Tension piles
Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics.
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 insufficient to support loads imposed and so they are transferred to deeper layers with higher bearing capacity.
Pile foundations are deep foundations. They are formed by long, slender, columnar elements typically made from steel or reinforced concrete, or sometimes timber. A foundation is described as 'piled' when its depth is more than three times its breadth.
Pile foundations are principally used to transfer compressive loads from a building's superstructures, through weak, compressible strata or water onto stronger layers of soil or rock at depth.
Tension piles, also known as uplift piles or anchor piles, are a type of pile foundation that is used to resist uplift forces that might otherwise cause it to be extracted from the ground.
Uplift forces can develop as a result of hydrostatic pressure, seismic activity or overturning moments. In particular, tall structures such as chimneys, jetties and transmission towers can be subject to high wind loads and hence, overturning moments. The compressive and tensile forces generated must be transmitted to the ground along the length of the pile.
Tension piles resist uplift forces through the action of friction along their length, by under-reaming (that is enlarging the end of the pile), by the action of helical bearing plates welded to the pile shaft, or by bonding the pile into rock.
Tension piles may also be used to resist the horizontal forces from sheet piling walls. See also: Ground anchors.
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