Ground engineering · Groundwater & dewatering
Dewatering: drawdown, pumping rate and neighbour settlement calculator
Excavation treated as an equivalent large-diameter well in an unconfined or confined aquifer: radius of influence (Sichardt), pumping rate for the required drawdown at the excavation, drawdown with distance, and the consolidation settlement of a compressible layer under neighbouring buildings caused by the increase in effective stress. Checks the angular distortion across the nearest building.
Standard: Dupuit–Thiem · Sichardt radius of influence · CIRIA C750 groundwater control · Burland & Wroth damage criteria
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Excavation length | 40 | m |
| Excavation width | 25 | m |
| Aquifer | Unconfined | |
| Permeability k | 0.0001 | m/s |
| Saturated thickness / head above aquifer base | 15 | m |
| Confined aquifer thickness | 8 | m |
| Drawdown required at the excavation | 6 | m |
| Radius of influence (0 = Sichardt 3000 s √k) | 0 | m |
| Yield per well / wellpoint header | 4 | L/s |
| Distance to near side of the neighbouring building | 8 | m |
| Distance to far side | 20 | m |
| Compressible layer thickness under the building | 4 | m |
| Coefficient of volume compressibility mv | 0.3 | 1/MPa |
| Tolerable angular distortion (1 in …) | 500 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Equivalent well radius | 17.8 | m |
| Radius of influence R0 | 180 | m |
| Pumping rate Q | 19.6 | L/s |
| Wells / headers at the stated yield | 5 | |
| Drawdown at the near side of the building | 4.80 | m |
| Settlement near / far side | 56.5 / 43.5 | mm |
| Angular distortion | 1 in 923 |
- ✓ Angular distortion ≤ 1/500 — 1/923
- ✓ Building within the radius of influence is monitored
Method, formulas and assumptions
- re = √(ab/π); Sichardt R0 = 3000·s·√k (k in m/s). Unconfined: Q = πk(H² − h²)/ln(R0/re); confined: Q = 2πkbs/ln(R0/re).
- Drawdown at r: unconfined H − √(H² − Q ln(R0/r)/πk); confined Q ln(R0/r)/(2πkb).
- Settlement = mv·γw·drawdown·Hc (effective-stress increase equals the fall in pore pressure); angular distortion between the two sides of the building vs 1/500 (Bjerrum / Burland).
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.