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.