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Wellpoint Dewatering Calculator

Points along the pit perimeter: count, spacing, header length and daily pumping.

Quick answer

With the default inputs: Pit length 20 m · Pit width 15 m · Point spacing 1.2 m etc.Number of wellpoints 59 bars

Edit the inputs below to recalculate.

Inputs

m
m
m
0.8-1.6 m typically; smaller in silt, wider in sand
m
Original water table down to 0.5 m below formation
m
m
m³/h
0.8-3 m³/h per point, 0.8-1.5 in silt

Result

Number of wellpoints
59bars
Pit perimeter70.0m
Actual spacing1.19m
Header pipe length70.0m
Wellpoint depth5.90m
Yield per point1.50m³/h
Total pumping rate88.5m³/h
Daily pumping2124m³/d
Dewatering stagesSingle stagestages
Pump capacity with 1.2 reserve106.2m³/h

Points are laid as a closed ring around the pit; count = ⌈perimeter ÷ spacing⌉, and you must back-calculate the actual spacing or the corners exceed it. A single-stage wellpoint system is limited by pump suction to roughly 5-6 m of drawdown; beyond that use two stages, ejector wellpoints or bored wells. Size pumps at 1.2× the steady pumping rate, since start-up and post-rain inflow run higher. Point depth = drawdown + screen + above-ground height.

Formula

PerimeterP = 2 × (length + width)
Point countn = ⌈P ÷ spacing⌉
Actual spacingd = P ÷ n
Point depthH = drawdown + screen + above ground
Total pumpingQ = n × yield per point
Daily pumpingQ = Q × 24
A 20×15 m pit at 1.2 m spacing, 4.5 m drawdown and 1.5 m³/h per point: 70 m perimeter, 59 points, 1.19 m actual spacing, 70 m header, 5.9 m point depth, 88.5 m³/h total and 2124 m³/day.

FAQ

Why can wellpoint spacing not simply be widened?
Because each point draws a cone of depression, and between two points a residual high remains. Space them too far and that midpoint never reaches the design level, leaving the middle of the pit wet. Normal spacing is 0.8-1.6 m: 0.8-1.2 in silt and muddy soil, up to 1.5-1.6 in permeable sand. Longer points and screens allow wider spacing.
When are two-stage wellpoints needed?
When the required drawdown exceeds what one stage can do. Single-stage wellpoints rely on pump suction — 10.3 m theoretical, but losses and imperfect vacuum limit the practical drawdown to 5-6 m. Beyond that, excavate a bench (or dewater one stage first) and lay a second ring on it, reaching 10-12 m. Deeper still calls for ejector wellpoints (8-20 m) or bored wells.
How much pump capacity?
About 1.2 times the calculated total pumping rate, with a standby pump. Early dewatering runs much higher because the head and gradient are large, and rainfall sharply increases recharge. Jet pumps typically deliver 8-30 m³/h each, used with a vacuum pump. Undersized equipment never gets the level down and costs more in stop-start running. Wellpoint systems must run continuously — stopping lets the water rise and fines migrate back into the screens.

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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.