A raft foundation is a type of foundation used to support buildings. They are typically used in weak soils where deep foundations are not feasible. A raft is a rigid structure that extends beyond the wall line and supports the building, negating tilting and preventing the building from settling.
The raft may be constructed in many ways, depending on the requirements of the design. Some common types of raft foundation include:
Cellular raft foundation (Box type)
This type of raft foundation is made up of boxes that contain beams connected by slabs at the top and bottom. They can resist high bending stress and are well suited for loose soils where settlement is uneven.
Raft foundations can be designed in accordance with IS 1080 to ensure the soil has a safe bearing capacity. They are a cost-effective alternative to piling for buildings if the soil has a high water table or if it is not suitable for piles.
Compensated raft foundation
A compensated raft foundation is formed in soft clay or soft sand conditions where the raft base is provided to a depth that the weight of excavated soil equals the weight of the raft slab plus the weight of the structure to be supported.
It also protects the raft from differential settlement caused by changes in underground conditions. Usually, this type of foundation is built to a minimum of 50 cm thick.
Deformed piled raft foundation
In the past, a piled raft foundation was only used on shallow foundations. However, new technology has made piled rafts a very effective method of supporting buildings in very complicated soil conditions.
To avoid the risk of excessive settlements in this type of foundation, a special pile-raft system with deformation adjustors was developed. The raft was then installed on the foundation and the deformation adjustors worked in tandem with the raft to coordinate the deformation of the raft and the soil.
The deformation adjustors were then fitted with eight bar stress meters to measure the stress distribution within the raft. The results of the measurement showed that the load-sharing ratio of the soils gradually stabilized as the deformation adjustors worked.
Nevertheless, the use of deformation adjustors is not without risk. The raft-soil interaction can result in severe straining actions at the pile head-raft connection, which may lead to a significant reduction in the load-sharing ratio between the piles and the raft.
Pile-Raft Foundation: Stable yet Expensive
The construction of a piled raft foundation is expensive, as the piles must be designed and cast in place, and the raft itself must be designed with sophisticated modelling and analysis tools. The piles are buried at the desired depth, and the raft is a poured concrete slab that connects the piles to the structure above.
It is important to remember that if a piled raft foundation is not designed correctly, it can lead to major problems for the structure. For example, if the piles are too close together, they can compress and crack the soil. Similarly, if the piles are too far apart, they can create a strong tension in the soil that can cause settlements and buckling.
