Investigation on foam stability and foam-conditioned soil properties under pressure in EPB TBM tunneling

Abstract

Proper soil conditioning is very important in earth pressure balanced (EPB) tunneling as it improves face stability and tunnel boring machine (TBM) performance. Foam is one of the most commonly used soil conditioning agents to modify the excavated soil properties. A critical characteristic of foam-conditioned soil is its stability, i.e., the ability to maintain the engineering properties throughout the residency time (30-90 min) in the mixing chamber. It is very important to understand the fundamentals of foam stability and foam-conditioned soil properties. This thesis examines foam stability under pressure through a novel foam generation – pressure chamber – foam capture testing system. A series of foam experiments was performed to examine the physical phenomenon of foam degradation and time-dependent foam properties under pressure. Test results suggest that liquid loss is not an effective indicator for characterizing foam stability, while foam volume loss is a more appropriate measure of foam stability. Results also reveal that foam liquid drainage is significantly retarded at higher chamber pressure. For the stability of foam-conditioned under pressure, a comprehensive suite of experiments was conducted for foam-conditioned soil to investigate the fundamentals of foam-soil interaction and engineering properties of foam-conditioned soil. A foam-soil capture device was used to capture bubble-grain images at a microscale under pressure. A pressurized testing chamber (PTC) was used to examine the stability of the mechanical properties of foam-conditioned soil. Test results reveal that changes in bubble size distribution for foam in foam-soil mixtures are much less than foam itself, indicating that soil particles help stabilize foam bubbles. Test results present that the engineering properties of foam-conditioned soil are relatively stable over 60 min. This thesis also investigates the mechanisms of foam-soil separation in the EPB mixing chamber through a series of soil conditioning tests. Parameters including molding water content wo, initial foam injection ratio FIRo, and fines content are varied to examine capacity for foam and foam-soil separation. Test results suggest that there is more expelled foam as molding water content and initial foam injection ratio increase. Test results also indicate that fines content increases the soil’s capacity for foam and water. In addition, results show that agitation and cyclic loading-unloading of pressure can induce foam-soil separation in conditioned soil.