A floating foundation, also known as a compensated foundation, is a specialized type of building foundation designed to balance the weight of a structure with the weight of the soil excavated during its construction.
Based on information from In construction: Foundations, a floating foundation consists of boxlike rigid structures set at such a depth below ground that the weight of the soil removed to place it equals the weight of the building. This unique design principle ensures that once the building is completed, the soil under it will bear the same weight as it did before any construction began.
Key Principle: Weight Compensation
The core idea behind a floating foundation is to achieve a state of equilibrium with the underlying soil. By excavating soil whose weight is equivalent to the anticipated weight of the entire building (including live loads), the net increase in pressure exerted on the soil below the foundation is ideally zero or minimal.
- Excavation Depth: The depth of the excavation is carefully calculated based on the building's expected weight and the density of the soil to be removed.
- Structure: The foundation itself utilizes rigid, often boxlike structures (like large raft slabs or deep basements) that distribute the building's load evenly over the excavated area.
- Result: This weight compensation means the soil doesn't experience a significant increase in stress, mitigating the risk of excessive settlement, especially on weak or compressible soil types.
Essentially, the building "floats" on the remaining soil because the pressure it exerts is no greater than the pressure previously exerted by the soil that was removed.
Characteristics of Floating Foundations
Floating foundations are distinguished by several key features directly related to their weight-compensating design:
- Rigid Structure: They typically involve substantial, rigid concrete structures, often forming deep basements or large caissons, which act as the "box" described.
- Deep Excavation: Construction requires significant excavation depth to remove the necessary volume of soil.
- Weight Calculation: Precise calculation of the building's total weight and the excavated soil's weight is critical for the design.
- Maintained Soil Pressure: The primary outcome is that the total load on the soil remains relatively constant before and after construction.
This method provides a stable base for structures, particularly in areas where conventional shallow foundations might fail due to inadequate soil bearing capacity or susceptibility to settlement.
