Effect of Grain Gradation on the Permeability Characteristics of Coarse-grained Soil Conditioned with Foam for EPB Shield Tunneling

Abstract

To avoid water spewing out of the screw conveyor, foam is often used to reduce the permeability coefficient of the soil during earth pressure balance (EPB) shield tunneling. Whether foam can effectively reduce the permeability or not strongly depends on soil gradation characteristics. A series of permeability tests were carried out on foam-conditioned soil with changing effective grain size (d10), curvature coefficient (Cc), and uniformity coefficient (Cu), further, three characteristic times were defined to evaluate the permeation safety of the conditioned soil, including critical time (tc), flow rate safety time (ta) and flow quantity safety time (ts). The test results show that with an increase in d10, the initial permeability coefficient of the conditioned soil increased greatly, however, it only changed slightly with increases in Cc and Cu. In addition, permeability coefficient was still much lower than that of the unconditioned soil, even though entered the slow growth period for a long time. It is shown that as the Cc and Cu increased, the ta and ts changed slightly, however, as the d10 increased, the ta and ts decreased significantly. The results indicate that d10 can greatly affect the permeability characteristics of conditioned soil, more specifically, the initial permeability coefficient and the permeation safety. By contrast, Cc and Cu have less effects. In addition, it is not reasonable to consider that the permeability coefficient of the conditioned soil will reach or be close to that of unconditioned soil when most, or all, of foam in the foam-conditioned soil dissipates.