Deep foundations, also called as pile foundations, are used when there is not sufficiently strong soil at a reasonable depth, and using shallow foundations would cause failure or excessive settlement of the structure, even if that same soil can be a sufficiently strong foundation material for a lighter project. For example, for a bridge, where people and cars travel, and the loads and turning effects are very high, no matter what type of foundation material you have, you will almost certainly need pile foundations, but for a building that is only a few stories high, if we have strong enough soil close to the surface, shallow foundation such as mat foundation or individual footings can be acceptable.
On top of piles, still the footings and or pile caps are made, so the cost and time of installing deep foundations (piles) are purely extra to the foundation cost in comparison to shallow foundations, which can be as high as few times of shallow foundation costs. Sometimes even using piles are not enough and ground improvement is also necessary, which further adds to the cost.
In pile foundations, in many instances more than one pile is used, which are joined by a pile cap that holds the individual piles together and enables them to serve as a unit and increases stability. These are for very heavy loads. Or for a very large building, all piles may be joined under one large mat foundation as well.
The selection of which pile foundation system, pile materials, installation depth and installation method to be used depend on factors such as the load requirements and design life of the structure above, availability of pile materials and driving or casting equipment, soil and groundwater conditions, and of course available construction time and budget. When calculating the cost of pile foundation system to compare different systems, the cost per bearing capacity of the entire foundation should be used, and not the cost per linear foot of pile, which can be misleading because construction methods, job conditions, obtained bearing capacities vary among different systems. What bearing capacity are you able to achieve, per unit cost, should be the bottom line that matters.
Pile materials can be of steel, concrete or even timber. Composite piles made of more than one of these materials are also seen but it is not common due to more difficult construction requirements and the difficulty of making joints to make both materials work together as one.
As far as their construction method, piles can be installed in two ways, driven or cast in place. Steel or timber piles are always driven piles, and sometimes precast concrete piles can also be driven. Driven means, a large equipment which produces a hammering action gradually drives the piles into the ground by blows to pile heads. Cast in place piles can only be made of concrete. To do this, first the hole in the ground is dug and the soil is extracted, and in many cases a casing is placed to serve as formwork for concrete, and also to prevent caving in of soil in many cases, and after the pile is poured, the casing form is extracted to be used in another pile foundation location.
Driven piles compress the soil around the pile, which is a good thing because denser soil means stronger soil and that in turn means more pile skin friction resistance. For this reason, driven piles are also called displacement piles, as they displace the soil around them. Cast in place concrete piles, do not cause such displacement action because the soil as a result of digging a hole is not pushed towards the sides but simply removed. For this reason, cast-in-place piles are also called non-displacement piles.
For large enough structures, before proceeding with pile installation, test piles are installed and load testing is also performed under actual conditions, in addition to design estimates.
Piles resist four different types of loads:
- Vertical downward,
- Vertical upward (uplift),
- Horizontal load and / or moment to top of pile,
- Horizontal load on the pile body itself. This fourth one is when piles are used as retaining structures.
Below we will first talk about these four cases and then move on to describe different pile foundation types.
In the next post of this series, we will discuss ” Loads that act on piles”