The extension of the Metro B line in Lyon represents a technical challenge combining difficulties with very few equivalents in pressurised TBM history. Alluviums with wide porosity and high permeability (up to 10-2 m/s) imply a risk of slurry loss leading to ground instabilities. To mitigate geological risks without using heavy ground treatment, two lines of research were followed:
i) A slurry development program, aiming to define and validate technical mud formulae reducing the losses and ensuring the continuous confinement pressure. Two types of innovative slurries were to be defined, a “technical” one for horizons with high permeability, and an “emergency mud” to secure the tunnel and constructions in the event of a sudden slurry loss.
ii) The use of a variably density TBM to use these technical slurries with a classic and an independent auxiliary hydraulic circuit. In addition, an embedded emergency mud supply was added to the TBM. The first phase implemented the slurry development lab, which consisted in the characterisation of a wide range of products entering in the slurry composition such as bentonite, mineral or organic clogging agents, polymers, gels, etc. The slurry formulae were then tested under pressure in permeameters of two different sizes. Solutions clogging the porosity and sustaining mud and air pressure have been developed. The execution phase proved, that the developed solutions were giving the expected results regarding controlling slurry losses in the ground, maintaining front confinement pressure and reducing surface settlement below expected values in a dense urban area.