Construction methods · Shallow foundations, cofferdams and dewatering

Dewatering: sumps, wellpoints and deep wells

Keeping an excavation dry below the water table. Sumps and pumps for small inflows in clay and rock; wellpoints for sands to about 5-6 m of drawdown per stage; deep wells with submersible pumps for larger drawdowns. Uncontrolled water is the fastest way to lose a footing excavation.

How bridges get built · Shallow foundations, cofferdams and dewatering · Dewatering: sumps, wellpoints and deep wells

Also called: groundwater control, wellpoint system, deep well dewatering

Concept diagram (TheBridgeEng)

How it is done

Estimate inflow from permeability and drawdown; choose the system and its spacing.
Wellpoints: jet 50 mm points at 1-2 m centres around the excavation, connect to a header and a vacuum pump; stage every 5 m of depth.
Deep wells: drill 300-600 mm wells around the excavation with screens and gravel packs, install submersible pumps, run continuously with standby power.
Monitor drawdown in observation wells; recharge or cut-off walls if neighbours are sensitive to settlement.

Where it fits

Sands and silty sands below the water table; footings, pile caps and cofferdam bases.

Where it does not

Dewatering next to structures on compressible soils without recharge: the settlement shows up as cracks next door.
Clays: pumping does almost nothing; use sumps and a cut-off.

Plant, pace and money

PlantWellpoint pumps and headers, drilling rig for deep wells, submersible pumps, generators, settlement tanks for discharge.

ProductivityA wellpoint ring for a pier excavation is installed in one to three days.

CostUSD 20,000-150,000 per pier excavation depending on inflow and duration.

Risks and controls

What goes wrong

Pump failure and flooding of the excavation; piping and boiling at the base in fine sand; settlement of adjacent ground; discharge permits.

Quality assurance

Observation wells read daily, standby pump and power tested, discharge turbidity checked.

Examples

Every river pier built in a sheet-pile cofferdam runs a dewatering system in parallel.