Alluvium
Alluvium is loose soil or sediments (such as clay, silt, sand, gravel and so on) that is eroded and carried in suspension by flood or river water before being deposited. The material of alluvium is may be unconsolidated, i.e. not formed together into solid rock, and can by picked up or eroded and carried away by moving water before being deposited elsewhere when the water flow slows down. Where the loose alluvial material is consolidated into a stone-like material (or lithological unit), this is known as lithification.
The fine-grained fertile soil that is deposited by water that flows over flood plains or river beds is known as alluvial soil.
Some of the characteristics of alluvial soils include:
- The soil morphology will vary according to the age of the alluvial deposit and how it was formed.
- The textural range of the soil can vary widely from gravel to silty clay.
- Drainage can vary from very poor to free.
- The texture of the soil can vary both vertically and laterally.
- It may contain a large amount of organic matter.
The presence of alluvial deposits may mean that the ground conditions are poor and so can require the construction of a raft foundation, or deep pile foundations. In these conditions, strip or pad foundations would require significant excavation.
Geoarchaeology, Using Earth Sciences to Understand the Archaeological Record, published by Historic England in 2015, defines states: ‘alluvium any water-borne sediment is technically alluvium, but the common usage is for fine-grained floodplain deposits’.
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