A lab-scale experimental approach to evaluate rheological properties of foam-conditioned soil for EPB shield tunnelling

In modern tunnel engineering, earth pressure balance (EPB) shield is the most frequently adopted type of tunnel boring machine (TBM). The EPB shield makes excavated soils properly conditioned by injecting additives and utilizes them as a face-supporting material to attain face stability and to facilitate tunnelling. Thus, the muck treatment process (i.e., soil conditioning) and improved soil properties have become influential factors in determining the overall EPB shield TBM performance. However, the mechanical behavior of conditioned soils has not been analyzed sufficiently owing to the difficulty in handling them. In this study, a laboratory pressurized vane shear test was adopted to examine the rheological characteristics of foam-conditioned sandy soils. The tests were conducted for different values of water content, foam injection ratio (FIR), and confining pressure to evaluate their effects on the rheological behavior of foam-conditioned soils. It was found that the yield stresses obtained from the flow curves correlated with the slumps under atmospheric conditions. Additionally, an engineering flowchart was developed based on the relationship observed between yield stress and slump. This flowchart allows to assess the applicability of conditioned soils when rotary vanes are installed inside the EPB shield TBM.