Vacuum dewatering behavior of foam-conditioned clay soil: Implications for foam optimization in earth pressure balance shield tunneling

Estimating the properties of foam-conditioned clay soils is important for both conditioning and recycling goals in earth pressure balance (EPB) shield tunneling. In this study, the vacuum dewatering behaviors of foam-conditioned clay soils were investigated, with their potential use as an alternative means to assess foam optimization being examined. A series of laboratory and field tests was conducted, including vacuum dewatering tests that considered the effects of filtration time and pressure, vane shear tests, and improved cone pullout tests under different gravimetric water content ( w ) and foam injection ratio (FIR) conditions. It was found that the filtrate loss (FL), which characterizes dewaterability, was increased by extended vacuum filtration time and elevated pressure. While increases in w and FIR enhanced FL, reductions were observed in the undrained shear strength ( c u ), tangential adhesion stress ( F s ), and normal adhesion stress ( F n ). Furthermore, a linear decrease in FL with increasing mechanical indices ( c u , F s , and F n ) was demonstrated by both laboratory and field data fitting results, regardless of w , FIR, and dewatering conditions. This study provides novel insights into the understanding of vacuum dewatering mechanisms in foam-conditioned clay soils, while a simple approach is proposed for evaluating foam conditioning effectiveness in EPB shield tunneling applications.