Very broadly, foundations can be categorised as shallow or deep:
- 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 sufficient to support the loads imposed by a structure and so those loads need to be transferred to deeper layers with higher bearing capacity.
Compensated foundations are a type of deep foundation, which work on the principle that if the weight of material excavated equals the weight of the building added, the soil is subjected to no additional stresses. They are also known as ‘floating foundations’ as the structure acts in a similar way to a ship’s hull.
The relief of stress due to the excavation is approximately balanced by the applied stress of the foundation, resulting in a negligible net stress. As a result there may be little consolidation settlement experienced.
Compensated foundations normally comprise a deep basement and/or are used to support tall buildings or swimming pools, where a very large amount of material is excavated.
- A simple construction process with minimal impact on adjacent structures.
- They can be integrated with other foundations in more complex situations, such as piling.
- They can contribute to protection from rising damp, thermal insulation, drainage, and so on.
 Compensated raft foundations
Where soil is compressible, the a raft foundation may be formed as a compensated foundation. In this case, the raft slab is provided to a depth that the weight of the excavated soil is equal to the raft slab weight plus that of the structure to be supported. This can be appropriate when constructing buildings on soft clay or loose sand, as settlement can be significantly reduced.
 Compensated piled raft foundations
Compensated piled raft foundations are typically used where the soil provides only modest bearing capacity, meaning that piles are necessary to carry some of the load. This can necessary, for example, in the case of soft clay, which may undergo settlement due to reclamation filling or dewatering processes, during which the soil may settle away from the raft base.
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