Geological description
Metamorphic rocks characterised by their foliation (parallel planes of weakness or slaty cleavage). Very common in Galicia, Asturias, the north of León, Extremadura and Sierra Morena. They are highly anisotropic rock masses: their mechanical properties change drastically depending on whether the load is applied parallel or perpendicular to the foliation planes.
Foundation ?
Shallow foundation if the stratum is competent. Strong bidirectional bracing is recommended due to the heterogeneity of the weathered rock.
Excavatability ?
Medium to difficult. Ripping (scarification) is usually effective in favour of the slatey cleavage planes, but against them it is tough.
Settlements ?
Low-to-medium. Very steep dip planes can favour differential settlements or slope stability problems.
Water table ?
Variable. Water usually circulates preferentially through the network of joints and foliation planes (fractured aquifers).
Seismic risk ?
Foliated metamorphic rocks can selectively amplify seismic waves in the direction perpendicular to the foliation planes. Their behaviour during earthquakes depends on the degree of alteration and on the orientation of the schistosity relative to the foundation. It is recommended to assess the foliation direction in the geotechnical study.
Construction advantages
- Adequate strength for conventional residential buildings.
- If the stone’s dip (foliation) is favourable, it is stable ground.
Site limitations
- Unstable and dangerous behaviour if excavated “along” the dip of the slate (risk of sliding over the site).
- Rapid degradation if the newly excavated stratum is left exposed to the open air and rainfall before concreting.
Where this soil is found
Alerts
Warning- Landslide risk: Take special care on slopes cut for basements if the bedding of the slate dips towards the excavation (imminent risk of planar failure). Side walls or rock bolting will be required.
- Avoid bearing the spread footings on both the ‘spurs’ of rock and the ‘voids’ of weathered clay simultaneously.
- The ground may undergo surface decompression and open up its planes upon oxygenation after excavation, losing bearing capacity within weeks if it is not cast in concrete.