- Project plans
- Project activities
- Legislation and standards
- Industry context
- Specialist wikis
Last edited 04 Sep 2020
How deep should foundations be?
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.
- 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.
- 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.
 Find out more
 Related articles on Designing Buildings Wiki
Featured articles and news
On Levelling-Up and Regeneration Bill.
Over 70 managers and organisations shortlisted for the 14 awards.
From biometric to electrical current, chemical and more.
Changes are due to come into force on 1st October 2022.
Heed advice and insight of this report IPA tells the government.
From the Commonwealth Association of Architects.
For the Levelling Up, Housing & Communities Committee.
BSRIA's Technical Director reflects on recent weather patterns.
A national valuation to fund old-age pensions.
The world’s largest Commonwealth memorial to the missing.
Long after the end of the defects liability period.
Occupant satisfaction and wellbeing in buildings.
From the simple to the complex.
And the UK Government guidelines.
Commitment agreed to by major built environment bodies.
Electrical skills, low carbon, high-tech and the building services revolution.
Ultra-deep drilling with millimeter-wave beam technology.
Looking at the built environment from space.
BSI standards 8671, 8672 and 8673.
Bringing life to burial grounds.
From failed modernism to twenty-minute neighbourhoods.
The gates process and change control.