# Wellpoint Dewatering Calculator

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

- Page: https://jianzhujisuan.com/en/wellpoint-dewatering
- Category: Construction (https://jianzhujisuan.com/en/c/construction)
- Chinese version: https://jianzhujisuan.com/zh/wellpoint-dewatering.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Pit length 20 m, Pit width 15 m, Point spacing 1.2 m, Drawdown required 4.5 m, Screen length 1.2 m, Height above ground 0.2 m, Yield per point 1.5 m³/h): **Number of wellpoints 59 bars**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Pit length | number | 20 | m |  |
| Pit width | number | 15 | m |  |
| Point spacing | number | 1.2 | m | 0.8-1.6 m typically; smaller in silt, wider in sand |
| Drawdown required | number | 4.5 | m | Original water table down to 0.5 m below formation |
| Screen length | number | 1.2 | m |  |
| Height above ground | number | 0.2 | m |  |
| Yield per point | number | 1.5 | m³/h | 0.8-3 m³/h per point, 0.8-1.5 in silt |

## Formula

- **Perimeter**: P = 2 × (length + width)
- **Point count**: n = ⌈P ÷ spacing⌉
- **Actual spacing**: d = P ÷ n
- **Point depth**: H = drawdown + screen + above ground
- **Total pumping**: Q = n × yield per point
- **Daily pumping**: Q = Q × 24

## Worked example

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.

## Questions

### 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.

## Related tools

- [Plaster and Render Calculator](https://jianzhujisuan.com/en/plaster-coat): Mortar, cement and sand by layer for internal plastering.
- [Mortar Mix Design Calculator](https://jianzhujisuan.com/en/mortar-strength): Trial strength, cement, water and sand, and the mix by mass.
- [Labour Days and Crew Calculator](https://jianzhujisuan.com/en/labor-productivity): Man-days, crew size and labour cost from output rates.
- [Line-of-Balance Duration Calculator](https://jianzhujisuan.com/en/schedule-flow): Rhythmic flow duration with overlap and break adjustments.
- [Material Storage Area Calculator](https://jianzhujisuan.com/en/material-storage): Net footprint, total area and a suggested bay size.
- [Dewatering Calculator](https://jianzhujisuan.com/en/dewatering): Pit inflow by the equivalent-well method, then well count.

---

Generated by BuildCalc. All calculations run in your browser; no data is uploaded. Formulas follow national standards and common trade practice; local codes govern.