Geological description
High-compacity granular deposits, typically associated with old fluvial terraces (Ebro, Duero, Tajo, Guadalquivir rivers) and Pleistocene glacis. They have no cohesion (they are not clays) but their angle of internal friction is very high, giving them a firm, stable behaviour that is well suited to traditional building, and allowing water to drain effectively.
Foundation ?
Conventional shallow foundation without problems. Raft foundation in the case of basements or if the ground intersperses silty lenses.
Excavatability ?
Easy. Standard backhoe. In strata of large rounded boulders (> 20 cm) there may be slight difficulty in extraction.
Settlements ?
Shallow layers. They are characterised by immediate elastic settlements; the ground settles instantly as the structure is raised, minimising long-term problems.
Water table ?
Exceptional aquifers. It is common to find the stabilised water table at a medium depth, linked to the base level of the nearby river.
Seismic risk ?
Compact, well-graded granular deposits transmit seismic waves without excessive amplification. Their seismic behaviour is generally favourable. The main risk is densification (dry settlement) in deposits with a certain proportion of fines under moderate accelerations.
Construction advantages
- The ideal ground for self-build construction. Sets instantly and does not cause problems in the long term.
- Excellent natural drainage; it does not retain moisture by capillarity.
- Very fast and economical excavation.
Site limitations
- The excavation walls (trenches, service connections, basements) collapse very easily when the sand dries out (low cohesion).
- Risk of collapse if there is a strong underground water flow that carries away the fines (siphoning).
Where this soil is found
Alerts
Neutral- Trench stability: Because it lacks cohesion (clays), the vertical walls of the excavations will collapse very easily when they dry out if they are not properly shored up.
- Take particular care regarding seepage (siphoning) at the base if excavation is carried out below the groundwater level. It is impossible to pump out water without entraining the fine sand.
- Constructively ideal ground, but it is necessary to check densities with periodic SPT as the excavation proceeds to greater depth.