Bentonite is a fine clay mixed with water which form the drilling sludge. These sludge allows the cooling of boring tool, the consolidation of tunnel walls and also facilitates drilling thanks to the pressure. They also contribute to evacuation of excavation materials before dragging the pipeline. It is possible to slightly modify the density of this product, sometimes meanwhile operating, to facilitate drilling and dragging. Drilling muds are generally recycled and used in a closed loop.