Geological description
Sedimentary rocks formed by the cementation of fragments of other pre-existing rocks. They include sandstones (cemented sand), conglomerates (cemented gravels) and the rhythmic alternation of sandstones and mudstones known as flysch. Very common in the Pyrenees, the Ebro Basin, the Cantabrian Cordillera and the Coastal-Catalan Ranges. Their geotechnical behaviour depends greatly on the degree of cementation: a well-cemented sandstone behaves like competent rock, whereas a friable sandstone breaks down into sand when excavated.
Foundation ?
Direct foundation is feasible in most cases. For flysch with an unfavourable dip or for friable sandstones, bidirectional bracing or a rigid raft is recommended to absorb heterogeneities in the subsoil.
Excavatability ?
Variable. Well-cemented sandstones require a breaker hammer; friable ones are excavated with conventional excavators. Cemented conglomerates are extremely hard and abrasive for the teeth of the bucket.
Settlements ?
Basement–lower levels. In general acceptable for conventional building. The flysch may present differential settlement if the foundation crosses the boundary between a hard sandstone stratum and a soft siltstone one.
Water table ?
Fissured aquifers of variable importance. Porous sandstones may host significant aquifers; interbedded mudstones act as an impermeable barrier, creating local spring outlets.
Seismic risk ?
Sandstones and cemented conglomerates exhibit intermediate seismic behaviour. Sound rock transmits the waves without notable amplification, but interbedded softer layers (shales, marls) can generate impedance contrasts that amplify the shaking locally. Slope stability in flysch requires special attention in seismic areas.
Construction advantages
- Generally competent ground conditions for conventional residential building.
- Good long-term stability where the foundations are appropriate.
- In areas of sound sandstone, shallow footings solve the problem without complications.
Site limitations
- Very variable behaviour depending on the degree of cementation: it can change from hard rock to loose sand within a few metres.
- In flysch with unfavourable dip, there is a serious risk of landsliding of the plot slope.
- Hardened/cemented conglomerates are extremely expensive to excavate (continuous percussive hammer).
Where this soil is found
Alerts
Warning- Risk of instability in flysch: The alternation of hard and soft layers with an unfavourable dip is one of the main causes of landslides on hillsides in the north of Spain. If the plot is on a slope, the dip orientation must be checked without fail.
- Friable sandstones lose strength rapidly when they become saturated with water. Do not leave excavation bases exposed to rain before concreting.
- Conglomerates may contain very large quartzite boulders that divert the pile auger or break the excavator’s teeth.