In construction, deep foundations are often necessary when building on soft, weak soils or when needing to reach stronger soil or rock strata. One popular type of deep foundation is the buoyancy raft or hollow box foundation, also known as a floating foundation. These foundations are designed to act as buoyant substructures, reducing the load intensity on the soil.
Buoyancy rafts are particularly useful when the load needs to be distributed over a larger area or when there are special concerns with the soil. They are constructed by creating a hollow box-like structure that is then placed on the soft soil. This box is then filled with a material that will provide the necessary buoyancy, such as polystyrene foam, and is anchored in place.
There are several advantages to using buoyancy rafts as a foundation. Firstly, they can distribute the load over a larger area, reducing the load intensity on the soil and minimizing settlement. They are also relatively easy and quick to construct, making them a cost-effective option. Additionally, because the foundations are floating, they can adjust to changes in the soil, reducing the risk of damage or failure.
Buoyancy Rafts or Hollow Box Foundations
Buoyancy raft foundations, which are also referred to as compensated foundations or deep cellular rafts, are a type of floating foundation. They are fully compensated foundations as the soil underneath the foundation is removed during their construction, which compensates for the weight of the whole superstructure. In other words, the weight removed from the soil is equal to the weight of the building, thereby creating a balance.
These types of foundations are typically used in cases where the soil bearing capacity is very low, which can make it difficult to support the weight of a building. In such instances, buoyancy raft foundations can help to distribute the weight of the structure evenly across a wider area, reducing the load on the soil and preventing settlement.
Buoyancy raft foundations can also be beneficial when the estimated building settlement is more than the safe limit. By using a floating foundation, it is possible to limit the amount of settlement that occurs, which can help to prevent damage to the building and ensure its long-term stability. Overall, buoyancy raft foundations are an effective solution for supporting buildings in challenging soil conditions and mitigating settlement-related risks.
Need for Buoyancy Rafts in Building Construction
In areas where there is a significant depth of soft soil, floating foundations are a more practical option compared to other types of foundations like pile foundations. This is because no other foundation can be as efficient in such conditions. In low-lying areas, the residents often face the problem of flooding which can lead to the collapse of houses. One solution to this problem is the construction of a buoyant foundation, which would elevate the house and reduce the risk of damage caused by floods. This is a flexible method, as the building would remain on the ground under normal conditions. However, when floods approach, the building would rise to a necessary height, as shown in figure-1.
Fig.1: Buoyancy Rafts or Hollow Box Foundation in case of FloodPlayNextUnmute
Types of Buoyancy Rafts or Hollow Box Foundations
There are two types of floating foundations available – the basement rafts and the buoyant rafts. It is important to note that the buoyant rafts differ from the basement rafts, and should not be confused with each other.
When it comes to the basement raft foundation, the process involves excavating the soil in such a way that its weight equals only a part of the weight of the building. This means that the weight of the entire building is not taken into consideration, and as a result, the foundation is referred to as a partially compensated foundation.
On the other hand, the buoyant raft foundation is quite different from the basement raft foundation. It is crucial to understand the distinction between the two. While the basement raft foundation involves excavation of soil whose weight is equal to only a portion of the building’s weight, the buoyant raft foundation operates differently.
Construction of Buoyancy Rafts or Hollow Box Foundations
When constructing buoyancy rafts, the shear strength of the soil in the location is often extremely low, making it challenging to build a stable foundation. In such cases, constructing a foundation by floating it on the water is the only viable option. This involves sinking cellular rafts in the shape of a box section to create a strong and stable raft foundation that is able to withstand the forces of buoyancy.
This type of foundation is highly rigid, which greatly reduces the amount of settlement that can occur over time. This is important as settlement can cause significant structural damage and compromise the overall stability of the buoyancy raft. By using a cellular raft foundation, construction teams can ensure that the raft remains stable and secure, even in areas with low soil shear strength. Overall, the use of a cellular raft foundation is an effective and reliable solution for constructing buoyancy rafts in challenging soil conditions.
