Foundations contractor Aarsleff Ground Engineering are equipped to work on several types of site delivering specialist ground engineering solutions to solve technically challenging ground related problems in the UK and abroad. Are you aware of the different types of foundations?
Broadly speaking, all foundations are divided into two categories: 1. shallow foundations and 2. 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 usually for large, heavy buildings.
Shallow foundations are also called spread footings or open footings. The ‘open’ refers to the fact that the foundations are made by first excavating all the earth till the bottom of the footing, and then constructing the footing. During the early stages of work, the entire footing is visible to the eye, and is therefore called an open foundation. The idea is that each footing takes the concentrated load of the column and spreads it out over a large area, so that the actual weight on the soil does not exceed the safe bearing capacity of the soil.
Shallow foundations comprise Pad Foundations, Strip Foundations and Raft Foundations.
Deep foundations include piles, piers and caissons or compensated foundations using deep basements and also deep pad or strip foundations. Deep foundations can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface.
What are Piles?
A pile is basically a long cylinder of a strong material such as concrete that is pushed into the ground so that structures can be supported on top of it. Piles transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. They are used when for economic, constructional or soil condition considerations it is desirable to transmit loads to strata beyond the practical reach of shallow foundations. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.
Pile foundations are used in the following situations:
- When there is a layer of weak soil at the surface. This layer cannot support the weight of the building, so the loads of the building have to bypass this layer and be transferred to the layer of stronger soil or rock that is below the weak layer.
- When a building has very heavy, concentrated loads, such as in a high rise structure.
There are two types of pile foundations, each of which works in its own way.
1. End Bearing Piles
End bearing piles are those which terminate in hard, relatively impenetrable material such as rock or very dense sand and gravel. They derive most of their carrying capacity from the resistance of the stratum at the toe of the pile.
2. Friction Piles
Friction piles work on a different principle. The pile transfers the load of the building to the soil across the full height of the pile, by friction. In other words, the entire surface of the pile, which is cylindrical in shape, works to transfer the forces to the soil. Their carrying capacity is derived partly from end bearing and partly from skin friction between the embedded surface of the soil and the surrounding soil.
In practice, however, each pile resists load by a combination of end bearing and friction.
Other types of Pile include: Settlement Reducing Piles, Tension Piles, Laterally Loaded Piles, Piles-in-Fill, Displacement Piles, Totally preformed displacement piles,Driven and cast-in-place displacement piles and Helical (screw) cast-in-place displacement piles.
There are many factors that can affect the choice of a piled foundation. All factors need to be considered and their relative importance taken into account before reaching a final decision. In coming to the final decision over the choice of pile, cost has considerable importance. The overall cost of installing piles includes the actual cost of the material, the times required for piling in the construction plan, test loading, the cost of the engineer to oversee installation and loading and the cost of organisation and overheads incurred between the time of initial site clearance and the time when construction of the superstructure can proceed.