Dewatering (Wellpoints and Sumps)

Dewatering lowers groundwater levels to allow excavation and construction below the natural water table.
Common methods include sump pumping, wellpoint systems, deep wells, and ejector wells.
Sump pumping collects water at the lowest point of an excavation and pumps it to a discharge location.
Wellpoint systems use closely spaced suction pipes connected to a header main and vacuum pump.
Uncontrolled dewatering can cause ground settlement, damage to adjacent structures, and slope instability.
Discharged water must be managed to prevent silt pollution of watercourses and surface water drains.
The Environment Agency may require an abstraction licence and discharge consent for dewatering operations.
Pumps, generators, and fuel storage near excavations create noise, fume, and spill contamination risks.
Dewatering must be continuous; pump failure overnight can flood the excavation and collapse support systems.
CDM 2015 requires that temporary works including dewatering are planned and managed by competent persons.

Prevent excavation flooding	Rising groundwater floods excavations, softens formation, collapses support systems, and drowns workers trapped below.
Ground settlement	Lowering groundwater draws water from soil beneath adjacent structures, causing settlement, cracking, and structural damage.
Environmental compliance	Discharging silty or contaminated water to drains or watercourses breaches environmental permits and leads to prosecution.

Do

Don't

Design the dewatering system for the actual groundwater conditions encountered on site.
Obtain abstraction licences and discharge consents from the Environment Agency where needed.
Monitor groundwater levels and pump performance continuously during dewatering operations.
Install settlement monitoring on adjacent structures before dewatering begins.
Use silt settlement tanks or filtration before discharging pumped water off site.
Maintain backup pumps and generators to prevent system failure during overnight periods.
Contain fuel storage for generators with secondary bunding to prevent spills.
Plan the dewatering as a temporary works scheme with design and TWC approval.
Brief all operatives on the emergency procedure if dewatering pumps fail.
Record water levels, pump rates, and discharge quality in a daily monitoring log.

DON'T discharge silty or contaminated water directly to drains or watercourses.
DON'T start dewatering without checking the licence and consent requirements first.
DON'T leave dewatering pumps unmonitored overnight without backup and alarm systems.
DON'T ignore ground settlement signs in adjacent buildings during dewatering operations.
DON'T enter flooded excavations to repair pumps; pump from a safe position first.
DON'T oversize the dewatering system and draw down water beyond what is needed.
DON'T store fuel for dewatering generators without proper bunding and spill kits.
DON'T switch off dewatering without confirming the excavation support can withstand water pressure.
DON'T assume dewatering from a previous project will work on this site.
DON'T position noisy pumps and generators near residential boundaries without assessment.

See also: Groundworks Safety Awareness | Slope Stability Awareness

Presenter		Project
Location		Date

Toolbox Talk Record