Fig.2: Arrangement of Buoyant Foundations
In Figure 2, it can be observed that the load exerted on the structure from the soil has decreased. As a result, the superstructure appears to be floating, similar to a boat. This effect is achieved by placing the bottom basement of the structure on the excavated area. By doing so, the load from the soil is reduced, allowing the superstructure to “float” on top of it. This method of construction may have benefits in certain situations, such as areas with weak soil or sites where the groundwater level is high.
Fig.3: Arrangement of Cellular Buoyant Raft Foundation
The given figure-3 shows a foundation system consisting of a bottom slab acting as the basement and a ground slab forming a raft foundation. This foundation can be designed to be cellular in structure, resulting in a hollow raft or cellular raft substructure. The advantage of using such a system is that the total load of both the building and foundation is reduced due to the excavation of soil. This reduction in load is significant, resulting in a fully compensated foundation design.
Caisson Type Buoyancy Rafts
The figure-4 depicts the use of cellular caissons, which is a cost-effective construction method. However, there is a potential issue during the sinking process that can lead to disturbances in the adjacent soil, causing further destabilization of the soil structure.
Fig.4: Buoyancy Rafts with Cellular Caissons
Rafts for construction can be made in specific areas or as strips that are later joined together for a stronger structure. By limiting their use in this way, the amount of excavation required in the construction area can be reduced. In Glasgow, a buoyancy raft or hollow raft foundation was constructed for a G+15 building, as shown in Figure 5.
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 design process for an excavation project involves several step-by-step procedures that must be followed. Firstly, the depth of the excavation is determined based on the building plan. This is a crucial step as it sets the size of the excavation.
Next, the center of gravity needed for the overburden removal is calculated. It is important to ensure that this removal is equal to sustain the structural buoyancy. This step helps to maintain the structural integrity of the excavation.
After calculating the center of gravity, the decision obtained from the basement design is compared with the client’s needs and opinions. This comparison helps to ensure that the design meets the requirements of the client.
The water pressure is then calculated to check for the flotation phenomenon. This step is crucial as it helps to prevent the excavation from floating, which can be disastrous.
Based on the needs and requirements obtained from the above four steps, a design is created that combines all the necessary elements. This design is aimed at creating an avoided foundation.
Finally, the design details for external walls, floors, and separating walls are prepared. The focus is on designing for earth pressure, bending moments, and shear forces. Special consideration is given to designing for flotation to ensure that the excavation remains stable. Overall, the design process for an excavation project is a complex but essential step in ensuring the safety and stability of the structure.
Advantages of Buoyant Raft Foundation
The construction of buildings at an elevated height is a common practice to ensure they remain above high water levels and reduce the chances of settlement. This is because the total weight of the building is equal to that of the excavated soil, which prevents any significant settlement. Even a slight increase in load would not exceed the settlement limit.
Using watertight materials for the foundations of buildings can increase their durability, particularly in underground structures. Skyscraper constructions often use foundations that have a height of 3 or 4 floors, which is more efficient than pile foundations in areas with very weak soils for a larger depth. This type of construction helps to maintain stability and prevent any unwanted movement or settling of the building.
Disadvantages of Buoyant Raft Foundation
Excavation is a costly process in construction, particularly when the depth of weak soil increases. As excavation depth increases, so does the amount of excavation required. If excavation goes beyond the critical depth of stability, it can lead to catastrophic movement possibilities.
When surrounded by soft soils, deep foundation support may experience settlement due to earth pressure. Additionally, hollow raft foundations cannot be used for any other purpose, resulting in wasted space. If uplift pressure affects the entire foundation arrangement, it can cause significant problems.
Hollow cells left unoccupied can result in water entering or seepage through the substructure or water pipelines. Unaware of such situations, there is a possibility of gasses leaking into these hollow cells, as they remain unventilated. This can lead to unexpected explosions, even with only minor chances of ignition in the surrounding area.