Pile 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 takes place simultaneously, the piles are called continuous fight augured (CFA) piles.
Mini piles (or micro piles/micropiles)
Mini piles are used where access is restricted, for example underpinning structures affected by settlement. They can be driven or screw piles.
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 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 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.
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 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.