The primary function of the foundation is to receive different types of load that occur on the structure and eventually distribute to the ground without any displacement. 

Different types of foundations are used based on the location, structure weight, and soil bearing capacity. The floating foundation is one among them.

What is a Floating Foundation?

A foundation where the weight of the structure built will be nearly equal to the soil excavated, including water from the structure location is known as the floating foundation.

Floating Foundation

Suitability of Floating Foundations

Floating foundations would be suitable if

  • The proposed building location has low shear strength and does not have hard rock strata to rest the foundation properly.
  • The soil settles largely and induces differential foundation settlements under heavy load.
  • The ground has good shear strength but has chances of differential settlements.

Foundation Settlement

A vertical displacement happening in the structure’s foundation is called foundation settlement. There are chances for a structure to displace in three ways, as mentioned below.

  • Uniform Settlement
  • Tipping Settlement
  • Different settlement

Foundation Settlements

The displacement of the foundation causes severe building damages and leads to the collapse of the entire structure. So to reduce or eliminate the movement that happens, the floating foundation is preferable. 

The other name of the floating foundation is balancing the raft foundation for the reason of zero settlement to the structure. 

Principle of Floating Foundation

The principle of the floating foundation is to equilibrate the excavated soil weight by the structure weight, which is to be constructed on the excavated soil location. 

Floating Foundation

  • This methodology is applied to avoid the settlement happening at foundation level. The soil pressure remains the same before and after the construction of the structure. 
  • The soil excavation has to be done below the water table, and the structure should be constructed in such a way that the weight equals the excavated soil and water.

The structure can not be constructed immediately after excavating the soil. There are slight settlement changes that will happen in the foundation.

Safety measures during construction of a Floating foundation

Soil Sliding – The sheet piles should be used to support the side of the excavation to avoid the soil sliding. The excavation work should be done more carefully and must ensure all safety measures.

Dewatering – The process of dewatering work should be done continuously when the depth is going below to the water level else; it impacts the adjacent structure and leads to its sliding. 

Critical Depth – The following formula is used to determine depth for low bearing capacity of the soil. The limitation of the greatest extent of the floating foundation is called critical depth.

  • Terzaghi equation

Terzaghi equation

Dc = critical depth     s = shear strength of soil 

B = width of foundation (if the width is less than the length of foundation “L”) = unit weight of soil


  • Skempton’s equation 

Skempton’s equation


Where Nc = Skempton’s bearing capacity factor.

The above two formulas are surging to find critical depth.

Bottom heave

The soil pressure will reduce after reaching a certain depth, and it results in heave formation at the bottom. So after the construction of the structure, displacement would happen at the beginning up to the formation heave. 

Bottom Heave

It may be prevented if the foundation work started soon after excavating the soil. Providing lateral support helps to avoid the formation of heave.

Advantages of the floating foundation

  • It can be applied where the soil has a low bearing capacity.
  • It can be applied where deep excavation is not required.
  • It can be used for highly moisturized soil.

Disadvantages of the floating foundation

  • In future, if a problem occurs on the electrical or plumbing line, it may be a little difficult to solve.
  • Due to lack of proper construction, it may crack of sudden impact and can’t be repaired.

Happy Learning 🙂


Bala is a Planning Engineer & he is the author and editor of Civil Planets.

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