# Dewatering Calculator

Pit inflow by the equivalent-well method, then well count.

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

## Quick result

With the default inputs (Pit area 200 m², Drawdown 3 m, Permeability K 5 m/d, Aquifer thickness 8 m, Well radius 0.2 m, Screen length 2 m): **Pit inflow 350.4 m³/d**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Pit area | number | 200 | m² |  |
| Drawdown | number | 3 | m | Original level to 0.5 m below formation |
| Permeability K | number | 5 | m/d | Silt 0.1-1, fine sand 1-5, medium 5-20 |
| Aquifer thickness | number | 8 | m |  |
| Well radius | number | 0.2 | m |  |
| Screen length | number | 2 | m |  |

## Formula

- **Equivalent radius**: r₀ = √(A ÷ π)
- **Influence radius**: R = 2S√(HK)
- **Pit inflow**: Q = 1.366K(2H−S)S ÷ (log(R+r₀) − log r₀)
- **Yield per well**: q = 120π r l ∛K
- **Number of wells**: n = 1.1Q ÷ q
- **Well spacing**: perimeter ÷ number of wells

## Worked example

A 200 m² pit with 3 m drawdown, K = 5 m/d and an 8 m aquifer: equivalent radius 7.98 m, influence radius 37.95 m, inflow 350.4 m³/d, 257.9 m³/d per well — two wells.

## Questions

### Why does the formula always give fewer wells than needed?

Because it checks that the flow can be pumped, not that the water level can be drawn down. Spacing limits the count too — bored wells rarely exceed 20 m apart or the level between them will not reach design — and corners of irregular pits need densifying. The formula also assumes a uniform aquifer, while lenses and layers vary permeability enormously, so a site pumping test is usually needed.

### Can dewatering affect neighbouring buildings?

Yes, and seriously. Lowering the water table raises effective stress and consolidates the soil, causing settlement out to one or two influence radii. In soft ground a 3 m drawdown can settle neighbours by 20-50 mm — enough to crack old buildings. Deep dewatering needs settlement monitoring, and often a cut-off wall or recharge wells.

### How is permeability determined?

A site pumping test first, a laboratory permeability test second, and empirical values only as a first guess: silt 0.1-1 m/d, fine sand 1-5, medium 5-20, coarse 20-50, gravelly 50-150. An order of magnitude changes the well count severalfold. Many dewatering failures trace back to treating silt as sand, sizing wells from a table and never getting the level down.

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