Dewatering for dry, stable excavations
Dewatering temporarily lowers groundwater so excavations and foundations can be built in dry, stable conditions. Depending on the soils and the depth, systems range from sump pumping to wellpoints and deep wells.
Suitable ground conditions and applications
Can be considered where…
- Excavations below groundwater for basements, pipelines, pump stations and foundations.
- Preventing base heave or piping in excavations.
- Keeping formation levels stable during construction.
Limitations
- Lowering groundwater can cause settlement of nearby structures, which is assessed and monitored.
- Discharge must be permitted and managed for sediment and salinity.
- Fine soils drain slowly and may need other methods.
- Pumping from open sumps in fine sands can draw soil into the excavation; wells are then preferred.
- Systems must run continuously, with standby power and pumps.
Suitability is confirmed from the site investigation and the performance requirement.
How it works
Sumps collect water inside the excavation for shallow, low-flow conditions. Wellpoints (closely spaced small wells connected to a suction header) suit sandy soils at moderate depth. Because they work by suction, the drawdown from one stage is limited; deeper excavations need more than one stage or deep wells.
Deep wells with submersible pumps lower groundwater over larger depths and flows. Together they draw down the water table around the excavation in a cone of depression.
Design inputs
- Site investigation: soil permeability and groundwater levels.
- Excavation depth and required drawdown.
- Duration of dewatering.
- Adjacent structures and settlement limits.
- Discharge location and permits.
Trial and calibration
- Pumping tests to measure permeability and response.
- Trial wells, where needed, to confirm yield.
Execution controls
- Groundwater levels in piezometers.
- Flow rates and discharge quality.
- Pump running hours and standby checks.
- Daily records.
Verification and acceptance
Design intent, production control and acceptance verification are kept separate, so it is always clear what was required, what was done and what was proven.
What is required
Target groundwater level below formation agreed with the consultant.
What was done
Piezometer levels and flow records.
What was proven
Confirmed drawdown before excavation; settlement monitoring of neighbouring structures.
Adjacent assets
Drawdown can extend beyond the site and cause settlement of neighbouring structures.
- Neighbouring structures are assessed and surveyed before pumping starts.
- Settlement and groundwater levels are monitored against trigger levels.
Deliverables
- Method statement and risk assessment
- Dewatering design and layout drawings
- Pumping test report
- Monitoring records
- Discharge records
Common questions
Which dewatering system is right for my excavation?
It depends on the soil permeability, the drawdown needed and the duration. Sumps suit shallow, low-flow conditions; wellpoints suit sands at moderate depth; deep wells suit larger drawdowns and flows.
Can dewatering affect neighbouring buildings?
Lowering groundwater can cause settlement nearby. The risk is assessed before pumping and monitored during the work.
Is a permit needed to discharge the water?
Discharge must meet the requirements of the local authority. This is arranged for each project.
Discuss whether Dewatering fits your ground conditions.
Share the site location, the investigation report and the performance requirement.