Geological description
Igneous and metamorphic rocks with very high mechanical strength. They form the most competent and stable substrate. They predominate in the Sistema Central, in Galicia, and in outcrops of the Hercynian basement. Although sound rock is excellent, it is vital to assess the degree of superficial weathering (jabre), which reduces local bearing capacity.
Foundation ?
Direct foundation using isolated spread footings bearing on sound rock or hard marl. Typical founding depth is between 0.5 and 1.2 m.
Excavatability ?
Very difficult. Sound rock will require the use of a breaker hammer (pica) coupled to the backhoe loader, or even controlled micro-blasting. However, the marlstone (jabre) can be excavated using conventional mechanical means.
Settlements ?
Practically negligible ( 5 mm). The material is not compressible under the usual loads from a residential or industrial building.
Water table ?
Generally deep or non-existent at foundation level, except in zones of fracturing or hollows where local fissured aquifers may occur.
Seismic risk ?
Massive igneous and metamorphic rocks transmit seismic waves rapidly but without significant amplification. Their high stiffness makes them very stable in the face of earthquakes, with minimal permanent deformation. The main risk is the fall of rock blocks in slopes if the rock is fractured or superficially weathered.
Construction advantages
- Excellent bearing capacity and no risk of settlement.
- Impermeable material, so there are no fluctuations in the groundwater table that affect the foundation.
- Standardised and economical foundation (shallow spread footings).
Site limitations
- Excavation of basements or swimming pools is extremely costly, as it requires continuous breaking hammering or blasting.
- If the rock is irregular, part of the dwelling may bear on mud and another part on monolithic rock (risk of differential settlement).
Where this soil is found
Alerts
Neutral- Carry out penetration tests or pressuremeter tests to characterise the thickness of the layer of weathered jabre before supporting the foundation.
- The rock is impermeable: rainwater or runoff will run over the rocky surface and tend to accumulate in the basement trenches. Provide a robust drainage network.
- If the rock outcrops on the mid-slope, there is a risk of differential settlement if part of the house bears on rock and another part on loose colluvial material.