The groundwater level, or water table, is the height to which the ground is fully saturated with water; it is effectively the top surface of the groundwater. It is measured against a fixed datum and varies with the season, rainfall and the water management of the area.
Why it matters
The groundwater level has a major influence on almost every construction project:
- Excavation and dewatering — if the excavation lies below the water table, the water must be temporarily pumped away (dewatering) to work in the dry.
- Basements — a basement below the water table must be built watertight and designed against uplift (upward water pressure).
- Foundations — a fluctuating water table can attack timber piles: pile heads that fall dry rot away (pile rot). A steady, sufficiently high level is therefore important for older timber foundations.
- Settlement — lowering the water table raises the effective stress and can cause settlement of the surroundings.
Measuring and controlling
The groundwater level is measured with a standpipe (piezometer): a perforated pipe in the ground in which the water level is read. To control water, builders use:
- Drainage — removing surplus water
- Dewatering — temporarily lowering it during construction
- Infiltration — replenishing or retaining water
At the start of a project, a site investigation with standpipes gives insight into the highest and lowest groundwater levels to expect.
Related terms
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