohesion enhances but its internal friction angle decreases. The changing trend of these properties is more prominent with the increase of consolidation time but they stabilize after 12 h. The starting torque of the shield machine in 3-h shutdown reaches 60% of the rated torque, while the starting torque in 6-h shutdown exceeds 80% of the rated torque. To avoid a large starting torque, we suggest that the cutterhead is idled for a short time within 12 h after the shield machine is shutdown. Finally, the theoretical calculation method of torque on cutterhead was developed, and the prediction by such method is in good agreement with the in situ measured data in the cutterhead. [ABSTRACT FROM AUTHOR]