The document discusses the concept and principles of floating foundations. A floating foundation is constructed by excavating soil such that the weight of the structure equals the weight of excavated soil and water, causing zero settlement. The main principle is balancing the weight of removed soil with an equal-weight structure. Problems in design include ensuring stable excavation sides, careful dewatering, avoiding exceeding the critical depth where soil shear failure may occur, and preventing bottom heave from lowering soil pressures. Floating foundations are suitable for low-bearing soils and minimize settlement issues.

1. CONCEPT OF FLOATING
FOUNDATION
MILIND RAO
(184104213)
MTech - GEOTECHNICAL ENGINEERING
IIT - GUWAHATI

2. INTRODUCTION
When the soil is so soft that even friction piles will not support the
building , the final option is the use of the 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 of the water in the soil before the
construction of structure.
Floating foundation is also called balancing raft and causes zero
settlement to the structure.

3. INTRODUCTION
A floating foundation
(Donald P. Cudoto- Foundation Design)

4. PRINCIPLES 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.
A floating foundation consists of box like rigid structures set at such a
depth below ground level that the weight of the soil removed to place
it equals the weight of the structure.
Thus, once the structure is completed, the soil under it will bear the
same weight it bore before the excavation was begun.

5. PRINCIPLES OF FLOATING FOUNDATION
Balance of stresses in foundation excavation
(NPTEL: Advance soil mechanics- Prof. T.G. Sitharam)

6. PRINCIPLES OF FLOATING FOUNDATION
While dealing with floating foundation, it is important that we
consider following two types of soils,
• Type 1: The foundation soils are of such strength that shear failure of
soil will not occur under the building load. But the settlements,
differential settlements in particular are so large that they constitute
the failure of the structure. In such cases a floating foundation is used
to reduce settlements to an acceptable value.

7. PRINCIPLES OF FLOATING FOUNDATION
• Type 2: The shear strength of the foundation soil is so low that
rupture of the soil would occur if the building were to be founded at
ground level. At a reasonable depth, in the absence of a strong layer
the building can only be built on a floating foundation. This reduces
the shear stresses to an acceptable value.
In the above mentioned cases, a rigid raft or box type of foundation
is required for the floating foundation.

8. PROBLEMS TO BE CONSIDERED IN THE
DESIGN OF A 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

9. PROBLEMS TO BE CONSIDERED IN THE
DESIGN OF A FLOATING FOUNDATION
Excavation:
• The excavation for floating
foundation should be done
carefully.
• The sides of the excavation
should suitably be supported by
sheet piling, soldier piles and
timber lagging or some other
standard method. (https://theconstructor.org/geotechnical/floating-foundation-
construction/21301/)

10. PROBLEMS TO BE CONSIDERED IN THE
DESIGN OF A FLOATING FOUNDATION
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.
(https://theconstructor.org/geotechnical/floating-foundation-
construction/21301/)

11. PROBLEMS TO BE CONSIDERED IN THE
DESIGN OF A FLOATING FOUNDATION
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.
(Terzaghi, 1943) where; γ= unit weight of soil
s = shear strength of soil
B = width of foundation
(Skempton, 1951) Nc = bearing capacity factor given by Skempton
2
7.5
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B
s
s
DC
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s
ND CC 

12. PROBLEMS TO BE CONSIDERED IN THE
DESIGN OF A FLOATING FOUNDATION
Bottom Heave:
• When the soil is excavated up to
some depth, the pressure of the
soil below this depth is lowered
which results the formation of
heave.
• The formed heave causes
settlement to the structure or
foundation.
(https://www.osha.gov/dts/osta/otm/otm_v/otm_v_
2.html)

13. ADVANTAGES OF FLOATING FOUNDATION
• Used for low bearing capacity soils.
• Minimizes the foundation settlement.
• Floating foundations can be used in on high moisture soils.
• Requires very less digging because deep footer trenches are needed.
• Most suitable for heavily used highways.

14. CONCLUSION
• Foundation settlement is the major problem faced by geotechnical
engineers. If the settlement is severe, it may lead to the complete
collapse of the structure even though the factor of safety against
shear failure is high.
• The floating foundation is the best choice for geotechnical engineers
to minimize the foundation settlement problems.

15. REFERENCES
• David J. D'Appolonia, (A.M.ASCE), T. William Lambe, (F.ASCE), Edmund
K. Turner (1971). Floating Foundations for Control of Settlement.
Journal of the Soil Mechanics and Foundations Division, 1971, Vol. 97,
Issue 6, Pg. 899-915.
• Donald P. Coduto, (2001). Foundation Design- Principles and
Practices. New Jersey: Prentice Hall. Inc.