What is pressure relief dewatering?
Pressure relief dewatering is pumping groundwater from a deeper, water-bearing sand layer to reduce the upward water pressure beneath an excavation or structure. The aim is not to make the excavation dry, but to prevent the confining layer below the base of the excavation from being pushed up by the water pressure and bursting open (base heave). In Dutch it is called spanningsbemaling. It is used, for example, for tunnels, cellars and deep excavation pits.
How it works
- Confined sand layer. In large parts of the Netherlands, below a poorly permeable layer of clay or peat, lies a sand layer in which the groundwater is under pressure. The piezometric head of that water can be higher than the base of the excavation.
- Base heave. As you dig the excavation, the soil layer above the sand becomes thinner and lighter. If the water pressure below is greater than the weight of that layer, the base can burst open or float up. Every metre of piezometric head above the underside of the confining layer gives roughly 10 kN/m² of upward pressure.
- Lowering the pressure. Using filter wells in the deep sand layer, water is pumped out so that the piezometric head drops to a safe level.
Application
- Ground investigation first. Use cone penetration tests and boreholes to establish where the confining layer and the aquifer lie and what the piezometric head is.
- Calculation by a geotechnical consultant. They determine whether pressure relief dewatering is needed and how far the head must be lowered.
- Permit or notification. Abstracting and discharging groundwater is usually subject to notification or a permit; check with your local authority or water authority well before the start.
- Monitor the surroundings. Lowering groundwater pressure can cause settlement in the surrounding area, and with a lower groundwater level the timber foundation piles of neighbouring buildings can start to rot. Install monitoring wells and monitor settlement and groundwater levels.
- Consider an alternative. Where dewatering is not permitted or too risky, a closed excavation pit with sheet pile walls and underwater concrete is often used.
Common mistake: switching off the dewatering too early. As long as the structure is not yet heavy enough or not anchored, it can still float up.
Related terms
- Dewatering
- Wellpoint dewatering
- Excavation pit
- Construction pit
- Sheet pile wall
- Groundwater level
- Settlement
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