Quantifying foam destruction during conditioning of fine-grained soils

This paper presents the development of a novel methodology and the results of an experimental study to quantify foam destruction ($\mathrm{FD}$) observed during the conditioning of fine-grained soils. A laboratory setup was designed to simulate pressurized mixing conditions prevalent in the cutterhead tool gap and excavation chamber of an earth pressure balance tunnel boring machine (EPBM). The developed $\mathrm{FD}$ quantification methodology utilizes the concept of back pressure saturation to determine the volume of the ruptured air from the foam bubbles during the conditioning of soil. The influence of major soil types, fines content ($\mathrm{FC}$), soil consistency index ($I_c$), foam injection ratio ($\mathrm{FIR}$), and foam liquid half-life ($t_{1/2}$) on $\mathrm{FD}$ were investigated. The results showed no $\mathrm{FD}$ in sand, 15–30% $\mathrm{FD}$ (% of injected foam volume) in silt, and significant $\mathrm{FD}$ of 30–85% in clay. $\mathrm{FD}$ was found to be directly proportional to $\mathrm{FC}$ and inversely proportional to $\mathrm{FIR}$ and $t_{1/2}$. No relationship was observed between $\mathrm{FD}$ and $I_c$. The study provides clear experimental evidence of the presence of $\mathrm{FD}$ phenomena in fine-grained soils and a methodology for calculating $\mathrm{FD}$. The quantification of $\mathrm{FD}$ range in major soil types and insights into influencing factors revealed in this study will help practitioners account for the expected $\mathrm{FD}$ in various soil types while deciding $\mathrm{FIR}$, and decide whether to use foam at all while preparing conditioning strategies for EPB tunneling projects in fine-grained soils.