Effectiveness of Foam Injection during Mechanized Excavation of Tunnels with TBM-EPB Technology

In the mechanized excavation of tunnels with Tunnel Boring Machines, an ever more frequently used technique worldwide is the Earth Pressure Balance (EPB) technology; during the excavation phases this technology requires continuous injection of chemicals under the form of foam to be mixed with the excavated soil. Since the features of the foam influence significantly the excavation performances and the propagation of induced effects such as settlements in the environment, the studies of the parameters affecting the quality of the foam have great importance for practical purposes. This study summarizes the results of several years of research performed in laboratories of Sapienza University of Rome with the contribution of the expertise provided by SELI Technologies and months of data acquisition derived from actual excavation projects. Results show the influence on the quality and stability of the injected foam for several factors such as: a) typology of foaming agents, b) dosage, c) pressure and flow of air and foaming solution, d) typology and geometry of the foam generator, and e) features of the injection plant. The experimental activity was performed using a particularly arranged test apparatus, a large number of chemical products from the main European suppliers commonly used in tunnelling projects and a series of mechanical elements originated directly from injection plants of several real tunnel-boring machines combined into the laboratories in different configurations. In parallel with the experimental activity, a series of field observations were collected and processed to take into account several practical details that influence the design and operation of the injection plant and on the quality of the foam injected. These data, from various projects worldwide, provide general indications on the elements that cannot be considered directly in the laboratory. The results obtained from laboratory tests combined with the on-site observations, provide both a useful input for an informed design of the mechanical elements of the plant and their arrangement as well as indications on the choice of chemicals and management of the conditioning parameters during the excavation process.