Non-stop conveyance of excavated material.

When tunnelling through sandy and gravely (loose) mixed soil with high water permeability, the excavated material is usually handled by a hydraulic transportation system. For this, either water or a bentonite suspension is used as the transportation medium.

When tunnelling through cohesive soil with low water permeability, such as loamy and clayey soils with a high fine-particle content, the excavated material is itself the support medium, which it why it has to have a plastic or extrudable consistency. This is either naturally the case or the soil has to be conditioned to make it like this by adding foam or polymers.