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 (C_u) but with a decrease in the coefficient of curvature (C_c). In the unloading stage, with higher d₆₀ and C_c but lower C_u, the void ratio and pore pressure initially stayed constant with a larger range of total vertical stress (σ_v), but the effective vertical stress ${\sigma }_v^{\mathrm{\prime }}$ sharply decreased to zero at lower σ_v. With further unloading, pore pressure (u) and total lateral stress (σ_L) started to decline significantly at lower σ_v. With higher d₆₀ and C_c but lower C_u, 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/e_max became larger than e_t/e_max, which was approximately equal to 1.0, regardless of soil gradation.