Geological description
Cohesive soils with a hard feel, sometimes with a rock-like appearance when dry (grey marls, pre-Pyrenean facies, deep Guadalquivir basin). They have a long history of geological compression in the past, which gives them high stiffness. They support structural loads very well, but as they are clays, they retain hydrophilic and plastic behaviour to some extent.
Foundation ?
Direct foundation is viable. Piling is rarely resorted to unless the firm stratum lies beneath deep layers of soft clays.
Excavatability ?
Medium-difficult. When dry, it is as hard as stone. When wet, it strongly greases mechanical equipment or vehicles on site (impassable mud tracks).
Settlements ?
Medium term, in the long run. Settlement due to secondary consolidation may take 5–10 years to die out completely.
Water table ?
Impermeable soil. It does not have a groundwater table per se; it acts as an aquiclude, preventing water from passing downwards.
Seismic risk ?
Hard clays and overconsolidated marls exhibit moderate seismic behaviour. Their stiffness limits wave amplification, but they may experience cracking and a partial loss of strength under cyclic loading. Slopes in these materials may undergo earthquake-induced landslides.
Construction advantages
- Hard ground ("false boulder") that allows isolated strip footings to be used without excessively oversized foundations.
- It has no problems with sudden collapse like sands.
Site limitations
- When it rains, the plot becomes an unworkable, sticky quagmire.
- Although they are tough, they are still clays: they develop settlements over a very long period (5 to 10 years) which can, over time, generate settlement cracking.
- If the ground is reduced significantly to construct the basement, the base may “expand” and lift slightly.
Where this soil is found
Alerts
Warning- As the original level is offset with tanks or cuts, the clay may attempt to react (residual expansion due to lithostatic unloading). Pour concrete quickly after excavation to prevent harmful relaxations or heave of the excavation base.
- Pay special attention to slopes: a saturated, inclined clay layer is the perfect slip-plane for large-scale slope processes.
- Its negative adhesion to pile shafts in active zones may require deep reinforcement in tension.