Buoyancy rafts or hollow
box foundations also known as the floating foundations is a type of deep
foundation is used in building construction on soft and weak soils. Types
of buoyancy foundations, their construction methods, and advantages are
discussed in this article.
The decision of
construction of a deep foundation is taken when the load has to be distributed
to an area either with soft weak compressible soils or to reach strata that
have strong soil or rock or any other special concerns. One such generally used
a type of deep foundation is called as Hollow box foundations or buoyancy rafts
foundation.
These types of
foundations are designed such a way that they behave as buoyant (floating)
substructures for the net loading over it. Hence reducing the load intensity
over the soil.
Buoyancy Rafts or
Hollow Box Foundations
The buoyancy raft
foundations are also known as compensated foundations or deep cellular rafts.
Whatever be the name they are known for, they come under the category of
floating foundations.
These are known as fully
compensated foundations as during their construction the soil underneath the
foundation is removed, whose weight is equal to the weight of the whole
superstructure. Hence the weight removed from the soil is compensated by the
weight of the building.
The buoyancy rafts are
adopted under the following cases:
The soil bearing capacity
is very low
The estimated building
settlement is more than the safe limit
Need for Buoyancy Rafts
in Building Construction
Those areas with soft
soil, layer in a huge depth is observed, it is economical to go for floating
foundation. No other foundation like pile foundation cannot be an efficient
replacement for these.
Occupants in low-altitude
areas face the problem of high floods resulting in the collapse of houses. The
construction of buoyant foundation would help in increasing the elevation of
the house.
This arrangement is a
flexible method, as the building would remain on the ground under the normal
conditions. When flood approaches, the building would rise to a necessary
height, as shown in the figure-1.
Fig.1: Buoyancy
Rafts or Hollow Box Foundation in case of Flood
Types of Buoyancy
Rafts or Hollow Box Foundations
Now the floating
foundation can be of two types,
The basement rafts, and
The buoyant raft.
The buoyant rafts differ
from the basement rafts and both should not be confused. The basement rafts,
unlike buoyant raft foundation, involve only excavation of soil whose weight
equals to a part of the weight of the building. The whole building weight is
not taken and hence called as a partially compensated foundation.
Construction of Buoyancy
Rafts or Hollow Box Foundations
In the case of buoyancy
rafts construction, the shear strength of the soil in the site is very low. Now
under such conditions, the construction by floating of the foundation is the
only way that works.
To have such a foundation,
cellular rafts are sunk in the form of the box section. These forms a raft
foundation that is rigid in nature, which reduces the settlement.
Fig.2: Arrangement of
Buoyant Foundations
As shown in the figure-2,
the load from the soil is reduced and hence the superstructure floats like a
boat. The bottom basement as explained is placed on the excavated area.
Fig.3: Arrangement of
Cellular Buoyant Raft Foundation
As shown in the figure-3,
the bottom slab can be treated as the basement for the foundation. This is
connected to the ground slab, which forms a raft foundation. The foundation can
be made cellular as shown.
By the usage of hollow
raft or cellular rafts substructure, the total load value contributed by the
building and the foundation itself is reduced to a lower value by the soil that
is excavated, which make us clear about the designation, fully compensated
foundations.
Caisson Type Buoyancy
Rafts
These forms cellular
caissons as shown in the figure-4. This method is found economical, except for
the cases during the sinking. The sinking may cause disturbances to the nearby
soil causing further loosening of the soil arrangement.
Fig.4: Buoyancy Rafts
with Cellular Caissons
The construction of these
rafts can be limited to certain individual areas or in the form of strips.
Later these are connected to have a proper bond. This limited use would help in
reducing the amount of excavation conducted in the construction area.
The figure-5 shows a
construction of a buoyancy raft or hollow raft foundation for a G+15 Building
in Glasgow.
Fig.5: Construction of a
Buoyancy Raft or Hollow Raft Foundation for a G+15 Building in Glasgow
Design Considerations
for Buoyancy Raft or Hollow Raft Foundation
The general design
involves following step by step procedures:
The initial step involved
is to determine the depth of the excavation. Its size is determined by the plan
of the building.
Next is to calculate the
center of gravity that is required for the overburden removal that must be
equal to sustain the structural buoyancy
Compare the decision
obtained in basement design with the client’s needs as well as with his
opinions.
Calculation of the water
pressure, to check for the flotation phenomenon
To bring up a design
combining all the needs and requirements obtained from above four steps. This
would give us avoided foundation
The final step in design
is to prepare to design details for external walls, floors as well as the
separating walls. The main concern is to design for earth pressure, the bending
moments and the shear forces. Special design for flotation must be considered.
Advantages of Buoyant
Raft Foundation
The building is elevated
to a required height, which makes them stay above high water levels.
The chances of settlement
are reduced. As the total weight coming is equal to the excavated soil, there
is no settlement. A slight increase in this load would not bring any drastic
change in exceeding the settlement limit.
Watertight material for
foundations would help in the durability of the underground structure.
Foundation having a
height of 3 or 4 floors are used for skyscraper construction. This construction
is found for efficient than pile foundation in areas with very weak soils for a
larger depth.
Disadvantages of Buoyant
Raft Foundation
As the depth of weak soil
increases, the amount of excavation also increases. In construction, the
excavation process is an activity found very costly.
Catastrophic Movement
possibilities- These occur when the excavation is done beyond the critical
depth of stability.
Support of deep
foundation may undergo settlement due to the earth pressure it is subjected to,
from the surrounding soft soils.
Space is wasted as hollow
raft foundations cannot be used for any other purpose.
Uplift pressure would
affect the whole foundation arrangement.
Leaving the cells
unoccupied would result in water entering or seepage. This may be through the
substructure or due to any complaints in the water pipelines going underground.
There are possibilities
of leakage of gasses into these hollow cells. As these situations are unaware,
they remain unventilated. This would cause unexpected explosions, even if there
are small chances of ignition anywhere around.
