Dewatering: sumps, wellpoints and deep wells

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

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

How it is done

Characterise the aquifer: layering, permeability from pumping or borehole tests, and the head to be removed; standard borings alone often miss thin permeable layers that control inflow and base stability.
Choose the system: sumps for small inflows in clay or rock; wellpoints for sands up to about 5-6 m of drawdown per stage; deep wells with submersible pumps for larger drawdowns or high flows; ejectors for silty fine sands.
Install wellpoints by jetting at 1-2 m centres around the excavation, connected to a header and vacuum pump; drill deep wells with slotted screens and graded filter packs at 10-30 m spacing.
Pump down ahead of the dig and confirm in observation wells that the water level stands at least 0.5-1 m below each excavation stage before excavating to it.
Run continuously with standby pumps and automatic standby power; in sand a stoppage of minutes can flood the base and loosen the formation.
Monitor drawdown, flow rate, discharge turbidity and settlement of neighbouring structures daily; use recharge wells or a cut-off wall where settlement is unacceptable.
Decommission in stages only when the structure is heavy enough to resist uplift: switch off, watch the recovery, then grout or extract the wells.

Key numbers

Wellpoints1-2 m spacing; about 5-6 m drawdown per stage (suction-lift limit).
Deep wells300-600 mm bores at 10-30 m spacing; drawdowns of 30 m or more are possible.
Ejector wellsused in low-permeability silty sands (k about 1e-5 to 1e-7 m/s) where wellpoints and deep wells yield little.
Target groundwater level0.5-1.0 m below the current formation level.
Base uplift checkweight of soil below formation against the underlying water pressure, factor of safety typically 1.2-1.5.

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.

Choosing it

Choose pumped dewatering over a sealed cofferdam when the excavation is shallow, the ground is permeable sand and gravel, and no settlement-sensitive structures sit within the drawdown cone.
Prefer a cut-off (sheet piles, secant piles or a diaphragm wall keyed into clay or rock) with internal pumping only, when neighbours stand on compressible soil or discharge volumes would be very large.
Prefer a tremie seal and pump only the leakage when the base is in open sand under a river and no cut-off stratum is within reach.
Prefer deep wells over wellpoints when drawdown exceeds about 6 m or the site needs a clear working area around the excavation.

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.

Origins

Pumping from sumps is as old as excavation; wellpoint systems came into general use in the 1920s and 1930s, and deep wells with electric submersible pumps became standard after the Second World War.

Examples

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

Case studies

Kabinakagami River Bridge replacement (Highway 11)Canada · 2008Abutment excavation inside a 15.5 x 7.4 m single-strut sheet-pile cofferdam in layered silt and silty clay; no seepage or pressure control was installed, the base heaved and the sheets kicked out at the toe. The review concluded that layered deposits need pumping and observation-well tests, not only borings.

Sikorsky Bridge (Merritt Parkway over the Housatonic)USA · 2003-2005Riprap stopped the cofferdam sheets from penetrating, so the cofferdam could not be pumped dry; a precast concrete 'bathtub' was set inside and the residual leakage at joints was handled by diesel pumps for about a month while the pier was cast.