<p>Earth Pressure Balancing Shields (EPBS) are commonly used for tunnelling in soft ground and loose soils. A crucial component of this technique is soil conditioning, which ensures tunnel face stability and enhances the performance of the tunnelling machine. The most versatile soil conditioning agents are foams since they can be used on any type of soil with the addition of anti-clay agents or polymers. Additionally, foam can serve as a standalone conditioning agent, fluidising the soil, forming a homogeneous soil paste, reducing permeability, reducing torque, minimising soil stickiness, and reducing abrasion. This study focussed on optimising the amount of foam that needs to be used in terms of Foam Injection Ratio (FIR) with respect to soil’s consistency. The objective of the study was accomplished through 3 phases of experimental work. The first phase established FIR ranges based on soil plasticity index: low plastic soils required 30–40% FIR, moderate plasticity needed 70–80%, and high plastic soils over 80%. The second phase optimised FIR considering soil plasticity index and water content, finding that foam is ineffective in soft to liquid or hard states but effective in medium stiff to very stiff soils. The third phase validated these findings with tangential adhesion tests, suggesting FIR ranges of 20–30% for medium stiff soils (consistency index 0.5–0.75), 30–50% for stiff soils (index 0.75–1.0), and 50–80% for very stiff soils. This study offers a valuable database for field engineers to optimise foam concentrations across different soil types.</p>