Dewatering Calculator
Pit inflow by the equivalent-well method, then well count.
Quick answer
With the default inputs: Pit area 200 m² · Drawdown 3 · Permeability K 5 m/d etc.Pit inflow 350.4 m³/d
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Result
Pit inflow
350.4m³/d
| Equivalent radius | 7.98m |
| Radius of influence | 37.95m |
| Yield per well | 257.9m³/d |
| Wells calculated | 1.49wells |
| Wells to install | 2wells |
| Yield per well as installed | 175.2m³/d |
| Area per well | 100.0m² |
| Daily pumping | 350m³ |
| Hourly pumping | 14.60m³/h |
Inflow uses the equivalent-well method, Q = 1.366K(2H−S)S ÷ (log(R+r₀) − log r₀), with the Kusakin radius R = 2S√(HK). Real design also depends on well spacing — 1-2 m for wellpoints, 10-20 m for bored wells — plus settlement effects on neighbours and whether a cut-off wall is needed. Deep dewatering must be designed by a specialist with settlement monitoring.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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 |
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.
How to work it out
- Compute the equivalent radius r0 = 0.29(a + b) for a rectangular pit of plan dimensions a and b.
- Radius of influence R = 2S√(KH) for an unconfined complete well, with S the drawdown, K the permeability and H the aquifer thickness.
- Inflow by the large-well method: Q = 1.366K(2H − S)S / (lg(R + r0) − lg r0). This is the complete-well formula; partial penetration needs a correction.
- Yield per well q = 120πrL·∛K with r the screen radius and L the screen length, or take the pump's rated flow.
- Wells n = 1.1 × Q / q, the 1.1 being standby, then lay them round the perimeter and check the spacing is workable.
Where it goes wrong
- Not distinguishing complete from partial wells — the formula assumes the well reaches an impervious layer; partial wells yield less but take a correction, and applying the plain formula overstates by 20-40%.
- Guessing K — one order of magnitude in permeability is one order in inflow, so it must come from a pumping test or the ground investigation.
- Checking inflow but not drawdown — the flow can be right while the excavation still floods; drawdown must reach 0.5-1.0 m below formation.
- No standby wells or pumps — dewatering runs 24 hours and one failed pump undoes the lot; allow 20-30% standby.
Material and cost
Deep wells 80-200 CNY per well per day including power and maintenance; wellpoint systems 60-150 per set per day. Dewatering runs for the whole basement period and often totals 100000-500000 CNY.
On site
- Start dewatering one to two weeks before excavation and dig in dry; dewatering as you dig causes base heave and slope instability.
- Dewatering settles the ground around it, so near buildings or services provide recharge wells or a cut-off wall.
- Settle and treat discharge before it enters the sewer; raw discharge blocks drains and breaches environmental rules.
Basis
- JGJ 120-2012 Technical specification for excavations, Chapter 7 (groundwater control)
- GB 50021-2001 Code for investigation of geotechnical engineering (2009 edition)
- GB 50202-2018 Code for acceptance of foundation construction, Chapter 7
FAQ
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Dewatering Calculator. 2026-10-25. https://jianzhujisuan.com/en/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.