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Building foundations

Revision as of 09:29, 12 November 2015

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 suitable for different applications, depending on considerations such as:

The nature of the load requiring support.
Ground conditions.
The presence of water.
Accessibility.
Sensitivity to noise and vibration.

Very broadly, foundations can be categorised as shallow foundations or deep foundations. Shallow foundations are typically used where the loads imposed by a structure are low relative to the bearing capacity of the surface soils. Deep 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.

Shallow foundations include:

Strip foundations (or footings). Used to provide a continuous strip of support to a linear structure such as a wall.
Pad foundations. Typically rectangular or circular pads used to support localised loads such as columns.
Raft foundations. Typically these are slabs that cover a wide area, often the entire footprint of a building, and are suitable where ground conditions are poor, settlement is likely, or where it may be impractical to create individual strip or pad foundations for a large number of individual loads. Raft foundations may incorporate beams or thickened areas to provide additional support for specific loads.

Deep foundations include:

Piles. Pile foundations are long, slender, columns typically made from steel or reinforced concrete and 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 takes place simultaneously, the piles are called continuous fight augured (CFA) piles.
Mini piles (or micro piles). Used where access is restricted, for example underpinning structures affected by settlement. They can be driven or screw 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. 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. Watertight retaining structures sunk into the ground by removing material from the bottom, typically this might be suitable for building structures below water level.
Compensated foundations. If a very large amount of material is excavated, (for example where there is a deep basement) this 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.