A mixed foundation is a foundation that combines two or more foundation types within a single building, for example a shallow foundation and a pile foundation. This approach is used when the subsoil beneath a structure varies significantly.
How a mixed foundation works
The bearing capacity of the ground is not the same everywhere. Where a firm sand layer lies close to the surface in one spot but much deeper in another, a single foundation type may be inadequate. A mixed foundation then combines:
- Shallow foundation – where the bearing layer is near the surface and the load can be transferred directly to the ground.
- Pile foundation – where the firm layer is deep and the load is carried down through piles.
The challenge is that different foundation types settle unevenly. Careful design and calculation are needed to prevent cracking caused by differential settlement.
Application
A mixed foundation is chosen for:
- Varying subsoil – with local differences in bearing capacity.
- Extensions and additions – where new build connects to existing structures with a different foundation system.
- Sloping terrain – where foundation depth varies across the building.
A geotechnical investigation with soundings is essential to determine the right combination.
Related terms
- Foundation
- Pile foundation
- Shallow foundation
- Pier
- Subsoil
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