Excavation dewatering and water control

One-line orientation

Dewatering keeps an excavation dry enough to work in when digging at or below the water table. It either blocks water or pumps it down. Do not confuse a cofferdam, which keeps water out of a work area, with a check dam, which slows runoff.

Key points

• The problem: working below the water table means groundwater seeps into the cut; it must be sealed out or removed to keep a dry, workable, stable excavation.
• Three strategies:
  • Waterproof enclosure (“bathtub”) — seal the excavation against water infiltration.
  • Pump water out from within the excavation as it seeps in (sumps/pumping).
  • Pump from the surroundings to lower the groundwater level and reduce inflow.
• Cofferdam: a watertight temporary enclosure (sheet piling, earth, or concrete) built in a water body and pumped dry so work can occur below the waterline — bridge piers, dock foundations. Keeps water OUT.
• Dewatering is temporary (construction phase). It is distinct from permanent foundation drainage/waterproofing, which protects the finished below-grade structure for its service life.

Confusions / comparison

	Cofferdam	Check dam
Purpose	Keep water OUT of a work area	Slow runoff, control erosion
Where	In a water body (river, lake)	Across a channel/ditch on a slope
Made of	Sheet piling, earth, concrete	Rock, sandbags, logs
Creates dry work zone?	Yes (pumped dry)	No

The three below-water-table strategies the card lists differ in how they keep the cut workable and in what they do to the groundwater:

Strategy	Mechanism	Effect on the water table	Soil / site fit
Waterproof enclosure (“bathtub”)	Seal the cut so water cannot infiltrate	Leaves the surrounding water table where it is — water is held out	Depends on soil and site
Pump out from within	Collect and pump away water as it seeps into the cut (sumps/pumping)	Does not draw down the regional table — only removes what reaches the excavation	Depends on soil and site
Pump from the surroundings	Pump from around the cut to lower groundwater before it reaches it	Lowers (draws down) the water table to reduce inflow	Depends on soil and site

Related

→ Shoring systems (this module): sheet piling / slurry wall also resist groundwater · Angle of repose (this module): saturated soil destabilizes a cut · Site utilities (this module): site stormwater vs construction dewatering.