Effect of Gradation on Undrained Compressibility of Foam-Conditioned Coarse-Grained Soils

To reduce pressure fluctuations in the excavation chamber during earth pressure balance (EPB) shield tunneling, it is necessary to inject foam into excavated soil to improve its compressibility. However, the effect of grain gradation on the undrained compressibility of foam-conditioned soil remains elusive. In this study, a series of one-dimensional undrained compression tests using a custom-designed large-scale compression device were carried out to study the role of grain gradation on the compressibility characteristics of foam-conditioned coarse-grained soils. The results demonstrated that the compressibility decreased with an increase in grain diameter (d60) and uniformity coefficient (Cu) but with a decrease in the coefficient of curvature (Cc). In the unloading stage, with higher d60 and Cc but lower Cu, the void ratio and pore pressure initially stayed constant with a larger range of total vertical stress (σv), but the effective vertical stress σ v ′ sharply decreased to zero at lower σv. With further unloading, pore pressure (u) and total lateral stress (σL) started to decline significantly at lower σv. With higher d60 and Cc but lower Cu, there were lower lateral pressure and total lateral pressure coefficients, indicating a lower workability of muck. Although the foam injection ratio of 20% was not sufficiently high to effect compression dominated by air in the early loading stage, the soil showed air-governed behavior in the unloading stage when e/emax became larger than et/emax, which was approximately equal to 1.0, regardless of soil gradation.