Geological description
Chemically formed sedimentary rocks produced by the precipitation of minerals dissolved in closed basins. Gypsum predominates (calcium sulphate) and, to a lesser extent, anhydrite and salts (halite). Extremely common in the Ebro Depression (Zaragoza, Tudela), the Madrid basin (Vallecas, Aranjuez), the south-eastern part of the peninsula and the Balearics. They constitute the most treacherous ground in Spain due to their solubility: groundwater literally dissolves the rock, creating cavities, conduits and sudden sinkholes that collapse without any warning.
Foundation ?
The foundation must be designed to withstand local loss of support due to dissolution. Hyper-rigid slabs with symmetrical reinforcement that bridge any possible voids, or piling down to non-soluble substrate. Isolated footings are prohibited.
Excavatability ?
Easy to Medium. The gypsum is excavated well with conventional backhoe. Compact anhydrite requires a breaker hammer.
Settlements ?
Tall and unpredictable. The progressive dissolution of the substrate creates voids beneath the foundation which cause sudden settlements and localised collapses without warning.
Water table ?
Critical. Water is the main destructive agent. Any seepage, pipe leakage, or variation in the groundwater level accelerates dissolution in an exponential way. Aquifers in gypsum are extremely aggressive chemically.
Seismic risk ?
Gypsum and salts exhibit a particular seismic behaviour: their solubility can generate cavities that collapse during an earthquake, similar to the karst risk in limestones. In addition, salts can flow slowly under cyclic loads (creep). In seismic areas, it is essential to rule out the presence of dissolution voids beneath the foundations.
Construction advantages
- Relatively straightforward excavation in pure gypsum.
- Generally level ground in the depressions where it appears.
Site limitations
- Water literally dissolves the ground beneath the house, creating voids and sudden subsidence.
- It forces the use of special sulphate-resistant cement throughout the foundation (additional cost of 15-25%).
- Any water leak on the plot (pipes, swimming pools, irrigation) can trigger an accelerated collapse of the subsoil.
- Anhydrite expands violently when it gets wet, potentially lifting ground slabs and cracking walls.
Where this soil is found
Alerts
Hazard- RISK OF COLLAPSE DUE TO DISSOLUTION: The main geotechnical hazard. Water (rain, leaks, groundwater level) dissolves the gypsum, creating voids that grow until the surface collapses. A geophysical study (ground-penetrating radar, electrical tomography) is mandatory in order to detect hidden cavities.
- Chemical attack on concrete: The gypsum sulphates attack conventional Portland cement. Use of sulphate-resistant cement (SR) or protective admixtures is mandatory throughout all the foundations and any buried drainage.
- Anhydrite hydrates by expanding by up to 60% of its volume on contact with water, potentially lifting slab surfaces and bursting retaining walls. Take maximum care to insulate against moisture.