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.
Let’s consider a ground with water
table at the top as shown in the above picture. The ground is excavated up to
certain depth which is below water table. Now in the next step, a building is
constructed which is as same weight as of the removed soil and water.
Even the depth of excavation is below
the table the total vertical pressure in the soil below the foundation is
unchanged because of its balancing weight. But here one point is to be noted
that we cannot build a structure immediately after the excavation.
During the time of construction, the
effective vertical pressure under the depth of excavation may slightly increase
because of unbalancing weight. So, this type of foundations can also be called
as partly compensated foundations instead of fully floating or compensated
foundations.
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:
Where,
Dc = critical depth
s = shear strength of soil
B = width of foundation (if width is
less than length of foundation “L”)
= unit weight of soil
2) Skempton’s equation for critical
depth is
Where Nc = Skempton’s bearing capacity factor.
By using any one the above two
equations, the critical depth or maximum depth of excavation can be determined.
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. We cannot prevent the formation of heave but there are some methods
to minimize the formation of heave.
Excavating narrow trenches and filling them with concrete soon after the excavation minimizes the heave formation. Installing friction piles before excavation or after the completion of half excavation can also prevents the formation of heave. By lowering water table also, it can be minimized. To prevent the heave formation in the side walls of excavated trench, lateral supports should be provided.
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