Classification of foam and foaming products for EPB mechanized tunnelling based on half-life time

During tunnel excavation with Tunnel Boring Machines (TBMs) and earth pressure balance (EPB) technology, conditioning agents are commonly used to modify the physical and mechanical properties of soils so that they are appropriate for the application of the front-face support pressure during excavation. The consistency of the conditioned soil must be stable during the time necessary to advance and install the lining. Hence, a stability analysis of the injected foams is an important aspect. To assess the stability of a foam a classification is proposed based on the results of a standard foam generation procedure and on half-life tests using products from the main suppliers. A further classification of the foaming agents used according to their ability to produce a stable foam in terms of Foam Expansion Ratio is also proposed.