What is excess pore water pressure?
Excess pore water pressure is a temporarily raised water pressure in the soil, caused when the ground is loaded or deformed faster than the water can escape. The gradual dissipation of that excess pressure is called consolidation.
How it arises
Saturated soil consists of grains with water in the pores between them. Load that soil and the load is initially carried by the pore water, not by the grain skeleton — water is barely compressible. In low-permeability soils such as clay and peat the water cannot escape quickly, so the water pressure rises.
As long as the water carries the load, the effective stress between the grains is low and the soil has little strength. Only once the water has drained does the grain skeleton take over the load, strength increases and settlement occurs.
Typical situations:
- Piling. Every driven pile displaces soil. In saturated sand or clay, excess pore pressure builds up around the shaft and beneath the toe, temporarily reducing friction between the grains. That is why the capacity of a driven pile keeps increasing in the days after driving — an effect known as set-up.
- Filling. A sand fill placed on clay or peat generates excess pore pressure in the subsoil. That is why a fill is given a consolidation period before anything is built on it, sometimes accelerated with vertical drains.
- Rapid water level change or vibration. Vibration from traffic, piling or an earthquake can also raise the water pressure in loose, saturated sand — in the extreme case leading to liquefaction.
Why it concerns you
- Measuring is the only certainty. Excess pore pressure cannot be read off the ground. For large fills on settlement-sensitive subsoil it is measured with piezometers, and the settlement with settlement plates.
- Waiting is building. The consolidation period is not lost time but part of the construction. Starting too early means residual settlement under the building or road.
- Excavation needs attention. Where a low-permeability layer overlies a water-bearing sand in an excavation, the pressure below can lift the base. This heave risk is assessed in the geotechnical report.
Related terms
- Consolidation
- Effective stress
- Piled foundation
- Ground investigation
- Strip foundation
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