Foundations & Building Site Excavation
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Foundations, also called footings or substructure, are the base for a building and support the walls and roof. The foundations are one of the most important and difficult aspects of a build but your builder or architect should be able to advise on what types of foundations are required.
The type of foundation depends on:
** The total load/weight of a building
** The quality of the subsoil
** The weather conditions
** The soil types - eg sandy or clay
If the soil is sandy, the ground can freeze and pushes up as it expands. If the soil is clay-like, in a drought the ground can shrink causing cracking and disturbance to the foundations.
Types of Foundations
Strip foundations provide good strength for a reasonable cost and are the most usual foundations employed. They are dug until a clay surface is reached to avoid damage from changes in various top soil types. Once a trench is dug, approximately 150mm of concrete is poured into the trench. Then block or brick and block cavity walls are built up to ground level and the trench is back-filled. There are three types of strip foundations:
Traditional Shallow Strip - The extra concrete required creates additional material expense but the process is easier and less time consuming. The trench is usually around 400mm wide and 900mm deep and can be reinforced.
Wide Strip - as above but wider and reinforced to save on concrete expense.
Deep Strip or Trench - narrower trench filled with concrete up to two bricks of ground level.
Raft foundations are used on soft soils such as those made of clay or peat. The soft soils can create problems of ground movement and sliding so Raft foundations are reinforced and have an apron edge.
On sinkable clay soils, infill or waste tip sites, slopes or sites with high water tables or poor drainage, traditional foundations would need to be so deep that they would be uneconomical. Stilts or columns are used to reach down to solid, load bearing soil. These columns can be 4m, even 10m deep.
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