Geological description
Fine to very fine granular soils with medium to loose compacity. Typical of floodplains and coastlines. Although they can support light buildings, they are treacherous soils linked to water processes: they may lose all their apparent structural strength when saturated or collapse instantaneously in the face of severe seismic vibrations (liquefaction).
Foundation ?
To avoid local punching, the foundation slab is used extensively to distribute the stress homogeneously under low loading.
Excavatability ?
Very easy. Manual excavation or light mechanical excavation without any difficulty.
Settlements ?
Medium-to-high. Significant risk of differential settlements due to lateral heterogeneities or density variations.
Water table ?
Highly superficial and destructive due to the dragging of fines (piping).
Seismic risk ?
Loose saturated granular soils are particularly vulnerable to liquefaction during earthquakes: the soil temporarily loses its load-bearing capacity and behaves like a fluid. This phenomenon is the most frequent cause of damage to foundations after an earthquake. It requires SPT tests and liquefaction potential analysis in accordance with earthquake-resistant regulations.
Construction advantages
- Extremely easy excavation (moves with a manual spade).
- Foundation suitable using reinforced slabs for one- or two-storey dwellings without insoluble problems.
Site limitations
- In areas at seismic risk and with a lot of water, they can “liquefy” and lose strength completely.
- Completely sensitive to pipe failure: flowing water washes the sand away and undermines the footing in days.
- Unstable excavation walls; it requires mandatory shoring.
Where this soil is found
Alerts
Hazard- Potential risk of seismically induced liquefaction: If located in areas of high seismic acceleration (Alicante, Murcia, Granada, Almería) and there is a high groundwater table, this soil may behave like a liquid during an earthquake, overturning the building.
- They are extremely sensitive to water: a prolonged leak from a pipe will locally cause the ground beneath the foundation to subside.
- Excavation for basements requires sheet piling and deep reductions in the groundwater level (well-points) to prevent liquefaction of the base.