# What is dewatering?
**Dewatering is the process of removing groundwater or surface water from a construction site by pumping, so that excavations, foundations and underground structures can be built in dry conditions.** It is an essential technique in areas with high water tables, ensuring that workers can safely access the excavation and that concrete and masonry are placed without water interference.
## How does dewatering work?
The basic principle is to lower the water table locally by extracting water through wells or drains and discharging it to a sewer, watercourse or infiltration system. Common methods include:
– **Wellpoint dewatering** — a series of small-diameter wells connected to a header pipe and vacuum pump, suitable for shallow excavations up to about 5 metres
– **Deep well dewatering** — individual submersible pumps in large-diameter boreholes, used for deeper excavations
– **Open sump pumping** — water is allowed to collect in a sump pit at the lowest point of the excavation and pumped out directly
– **Drainage blankets and trenches** — permeable layers or channels that guide water to collection points
## Application
Dewatering is required in:
– **Basement and cellar construction** — keeping the excavation dry during pouring of floor slabs and walls
– **Pile foundation work** — ensuring dry conditions around pile caps
– **Sewer and pipeline installation** — trenches below the water table must be dewatered before pipes are laid
– **Underground car parks and tunnels** — large-scale dewatering during the entire construction phase
### Important considerations
– A dewatering permit or licence is often required from the local water authority
– Extracted water must be discharged responsibly to prevent flooding or pollution
– Lowering the water table can cause settlement in surrounding buildings, particularly on soft soils — monitoring is essential
– Dewatering must continue uninterrupted; pump failure can flood the excavation within hours
## Related terms
– Groundwater
– Excavation
– Water table
– Wellpoint
– Settlement
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