Clay is a soil made up largely of very fine mineral particles. In soil science the boundary is: clay particles are smaller than 2 µm (0.002 mm). Because of those tiny, plate-shaped particles, clay sticks, holds water for a long time and lets water through very poorly.
For building, clay has two faces: as ground it is awkward material, as a raw material it is excellent.
Clay as ground
- Compressible — clay settles under load, and that can go on for years
- Shrinks and swells — in dry weather clay shrinks and cracks open; when it wets up it swells again. That damages paving, terraces and shallow-founded structures
- Almost impermeable — rainwater stands on clay soil; soakaways barely work
- Shaft friction — a pile in clay develops substantial shaft friction, but in settling layers also negative shaft friction
In large parts of the western and northern Netherlands, clay or peat lies at the surface. That is why building there is traditionally done on piles.
Clay as a building material
- Brick, roof tiles and floor tiles — clay is shaped, dried and fired
- Water retention — a clay layer or clay cut-off in a dike makes the structure watertight
- Clay plaster and earth building — unfired clay as an internal finish or as a solid wall
- Bentonite — a strongly swelling clay used for seals and drilling fluid
Practical tips
- Have the ground investigated — for an extension or a garden wall on clay, the soil test decides whether shallow foundations will do or piles are needed
- Expect settlement — hard surfaces on clay keep dropping in the first few years; lay terraces with a little extra height on a thick, well-compacted sub-base
- Do not improve garden soil with sand alone — a layer of sand on clay acts like a bathtub; work organic material into the topsoil instead
- Do not work clay when wet — wet clay smears, and the structure is then hard to restore
Related terms
- Soil type
- Soil mechanics
- Clay cut-off
- Claystone
- Brick
- Shaft friction
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