Water is the most expensive thing on a construction programme that nobody budgets for. A flooded excavation stops every trade at once, softens formation that then has to be dug out and replaced, and turns a two-day foundation pour into a two-week argument. Dewatering is the cheapest insurance on the site, provided the method matches the ground.
Three methods, three ground conditions
Sump pumping
The simplest approach: dig a sump in the lowest corner of the excavation, let water run to it, and pump it out. It works in clays, silts and rock where inflow is modest and the ground is stable.
Its limit is that it lowers the water table only at the sump, so water continues to flow through the excavation face on its way there. In sandy or silty ground that flow carries fines with it, undermining the batter and sometimes causing a base heave or a boiling condition. If the sump keeps producing dirty water, sump pumping is the wrong method for that ground.
Wellpoint dewatering
A line of small-diameter wellpoints is jetted into the ground around the excavation and connected to a header main under vacuum from a wellpoint pump. Instead of collecting water inside the hole, the system lowers the water table around it before the dig reaches that depth, so the excavation is worked dry and the faces stay stable.
A single stage of wellpoints lowers the water table by about 5 to 6 metres – the practical limit of suction lift. Deeper excavations need multiple stages, each on a bench, which is why wellpoint work has to be planned into the dig sequence rather than added later.
Deep well dewatering
For deep excavations or high-permeability ground, individual bored wells each fitted with an electric submersible pump. Each well is drilled, screened and gravel-packed, and there is no suction-lift limit because the pump is at the bottom. It is the most expensive method and the only one that works for basements in water-bearing sand or fractured rock.
| Method | Drawdown | Best ground | Relative cost |
|---|---|---|---|
| Sump pumping | 1 – 2 m effective | Clay, silt, rock, low inflow | Low |
| Single-stage wellpoints | 5 – 6 m | Sand, sandy silt, gravelly sand | Moderate |
| Multi-stage wellpoints | 10 – 18 m | As above, benched excavations | High |
| Deep wells | Unlimited in practice | High-permeability sand, gravel, fractured rock | Highest |
| Eductor / ejector | Up to 30 m | Low-permeability silts where wellpoints struggle | High |
Sizing: two flows, not one
Dewatering capacity must cover two separate things, and construction sites regularly size for only the first.
- Groundwater inflow – a steady, predictable rate estimated from the soil permeability, the excavation dimensions and the drawdown required. A geotechnical report gives you the permeability; a dewatering specialist turns it into litres per second.
- Storm water – an intense, short-duration load. A Highveld thundershower dropping 40 mm in an hour on a 30 by 40 metre excavation plus its catchment is well over 30 000 litres in that hour, arriving all at once.
The usual answer is a duty pump sized for groundwater inflow running continuously, plus standby capacity sized for the storm event. Sizing one pump for the combined worst case gives you a machine that runs far off its curve for 95 per cent of the programme.
Choosing the pump
- Electric submersible drainage pumps. The default for sump duty where there is site power. Quiet, compact, no priming, and they handle the silty water that construction produces. Fit a dry-run cut-out.
- Self-priming diesel trash pumps. The workhorse for sites with no power and for storm duty. They re-prime themselves after the sump runs dry, pass solids, and keep working in a downpour. Check the fuel tank endurance if they must run unattended overnight.
- Wellpoint vacuum pumps. Purpose-built to handle the air-water mixture a wellpoint header produces. A standard centrifugal will not do this job.
- Borehole-type submersibles for deep wells – the same machines described in how borehole pumps work, sized for the well yield.
- Hydraulic submersibles where the water is very abrasive or the duty is continuous and heavy.
Whatever the type, solids handling matters. Construction water is not clean water – it carries sand, cement fines and clay, all of which are abrasive. A pump with hardened or replaceable wear parts will outlast a clean-water pump many times over on the same duty.
Head, hose and the mistake that wastes half the pump
Dewatering head is usually dominated by the horizontal discharge run, not the lift out of the hole. A 6 metre lift is nothing; 250 metres of undersized lay-flat hose to the nearest legal discharge point is everything.
Velocity in lay-flat should stay under about 3 metres per second. Going one hose size up frequently gives more delivered flow than going one pump size up, for a fraction of the cost. Work the friction through as in how to calculate pump head before committing to hose sizes.
Discharge: the compliance problem
Where the water goes is a regulated question. Silty discharge into a storm water system or a watercourse is an offence under the National Water Act, and municipalities increasingly enforce it on urban sites.
- Settlement tanks or lagoons to drop suspended solids before discharge. Size for retention time, not just volume.
- Flocculant dosing where the fines are clay-sized and will not settle on their own.
- Discharge consent from the municipality or the Department of Water and Sanitation, obtained before the pumps start rather than after a complaint.
- Recharge or reuse – on larger sites, returning water to the ground away from the excavation or using it for dust suppression is often cheaper than treating it for discharge.
Running it properly
- Never stop. Dewatering runs continuously until the structure is heavy enough to resist uplift. Switching off over a long weekend has floated more basement slabs than any other single error.
- Standby power. Load shedding is a design input. Diesel standby or a generator on automatic changeover is not a luxury on a deep excavation.
- Monitor the water table, not just the pump. Piezometers around the excavation tell you whether the system is achieving the drawdown, and give early warning when a well silts up.
- Watch for settlement. Lowering the water table can settle adjacent structures. On urban sites this needs monitoring and sometimes a recharge system.
- Keep a spare on site. A replacement pump three hours away is a flooded excavation.
Rent rather than buy
Dewatering demand is intense and temporary, which makes it the classic rental application. Renting gives you the right pump for each phase, standby units without owning them, and somebody else’s maintenance obligation. Our fleet covers submersible drainage pumps, diesel trash pumps, wellpoint sets, hose and settlement tanks – see equipment rentals, or our dewatering pump range if the requirement is permanent.
Dewatering an excavation? Send us the excavation dimensions, the geotechnical report and the programme, and we will size the system, supply the plant and set it up on site. Request a quote or call +27 (15) 516-4991.