How deep should foundations be?

Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics to support them.

There is no established depth for foundations; nor is there a simple of of determining suitable depths. Instead there are a very wide range of foundation types and depths suitable for different applications, depending on considerations, such as:

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

A suitably-qualified engineer should be consulted to determine the type and depth of foundations required for a particular situation.

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 might include; pad foundations, strip foundations, trench fill foundations, raft foundations and so on. They are typically shallower if they are on sites with rocky/stony ground, whereas it is normal in ‘shrinkable’ soils such as clay for foundations to require a depth of at least 1 m. This is because moisture content in the soil can lead to expansion and contraction, typically to a depth of around 0.75 m, so foundations must be deeper to avoid being affected by this ground movement. If clay soil areas include trees in close proximity, foundations may need to be significantly deeper.

In more difficult ground conditions, deeper foundations may be required. However, at a depth of around 2.5 m it becomes too dangerous and impractical, to construct shallow foundations, with the amount of concrete required becoming expensive. In these circumstances, piling (or concrete rafts) are often used instead. This is also often the case where the site is on an area of landfill, and foundations several metres deep will be required.

If the foundations are to be in close proximity to an adjacent structure, care must be taken to ensure the excavation works do not undermine it. The foundation may need to be at least the same depth as the adjacent building’s foundation base.

Proximity to underground drains and sewers will also influence the depth of a foundation. Building load is transferred to the ground from the foundation at a 45-degree angle from the footprint. There is a potential for cracking to sewers or drains if they are situated within that 45-degree area. This means that the foundation is typically excavated to a depth that is at least the same as the bottom or deepest part of the drain, sewer, or its trench.

Pile foundations 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 loads through weak, compressible strata or water onto stronger, more compact, less compressible and stiffer soil or rock at depth. They are typically used for large structures, and in situations where soil is not suitable to prevent excessive settlement.

Piles may be classified by their basic design function (end-bearing, friction or a combination) or by their method of construction (displacement (driven) or replacement (bored)).

For more information, see Pile foundations.

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[edit] Related articles on Designing Buildings Wiki

• Building foundations.
• Cracking and building movement.
• Excavation.
• Groundworks.
• How to design a pad foundation.
• Pad foundations.
• Pile foundations.
• Raft foundations.
• Stepped foundation.
• Strip foundation.
• Trench fill foundation.