Effects of the physical characteristics of foams on conditioned soil’s flow behavior: A case study

Abstract

Earth pressure balance shield (EPBS) is the excavation technique responding to the technical construction complexity of the future line 16–1 of the Grand Paris Express project. Its principle is based on the capacity of the excavated ground to transmit the forces needed to counterbalance the earth and water pressures of the working face. Additives are injected into the soil; on the cutting wheel, in the excavation chamber and in the screw conveyor. Their role is to assure the soil conditioning by modifying the soil’s workability and its hydromechanical and rheological parameters. Therefore, the soil conditioning produces a plastic medium able to transmit the pressure to maintain the front face. In addition, as a second objective, the additives aim to overcome uncertainties in the soil. Depending on the case, the additives can reduce the clogging, internal friction, abrasiveness, and permeability of the soil. The additives can include water, surfactant, polymer and bentonite. The combination of water and surfactant with or without polymer produces the foam. In order to increase the excavation perfomance, laboratory prelimenary tests are performed to deduce the optimal foam parameters. Since no test measuring the foam characteristics is standardized yet, this article sums up the tests mostly used in foam characterisation; half-life measurements, rheological tests, microstructural observations. As for the conditioned soil, slump test is frequently used by the tunnelling industry to recommend the optimal soil conditioning parameters in terms of workability. This test has been adapted in this study and the results are verified by rheological tests, then compared to the real foam injection data used for the project and their effect on mechanical parameters. For a specific type of excavated soil, the quality (type) of foam recommended by the laboratory work falls into the same choice of additive based on in-situ tunnelling excavation data. These findings are explained by the foams characteristics. Therefore, a correlation between foam and the conditioned soil (sandy soil with silt and low percentage of clay) with foam is deduced at different levels: micro, meso and macro (real) scales.