Dewatering Calculator

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.

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.

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.