A soil type is a class of ground defined by its grain size and composition. The main soil types are sand, clay, peat, loam and gravel. The soil type beneath a building largely governs its bearing capacity, its settlement and the choice of foundation.
The main soil types
- Sand — loose, coarse-grained soil that drains well. It bears well when compacted and is often used as fill and foundation material.
- Clay — fine-grained, cohesive soil that drains poorly. Clay can shrink and swell with changing moisture and is compressible (prone to settlement).
- Peat — organic soil formed from plant remains. Soft, light and highly compressible; building on peat almost always calls for a piled foundation.
- Loam — a mix of sand, silt and clay; moderately permeable and reasonably load-bearing.
- Gravel — coarse-grained and very free-draining; strong and well load-bearing.
Why it matters
The soil type determines, among other things:
- Bearing capacity and foundation — sand or gravel may allow a shallow (strip) foundation, while clay and peat usually need piles
- Settlement — how much a building sinks; clay and peat settle a lot, sand very little
- Drainage and dewatering — how quickly water soaks away or stands
- Excavation and slopes — how stable an excavation stays
The soil type and the make-up of the subsoil are established by a site investigation, using probes and boreholes.
Related terms
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