Active earth pressure control with foam

Nowadays, foam ground conditioning allows the use of earth pressure balanced shields in mixed and coarse grained grounds even in presence of high groundwater pressures. From the process technology point of view it is possible to actively govern the confinement pressure on the tunnel face by means of the pressure–volume controlled foam injection, in the same way as the air bubble principle in slurry shields does. Experiences in the Botlek Railway Tunnel (Betuwe route/Rotterdam NL) show that the danger of face collapse is relatively low and that high advance rates are attainable. The limit of the application range is defined by permeable non-cohesive grounds under water pressures over 2.5 bar. The earth pressure balanced procedure reaches here its applicability limit when restrictive requirements are set to the admissible surface settlements.