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Deep Well Dewatering Calculator

When drawdown exceeds 5-6 m, wellpoints run out of suction - size deep wells instead.

Quick answer

With the default inputs: Pit length 30 m · Pit width 20 m · Drawdown 6 m etc.Deep wells (with 1 spare) 4 wells

Use your own dimensions ↓

Inputs

Water table down to 0.5 m below formation

Result

Deep wells (with 1 spare)
4wells
Pit area600m²
Equivalent radius13.82m
Radius of influence107.3m
Total pit inflow1031m³/d
Yield per well490m³/d
Wells required3wells
Total capacity with spare1960m³/d
How this is worked out

Deep wells take over where wellpoints lose suction, roughly below 5-6 m of drawdown. Total inflow uses the large-well method: an equivalent radius r₀ = √(area/π), the Kusakin radius of influence R = 2S√(kH), then the unconfined full-penetration formula Q = 1.366·k·(2H−S)·S / lg(R/r₀). Per-well yield is the empirical q = 65π·d·L·∛k, and the well count is inflow ÷ yield rounded up plus one spare - a dewatering ring without a spare fails the moment one well pumps out or a screen clogs. H is only the dewatered depth, not the whole formation thickness.

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Formula

Equivalent radiusr₀ = √(area/π)
Radius of influenceR = 2S√(kH) (Kusakin, k in m/d)
Total inflowQ = 1.366·k·(2H−S)·S / lg(R/r₀)
Per-well yieldq = 65π·d·L·∛k (d, L in m)
Well countn = ⌈Q/q⌉ + 1 (spare)
A 30×20 m pit, 6 m drawdown, medium sand k=8 m/d, 10 m aquifer, 400 mm wells, 3 m screen: r₀ = 13.82 m, R = 2×6×√(8×10) = 107.3 m, inflow Q ≈ 1031 m³/d; per-well yield q = 65π×0.4×3×∛8 ≈ 490 m³/d, so 3 wells plus one spare = 4.

How to work it out

  1. Work out the equivalent radius r₀ = √(area/π), treating the whole pit as one large well.
  2. Estimate the radius of influence with Kusakin's R = 2S√(kH); k is permeability (m/d), H the aquifer thickness.
  3. Total inflow by the large-well method: Q = 1.366·k·(2H−S)·S / lg(R/r₀), with S the drawdown.
  4. Per-well yield by the empirical q = 65π·d·L·∛k; well count = ⌈Q/q⌉ rounded up.
  5. Add one spare, place wells evenly around the pit, and size pumps at 1.2× total capacity.

Where it goes wrong

Material and cost

Deep wells run a few thousand to ten-plus thousand each (hole, screen, pump, power) - few holes but dear per unit. Wellpoints rent as a set at a few hundred to a thousand-plus a day. For deep, long programmes deep wells often come out cheaper overall: fewer holes, less attention.

On site

Basis

FAQ

When do I use deep wells instead of wellpoints?
Look at drawdown and soil. Wellpoints rely on vacuum suction and only manage 5-6 m of practical drawdown; beyond that use deep wells. Wells also deliver far more per unit (tens to hundreds of m³/d) and suit permeable sands and gravels. In silt and clay the permeability is so low the water will not move, and wells do worse than wellpoints or open pumping. Decide on permeability first - deep is not automatically a well.
Why add a spare well?
A dewatering system runs continuously, and any one failure - a clogged screen, a dead pump, a power cut - lets the level recover locally, and that is where flooding and slope failures start. The spare runs cold and cuts in the moment something trips. More wells mean any one failure matters less, so on a large pit the spare ratio can reach 15-20%; this tool starts at +1 and larger jobs should add proportionally.

Cite this page

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

Building Calculator. Deep Well Dewatering Calculator. 2026-10-25. https://jianzhujisuan.com/en/deep-well-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.