In the construction field, the requirement for innovative solutions to handle challenging soil conditions and environmental factors has resulted in the development of specialised foundation systems. One such solution is the Buoyancy Raft Foundation, commonly called floating foundations. These systems are designed to elevate structures above floodwaters and mitigate the risks associated with constructing on soft or weak soils.
In today’s Brick & Bolt blog, you are going to explore its definition, types, construction, challenges, applications, mechanisms of action, and design considerations of Buoyancy Raft Foundations.
What is the Buoyancy Raft Foundation?
Buoyancy Raft foundations, also known as floating foundations, are an innovative solution that prevents houses from flooding. It is a reinforced concrete foundation designed to elevate the house above flood waters and is widely used in building construction on soft and weak soils. Apart from flooding, the foundation is highly preferred when there is a need for load distribution over soft, weak, compressible soils. This feature negates the need to permanently raise the house high off the ground. Under the structure, floatation blocks are utilised in conjunction with steel frames and vertical poles.
As a result, the house remains grounded during normal times. During inundation, the blocks lift the house, and the poles help it to rise and fall safely in water without much mobilisation or destabilisation. The inclusion of such a foundation is a cost-effective way to protect homes from being deluged, avoiding issues that may arise from permanently raising buildings, and allowing them to remain grounded during normal conditions.
Types of Buoyancy Rafts
Buoyancy Raft Foundation, also known as Hollow Box Foundation, can be categorised into two types. They are,
1. The basement rafts, and
2. The buoyant raft
It is critical not to confuse these two types in construction. Unlike buoyant raft foundations, basement rafts involve only the excavation of soil. The weight of exhumed soil equals a part of the weight of the building.
Construction of Buoyancy Rafts
Whenever the shear strength of the soil in the site is very low, the buoyancy raft construction is introduced. In such cases, floating the foundation is the only way that works. To facilitate this, cellular rafts are sunk in the form of the box section. This reduces the settlement and forms a rigid raft foundation. This in turn, reduces the load from the soil and makes the superstructure float like a boat. Then after, the bottom basement is placed on the excavated area. The cellar for the foundation is the bottom slab. The bottom slab, in connection with the ground slab forms a raft foundation. This foundation can be made cellular if required. Using hollow rafts or cellular rafts in the substructure reduces the load value contributed by the building and the foundation. When it comes to the construction of Caisson-type buoyancy rafts, this type is limited to certain individual areas or in the form of strips.
Challenges in the Construction of Buoyancy Raft
In practice, it is not easily achievable to balance the load to ensure no additional pressure is applied to the soil. The foundation’s buoyancy is often altered by the fluctuation in the water table. In most of the cases, the intensity and distribution of loading cannot be accurately predicted. The reconsolidation of swollen soil caused by the elimination of overburden pressure during substructure excavation contributes to the settlement of a buoyant foundation. When loading is replaced on the soil, any swelling caused by elastic or long-term movements must be followed by reconsolidation. After the total dead load of the structure and its full live loading have been attained, it is usual practice to allow some net additional load to come onto the soil. Despite many buoyancy raft foundations evading bearing capacity failure, there is a danger of suffering an eventual limit condition owing to its floatation of the completed substructure. An overestimation of soil density and ground-water table height could result in excessive settling. The weights of construction materials and wall thickness in multi-cell buoyancy rafts can be very critical. Placing fill around a semi-buoyant substructure can be critical if placed on one side and may cause tilting.
Application of Buoyancy Rafts
The major application of Buoyancy rafts is to support structures in challenging soil conditions. Some of the most commonly used situations include areas where ordinary footings are not suitable. This could be used in areas where there is a significant chance of differential settlement. The rafts can also be retrofitted to the existing houses to allow them to float on water during floods.
How Buoyancy Raft Work
It acts like a floating dock, where a steel frame supports the floating blocks attached to the underside of the house. Near the corners of the house there will be four “vertical guidance” poles. All the poles’ heads are attached to the frame. The pole telescope facilitates the movement of the house up and down. At the time of inundation, the foundational blocks lift the house. This step includes the steel frame transferring the force between the house and the blocks. The presence of vertical guidance poles prevents the house from going anywhere except straight up and down on top of the water.
Design of Buoyancy Rafts
The step-by-step process involved in designing a Buoyancy raft is,
1. The first step is to determine the depth of the excavation and size according to the plan of the building.
2. To sustain the structural buoyancy, the overburden removal is made by calculating the centre of gravity.
3. Following this, the basement design obtained must be compared with the client’s needs as well as with his opinions.
4. To assess the floatation phenomenon, the water pressure should be calculated.
5. The final step is to prepare a design for external walls, and floors as well as separating walls. Besides, a special design for floatation must be considered.
Design Consideration for Buoyancy Rafts
Before skimming through the text, it is essential to understand the differences between a basement and a buoyancy raft. Not all types of buoyancy rafts are constructed solely for buoyancy foundations. The two main purposes of the basement are to provide more space in the building and to lower the net bearing pressure due to the weight of the displaced soil. The role of the basement in decreasing net bearing stresses is taken as an advantage to create additional substructure floor area.
The genuine buoyancy raft is a foundation of a structure that uses the buoyancy provided by the displaced earth, with no consideration for other uses of the space. To achieve this goal, the raft design should be as light and rigid as possible. This structural form will limit the excessive need for space within the substructure and accommodate any pipework or service ducts passing through holes in the walls of the cells. To avoid problems inherent in the design and construction of buoyancy rafts, the rafts will be supplanted by various types of piling. Another troublesome factor is that maintaining buoyancy in underground conditions might cause issues to the cells that need to be waterproof. Interior cell walls should have openings to drain out water using an automated pump. The cells must be sealed to prevent pervasion of dangerous gas accumulation within the substructure.
In an open excavation, the buoyancy rafts can be constructed either as open well caissons or in situ. The soft soil often demands the need for the caisson method, as the soil within the cells can be grabbed as the rafts sink under their weight. This method has a limitation for ground conditions when rigidity and weight are required to facilitate sinking through obstructions. Construction in open excavations is appropriate where the groundwater level can be maintained by pumping without the risk of boiling.
Conclusion
Due to the challenging soil conditions, the traditional foundation methods have become obsolete. The buoyancy raft foundation provides a unique and effective solution for construction projects, even for low-bearing capacity or high-water tables. By incorporating the principle of buoyancy, these foundations provide a stable and durable base for structures even in adverse environments.