Geological description
The great queens of pathology in civil building in Spain. They are frequently found in the lands of barros of Córdoba and Badajoz, the vegas of the Guadalquivir (Mairena, Carmona), the Miocene basin of Southern Madrid (Getafe, Móstoles, Esquivias). Clays with smectitic silicates act like an accordion: they absorb enormous quantities of water, swelling dramatically in winter and contracting, cracking the ground, in the summer heat (crevasse).
Foundation ?
Inviable the pure shallow isolated footing. Either a symmetrically designed reinforced hyper-rigid slab calculated for the ‘soil lifting’, or (preferably) piles to depths that cannot be altered under natural moisture conditions (> 4 m) with ground floor slabs raised over a void chamber.
Excavatability ?
Easy in principle, but dramatic if it rains.
Settlements ?
More than settlements: Heave. Clay can physically lift a house and rupture its ground slab if it takes on water (reversible seasonal movements).
Water table ?
Dramatic variability in the first metre. It is deep-lying, impermeable, but the surface zone captures all the rainfall.
Seismic risk ?
Soft, expansive clays amplify low-frequency seismic waves, which increases surface displacements. Under cyclic loading they progressively lose strength (cyclic degradation). In seismically active areas, deep foundations or ground improvement are recommended to mitigate the combined effects of expansiveness and seismicity.
Construction advantages
- Soft ground, very easy to excavate and work with in the dry season.
Site limitations
- The most hazardous ground conditions for a self-developer’s property in Spain.
- They lift and settle the dwelling cyclically every winter and summer, breaking partitions and ground slabs.
- They make it mandatory to drastically oversize the foundations (deep piles, massive ground-bearing slabs), increasing the cost of the work by up to 20%.
- They require all perimeter pavements to be cleared of contaminants and sealed against rain.
Where this soil is found
Alerts
Hazard- GREATER STRUCTURAL RISK: Cause diagonal cracks in walls from around doors and windows and at wall bases. An edometric and free swelling test is essential (Lambe, Presley).
- Watertight perimeter footpaths of at least 1.5 m width must obligatorily surround the property to protect the level of seasonal moisture.
- Trees with powerful roots are prohibited near the façade; they will suck up the localised water and cause the clay to shrink, destroying the corner of the house.