Characterization of foams used in mechanized tunnelling with TBM-EPB technology

Currently, TBM-EPB technology is the most common method to perform mechanized tunnel excavation in soil, particularly in urban environment. The development of technologically improved machines and the conditioning process of excavated soil through specifically tailored foams have made this technology more effective in many geological contexts. This technology requires the continuous injection of foam to modify the excavated soil features. The characteristics of the injected foam affect the effectiveness of the injection and consequently the excavation performances. This paper describes the most significant results obtained in years of laboratory activities on the factors affecting the quality of the foam, from the chemical composition up to the foam generation modes. Based on the results obtained performing half-life time (hlt) tests performed in standard condition of temperature, pressure and generation pressure, employing a modern and effective laboratory foam generation system on more than 25 commercial products using different values of Foam Expansion Ratio (FER) and chemicals dosages, a classification scheme of the products is proposed according to the ability to generate a stable foam. The proposed classification allows to define 5 classes, ranging from Class I to V, particularly useful to assess the stability of a foam, to select a product able to guarantee the generation of adequate stable foams and to evaluate the direct correlation between chemical composition and stability of generated foam.