Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics.
There are a very wide range of foundation types available, suitable for different applications, depending on considerations such as:
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The nature of the load requiring support.
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Ground conditions.
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The presence of water.
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Durability of the materials.
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Cost.
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Sensitivity to noise and vibration.
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Proximity to other structures.
PILING FOUNDATION
Foundations
Broadly speaking, all foundations are divided into two categories: shallow foundations and deep foundations. The words shallow and deep refer to the depth of soil in which the foundation is made. Shallow foundations can be made in depths of as little as 3ft (1m), while deep foundations can be made at depths of 60 - 200ft (20 - 65m). Shallow foundations are used for small, light buildings, while deep ones are for large, heavy buildings.
Trench fill foundations are a type of shallow foundation that avoids bricklaying below ground by instead almost completely filling the trench excavation with concrete.
Typically, concrete is poured to within 150 mm of the surface ground level. This type of foundation minimises the excavation required, as bricklayers are not required to access the trench to lay bricks or blocks. It can also help to reduce the width of the foundation where below ground brickwork would require a wider footing.
Trench fill is often used when soil is loose or in areas with a high water table, although in loose ground it can be difficult to predict the quantity of concrete required, and the quantity can be quite high if trenches meet or cross at right angles.
TRENCH FILL FOUNDATION
Raft Foundation
Pad foundations are used to support individual or multiple columns, spreading the load to the ground below. They are generally square or rectangular in plan, with the plan area being determined by the permissible bearing pressure of the soil. The shape in plan will be dictated by the arrangement of the columns and the load to be transferred into the soil.
The thickness of the slab must be sufficient to ensure distribution of the load. The top of the pad may be sloping (i.e. the pad is thicker in the centre than it is at the edge). This is an economic solution, although there may be construction problems involved with casting the slope.
In simple cases the pad may be constructed from mass concrete. In general, some reinforcement (either welded steel fabric or reinforcing bars, depending on the loads involved) will be required in both directions. For design purposes, the pad is treated as if it were an inverted cantilever carrying the soil pressure and supported by the column.
STRIP AND PAD FOUNDATIONS
Raft Foundation
Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics.
Typically, raft foundations are formed by reinforced concrete slabs that cover a wide area, often the entire footprint of a building. They spread the load imposed by a number of columns or walls over a large area, and can be considered to ‘float’ on the ground as a raft floats on water.
They are suitable where:
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Ground conditions are poor and strip or pad foundations would require significant excavation, for example on soft clay, alluvial deposits, compressible fill and so on.
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Settlement, or differential settlement is likely.
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Where it may be impractical to create individual strip or pad foundations for a large number of individual loads. In very general terms, if strip or pad foundations would cover 50% or more of the floor area, then a raft may be appropriate.
Raft Foundation
RAFT FOUNDATION
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Raft Foundation