Effect of dispersant on the tangential adhesion strength between clay and metal for EPB shield tunnelling

When Earth Pressure Balance (EPB) shields pass through clayey ground, the working tool is frequently clogged, causing a low tunnelling rate and accelerating cutter disk abrasion. Clay clogging is considered to be due to the high tangential adhesion strength between the clay and metal. This paper investigates the changes in the tangential adhesion strength with the consistency index under different total normal stresses and with different dispersant contents using a mixture of bentonite and kaolin (mass ratio of 1:1). The results showed that the tangential adhesion strength first decreased significantly and then remained almost constant with an increase in water content. Injection of the dispersant could decrease the tangential adhesion strength of clay. The tangential adhesion strength decreased gradually with a lower dispersant content ( < 4%) and then remained nearly unchanged. The tangential adhesion strength increased slowly with a consistency index lower than 0.5-0.6 and then increased significantly. Considering the effect of dispersant on the Atterberg limits of clay, the tangential adhesion strength between the soil and metal with different dispersant contents was virtually identical at the same consistency and under a normal stress. The tangential adhesion strength showed an exponential relationship with the normal stress and consistency index when the consistency index of clay was lower than 1. The reduction of average tangential adhesion strength showed a linear increase as that of liquid limit increased. The slope of the line increased when the normal stress increased. These results indicate that changes in the liquid can represent the effect of dispersant on the adhesion of clay. From the test results, it can be considered that a supporting medium with a consistency index lower than 0.5 is suitable for EPB shields. Since soil with a consistency index lower than 0.4 is easily squeezed out of a screw conveyor, a supporting medium with a consistency index of 0.4-0.5 is ideal for EPB shields, which is in agreement with the conclusion proposed by Hollmann and Thewes.