The study of foaming agent concentration choice in EPB shield soil conditioning

Abstract

Foam is widely used in soil conditioning during EPB shield tunnelling to ensure the efficiency and safety of face support and muck flow. Currently, the concentration of the foaming agent solution is chosen based on the recommendations of foaming agent suppliers, the half-life time of foam or the properties of the foam-soil mixture. This paper studies the foaming agent concentration (C_f) choice taking surface tension into consideration. Test results show that the C_f should be higher than the critical micelle concentration (CMC) to offset the influence of the expanding surface area during foam generation in shield tunnelling. The comparison between the capillary rise method and the platinum plate method shows that this offset can almost be accomplished when C_f is higher than 1% and fully accomplished when C_f is higher than 3%. Foam generation tests show that despite the differences in foam generation methods, the foaming ability of foaming agent solution and the stability of the generated foam are close to optimum at C_f of 1% and reach optimum at C_f of 3%. Slump tests show that foam with C_f lower than 1% is not stable enough in the foam-sand mixture for shield tunnelling. Both slump tests and field validation show that when C_f is higher than 3%, the increase of C_f has limited influence on soil conditioning performance.