A More Comprehensive Way to Analyze Foam Stability for EPB Tunnelling—Introduction of a Mathematical Characterization

In the tunnelling industry, a large share of the market is occupied by EPB (Earth Pressure Balance) machines. To operate this kind of machine, a radical change in the rheological behaviour of the excavated soil must be performed, and this is achieved by adding water, foam, and, eventually, polymers. The stability of the foam is assessed through a half-life test. The main limitation of this test is that only one value is used in the characterization of the foam degradation process, which is insufficient to describe the whole evolution of the phenomenon. The results of more than 270 tests were modelled through a five-parameter mathematical formulation that suited the experimental data. The results show that the influence of concentration on the stability of the foam is not always present and that the flow rate used during production bears an influence on the characteristics of the foam.