Physical Foam Stability of Loose Sandy-Clay: a Porosity Role in the Conditioned Soil

To prevent the collapse and settlement of fine granular soil during earth pressure balance (EPB) tunneling or tunneling balance machine (TBM) operations, foaming agent mixing is one of the most efficient techniques. This work focuses on the stability of loose sandy-clay soil conditioned with the foaming agent. Four reconstituted loose sandy-clay soils were conditioned with the foaming agent CLB F5/AC. Using optical observation, Turbiscan analysis, and foam volume determination, the stability of foam mixed with fine soils was studied based on drainage coalescence and coarsening phenomena that occurred over time. Additionally, foam stability under gravity drainage was analyzed through a (one-dimensional) 1D column experiment. Variation in the FIR (foam injection ratio) shows that there is a limit FIR value from which the foam begins to be observed in the soil sample, involving a discontinuous porosity increase to the detriment of the continuous porosity decrease. An approach that considered this discontinuous porosity generated by the inclusion of gas bubbles was used to describe foam bubble degradation. Tests carried out on foam alone showed its rapid degradation compared with that of the foam added and mixed in the soil. Fine particles of clay allow foam stability even in the soil mixture. The foam volume in the soil decreases according to constant volume stages separated by a fast volume decrease. Both drainage and coalescence phenomena have occurred significantly for high values of FIRs. From the 1D column experience, foam bubbles modify the drainage kinetics by accelerating the drainage velocity compared with that of unconditioned soil. The foam behavior evolution over time when mixed with the soil can be analyzed using the same theory of the behavior of the foam alone.