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**What is a Floating Foundation?**

A floating foundation is a type of foundation constructed by excavating the soil in such a way that the weight of structure built on the soil is nearly equal to the total weight of the soil excavated from the ground including the weight water in the soil before the construction of structure. Floating foundation is also called balancing raft and causes zero settlement to the structure.
## Principle of Floating Foundation

The main principle of floating foundation is to balance the weight of removed soil by a structure of same weight which causes zero settlement to the structure. So, this foundation is also called as balancing raft foundation.## Suitability of Floating Foundations

Floating foundations are desirable for the following types of soils:- Soils which are having good shear strength but having a problem of large settlements and differential settlements under heavy loads. In which case, floating foundation can reduce the settlement values to greater extent.
- Soils which are having low shear strength and there is no hard layer of soil at reasonable depth. Here also, floating foundation helps to reduce the shear stresses to satisfactory level and hence settlement is prevented.

## Difficulties in Construction of Floating Foundation

During the construction of floating foundation some problems may arise which can be prevented by taking proper care. The problems will arise during- Excavation
- Dewatering
- Critical depth
- Bottom heave

### Excavation

The excavation for floating foundation should be done carefully. The walls of excavated part should be supported using sheet piles or timber piles or soldier piles etc.### Dewatering

If the depth of the excavation is below the water table then dewatering is essential. It is better to examine the water table level prior to the excavation. Dewatering should be done cautiously otherwise it may cause lowering of water table which disturbs the adjoining structures.### Critical Depth

If the soil is of low shear strength as specified above, there is a limit to the depth of excavation which is known as critical depth. There are two equations are available to determine critical depth of excavation given by Terzaghi and Skempton respectively. 1) Terzaghi equation for the critical depth is:_{c}= critical depth s = shear strength of soil B = width of foundation (if width is less than length of foundation “L”)

_{c}= Skempton’s bearing capacity factor. By using any one the above two equations, the critical depth or maximum depth of excavation can be determined.