Introduction Building foundation
Building Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics to support them. There is a very wide range of Building foundation types suitable for different applications, depending on considerations such as the nature of the load requiring support.
- Ground conditions.
- The presence of water.
- Space availability.
- Accessibility.
- Sensitivity to noise and vibration.
Very broadly, Building foundations can be categorized as shallow Building foundations or deep Building foundations.
Shallow Building foundations are typically used where the loads imposed by a structure are low relative to the bearing capacity of the surface soils.
Deep Building foundations are necessary where the bearing capacity of the surface soils is not adequate to support the loads imposed by a structure and so those loads need to be transferred to deeper layers with higher bearing capacity.
Types of shallow Building foundations
- Trench fill Building foundation.
- Rubble trench Building foundation.
- Raft Building foundation.
Strip Building foundations (or footings)
Strip foundations provide a continuous strip of support to a linear structure such as a wall. For more information, see Strip Building foundation. Trench fill Building foundations are a variation of strip Building foundations, in which the trench excavation is almost completely filled with concrete. Rubble trench Building foundations are a further variation of trench fill Building foundations and are a traditional construction method that uses loose stone or rubble to minimize the use of concrete and improve drainage.
Pad Building foundations
Raft Building foundations
Types of deep Building foundations
Piles Building foundations
Pile Building foundations are long, slender, columns typically made from steel or reinforced concrete, or sometimes timber. Generally, piles are classified as; end-bearing piles (where most of the friction is developed at the toe of the pile, bearing on a hard layer), or friction piles (where most of the pile-bearing capacity is developed by shear stresses along the sides of the pile, suitable when harder layers are too deep). Piles are most commonly; driven piles prefabricated off-site and then driven into the ground or bored piles that are poured in situ. If the boring and pouring take place simultaneously, the piles are called continuous flight augured (CFA) piles.
Mini piles (or micro piles/micro piles)
Mini piles are used where access is restricted, for example, underpinning structures affected by settlement. They can be driven or screw piles. For more information, see Micro piles.
Pile walls
By placing piles directly adjacent to one another, a permanent or temporary retaining wall can be created. These can be closely-spaced contiguous pile walls, or interlocking secant walls, which depending on the composition of the secondary intermediate piles can be hard/soft, hard/firm, or hard/hard secant walls.
Diaphragm walls
Diaphragm walls are made by excavating a deep trench that is prevented from collapsing by being filled with engineering slurry such as bentonite and then the trench is filled with reinforced concrete panels, the joints between which can be water-tight. This is commonly used for top-down construction, where a basement is constructed at the same time as above-ground works are carried out.
Caissons
Caissons are watertight retaining structures sunk into the ground by removing material from the bottom, typically this might be suitable for building structures below water level. For more information, see Caisson.
Compensated foundations
If a very large amount of material is excavated (for example, where there is a deep basement), it may be sufficient that the relief of stress due to the excavation is equal to the applied stress from the new construction. As a result, there should be little effective change in stress and little settlement.
Ground anchors
Ground anchors transfer very high loads by using a grouted anchor to mechanically transfer load from a tendon to the ground. They can be pre-tensioned or can be tensioned by the applied load.
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