Geological description
karstic structures (caves, sinkholes, dolines) that pose the main geotechnical risk.
Foundation ?
In general, direct foundation using braced strip footings. If shallow cavities are detected, micro-piling or grouting infill may be required.
Excavatability ?
Difficult. Requires heavy machinery with a percussive hammer for excavations of basements or swimming pools.
Settlements ?
Foundations in normal conditions. There is a risk of sudden collapse if the foundation accidentally bears on the thin vault of an undetected karst cavity.
Water table ?
Highly dynamic karst aquifers. The water level can fluctuate by dozens of metres rapidly following episodes of heavy rainfall.
Seismic risk ?
Limestones and dolomites transmit seismic waves well without amplifying them under normal conditions. However, karstic cavities may collapse during an earthquake, causing sudden sinkholes. In karstic areas with seismic activity, it is essential to rule out voids beneath the foundation.
Construction advantages
- High mechanical strength base.
- Good support for conventional footings if the matrix is sound.
Site limitations
- Risk of cavities or hidden karst caves beneath the footprint of the dwelling that may collapse.
- Frequent boulders of hard rock embedded in soft clay (terra rossa) that complicate uniform foundation.
- Difficult basement excavation.
Where this soil is found
Alerts
Warning- Risk of karst collapse: The geotechnical study must place particular emphasis on ruling out cavities or sinkholes beneath the footprint of the building (using geophysics or denser ground investigations).
- Take particular care regarding backfills of 'terra rossa' (decalcification clays) concealed within larger fissures, which act as compressible soft spots.
- Excavability becomes drastically more expensive when encountering sound rock at formation level.