Dewatering comprises all measures for controlling and lowering the groundwater table in excavation pits, trenches, and tunnel structures. Without dewatering, water-bearing soil layers would flood the excavation pit and make dry working impossible. The method depends on soil permeability and required lowering depth.
Open dewatering: Pumps in the trench area (submersible pumps, slurry pumps) directly extract water that has entered. Simple, but effective only with low inflow. Closed dewatering: Wells outside the excavation pit lower the groundwater table over a large area before the excavation pit is opened — typically via vacuum wellpoint system or deep wells.
Wellpoint systems consist of closely spaced filter lances (1–2 m spacing) around the excavation pit, which extract groundwater via collector pipes and a vacuum pump. Lowering depths up to 6 m are possible with a single stage; deeper lowering requires multi-stage systems or deep wells with submersible pumps. The pumped water must be properly discharged in accordance with water legislation (WHG).