The three common methods
Sump pumping collects water in pits inside the excavation and pumps it out. It is simple, but water flows towards the excavation, which can carry fine soil with it.
Wellpoints are small wells at close spacing around the excavation, connected to a header pipe and a vacuum pump. They lower the water table before and during excavation.
Deep wells are larger drilled wells, each with its own submersible pump. They are spaced more widely and can lower groundwater to greater depth.
Side-by-side comparison
| Factor | Sump pumping | Wellpoints | Deep wells |
|---|---|---|---|
| Typical use | Shallow excavations with low inflow | Sands at moderate depth | Deep excavations and permeable soils |
| Drawdown | Small | Limited per stage by suction lift | Large |
| Deeper excavations | Not suitable | Extra stages on benches | Suitable |
| Main risk | Loss of fine soil and unstable slopes | Less effective in low-permeability soil | Wider drawdown beyond the site |
What the design needs
- Soil layers and their permeability, ideally from pumping tests.
- Groundwater level and how it varies with season or tide.
- Excavation depth, plan size and duration.
- Any confined water-bearing layer below the base that could cause uplift.
- Where the pumped water can be discharged, and its quality.
Effects outside the site
Lowering groundwater increases the load carried by the soil, which can cause settlement of neighbouring structures. The likely settlement is assessed at design stage and monitored during pumping.
Where the drawdown outside the site must be limited, a cut-off such as a secant wall or grouting reduces the flow, and sometimes water is returned to the ground through recharge wells.
What is monitored
- Groundwater levels in observation wells inside and outside the excavation.
- Flow rates, and the amount of fine soil in the discharge.
- Settlement of neighbouring structures against agreed limits.
- Standby pumps and power, because a pumping stop can flood or destabilise the excavation.