According to the building regulations, the minimum depth of a foundation means the least depth to which the foundation of a building must extend below the ground level. This level of depth ensures that the building has enough structural strength to protect the foundation from frost heave and earthquakes and improve the soil’s load-bearing capacity.
The depth of the foundation is considered the bedrock of your building’s future. With a well-planned foundation, a building can handle its weight, resist shifting ground and withstand difficult weather conditions.
What Should Be the Minimum Depth of a Foundation?
Building regulations in the UK stipulate that the depth of the foundations must be at least 450 mm below the ground level to protect the structure against frost damage in winter.
The depth of the foundations depends on the type of construction project. Depending on the building’s load, weather conditions, and the soil type, it can range from 0.6 to 2.5 metres.
Factors Affecting the Foundation Depth
Soil Types
As mentioned above, the depths of building foundations also depend on the soil type:
- For frost-susceptible soil, the minimum foundation depth must be 450mm below the ground level to prevent it from frost heave.
- In case of clay soil, the foundation depth may be at least 9 metres, due to the poor load-bearing capacity of the clay soil and its tendency to swell and shrink.
- If the soil is sandy and gravel, the foundations can be shallower in depth, ranging from 6m to 0.9m.
Environmental Factors
Environmental factors such as trees and frosts also impact the foundation depths of buildings in that area.
- Trees:
In tree areas, building foundations must be deep enough to mitigate the damage caused by soil movement due to tree roots. Foundations in tree areas should be around 2m deep.
- Frost:
In colder regions, frosts can penetrate up to 450mm below the ground level in the snowy season. This frost can then affect the groundwater beneath and destabilise the building structure. Therefore, building the foundation depth of at least 450mm is necessary to ensure a safe and sturdy build.
Foundation Types
The types of foundation play a vital role in the depth of a building’s foundations:
- Strip Foundation:
A strip foundation is used for load-bearing walls in a building. They are concrete-filled footings, and they can spread over a larger area. Strip foundations help in the even distribution of the building’s structural weight, reducing the weight on the soil.
Again, the minimum foundation depth should be at least 450mm for a strip foundation, or until the depth reaches the level of frost penetration beneath the ground.
- Trench Foundation:
A trench foundation is suitable when the soil is not conductive for higher loads and the building structure warrants a higher load. Trench foundations are entirely concrete-filled foundations.
In case of a trench foundation, the minimum foundation depth must be at least 1000mm, and the wall width should be 300mm overall (150mm each side) to stabilise the building structure.
- Raft Foundation:
Raft foundations are suitable for all types of building loads as their size depends on the structural load and dimensions of the building.
The minimum depth of a raft foundation ranges from 500mm to 2500mm. The thickness of the raft foundation ranges from 300mm to 1000mm, depending on the structural load and the nature of the soil.
- Pad Foundation:
Pad foundations are a simple type of foundation used for smaller building structures, and they can adapt to various soil conditions. Pad foundations provide structural stability and strength to buildings in areas where the foundations are shallow.
The minimum depth required for the pad foundation typically ranges from 600 mm to 2000 mm beneath the ground level.
Building Height:
The depth of the foundation also depends on the number of storeys in a building. A building with multiple storeys will put a much greater load on its foundation than a single-storey building:
- Single-Storey:
For a single-storey building, a concrete foundation of 150mm is considered sufficient to stabilise the structure. For a trench foundation, the depth should be up to 1000mm (1 metre) below the ground level.
- Double-Storey:
When it is a double-storey building, a concrete foundation with a depth of at least 200mm is required under the ground. This is because the foundation under a double-storey structure bears more load capacity than a single-storey structure.
For a double-storey building, the minimum depth of the trench foundation should be 1000m.
Conclusion:
To get the foundation depth right, various factors are taken into consideration, like soil type, type of construction project, environmental factors, ground condition, height of the building, etc.
Building regulations are the guidelines that ensure the safe execution of a building’s construction. Following the regulations helps us have a long-lasting structure and comply with government standards.
