Chemical interaction between fine-grained soil and foaming agents in tunnelling with TBM-EPB

The use of Tunnel Boring Machines (TBM) and particularly the Earth Pressure Balance (EPB) technology is one of the most commonly used way to perform mechanized tunnel excavation. The possibility to perform the excavation with high performance, avoiding risk for workers and reducing the induced subsidence, particularly important in urban area, are the main reason of the growing of this technology. The success of the excavation is manly related to the quality of the soil conditioning process obtained by injecting chemicals under the form of foam to the front of excavation and mixing it to the excavated soil. For fine grained soils the interaction between clayey particles and chemicals lead to the variation of the soil features and, particularly, to the reduction of the natural adhesion between soil and cutterhead metallic surface. This paper reports the results of an experimental research activity carried out to measure the temporary and permanent effects of chemical interaction between the chemicals and the clay particles both in terms of physical and mechanical properties. To this aim different clayey soil samples of different mineralogical composition, from European tunnel excavation projects, are treated with foams generated employing different commercial products. Laboratory apparatuses and test procedures specifically developed are employed to quantify accurately the adhesion between soils and metallic surfaces. The systematic comparison between the results obtained on natural and conditioned soil sample provide useful insights on the effects of chemicals on adhesion and on permanent effects on physical and mechanical properties.