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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 · Pit width 15 · Point spacing 1.2 etc.Number of wellpoints 59 bars

Edit the inputs below to recalculate.
Spacing sRun L
Wells close a ring around the pit at 0.8-1.6 m spacing, and the pipe length runs to the bottom of the filter, not just the burial depth.

Inputs

0.8-1.6 m typically; smaller in silt, wider in sand
Original water table down to 0.5 m below formation
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.

How to work it out

  1. Count the wellpoints: laid round the perimeter, number = perimeter ÷ spacing, rounded up. Spacing is normally 1.2-1.6 m, tightening to 0.8-1.2 m in silty sand.
  2. Point length = drawdown + screen length + height above ground. The screen must sit below the water table and runs 1.0-1.5 m.
  3. Header length = pit perimeter plus the run to the pump house, usually taken as 1.1 times the perimeter.
  4. Daily extraction = yield per point × number of points × 24 × a factor of 0.8-0.9 for intermittency and efficiency.
  5. Check the drawdown: single-stage wellpoints are limited to about 6 m below ground; deeper needs two stages or deep wells.

Where it goes wrong

Material and cost

Wellpoint systems 60-150 CNY per set per day including points, header, pump and labour, with a set carrying 20-30 points. Dewatering typically runs one to three months over the basement works.

On site

Basis

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.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Wellpoint Dewatering Calculator. 2026-10-25. https://jianzhujisuan.com/en/wellpoint-dewatering

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.