Types and Advantages of Raft Foundations

A raft foundation is a strong concrete base that helps hold up bridges, buildings, and other construction structures. The flat block of the Raft foundation helps to spread the overall weight of the construction structure, as a result, the construction structure does not sink into the ground. This type of foundation is normally used in such places where the soils are like soft clay or sand. Raft foundations are floated on the soil and it is useful in the case where the other types of foundations do not work properly.

A thick and strong piece of concrete which is held up by the grid of steel bars is used for constructing the Raft foundation. The thick and strong nature of the Raft foundation helps to prevent bending or broken incidents due to the heavy weights thus preventing the sinking of the construction structure into the ground. The thickness and deep nature of the Raft foundation is depended upon the nature of the ground and the load of the construction structure.

Materials that have been used for constructing the Raft Foundation

➢ Concrete is mainly used to construct the raft foundation. In this case, steel has been reinforced this foundation to enhance the strength and durability of the foundation structure.
➢ For reinforcing the concrete of the raft foundation, a steel bar has been used. These materials help to add strength to the foundation structure thus handling the tensile stress.
➢ Wood, plastic or metal is also used by the builders for making formwork which acts as a temporary mold and provides shapes to the concrete.
➢ A waterproofing membrane has been also applied to the Raft foundation’s surface which helps to prevent water ingress. The use of a waterproofing membrane is important in those areas where groundwater level is high.

The process of constructing the Raft Foundation

The preparation of the site

This is the first step of constructing a foundation, in which, the debris is cleared, and the grounds have been levelled which helps to ensure a stable base for the raft foundation.

Installation of the formwork

After preparing the site, the formwork has been installed to contain the concrete. In this case, formwork has to be strong and stable to maintain the proper shape of the concrete until the concrete has hardened.

Reinforcement

Builders are involved in laying the steel bars in the formwork. This reinforcement helps to provide the strength to the foundation which is needed for supporting the foundation’s load.

Concrete pouring

The next step for making the raft foundation is pouring the concrete into the formwork. After pouring the concrete, it has been spread evenly which helps to ensure that there is no gap or void in the concrete.

Curing the concrete

Proper occurring is necessary for achieving the full strength of the foundation. This process helps to keep moisture, time and temperature.

Removing formwork

After hardening the concrete, the formwork was removed and the final structure of the foundation was inspected for detecting issues or defects.

Advantages of constructing the Raft Foundation

There are numerous advantages of using Raft foundations in the construction site which include:

➢ Raft foundation helps to reduce the pressure of the soil and the risk of soil failure by spreading the loads of the construction structure.
➢ Raft foundation can be used in soil which has low bearing capacity.
➢ The differential settlement that can damage the building structure has been limited by using raft foundations.
➢ The need for footing for each column has been also eliminated by the use of a raft foundation which is another important advantage of using this foundation structure. This helps to reduce the time and the labour cost in the construction process.

Different types of the Raft Foundation

There are different types of raft foundations which include

Flat Plate Type Raft Foundation

In this type of raft foundation, a reinforced concrete slab which can cover the whole bearing area has been consisted. This type of raft foundation has been appropriate in the case of a column has been placed in a uniform space and carries small loads. In this type of raft foundation, two mesh are reinforced.

Thickened Flat plate Type Raft Foundation

In the case of the column, the load is very heavy, the flat plate-type raft foundation is not appropriate. So, the the thickness of the flat plate-type foundation needs to be increased which helps to receive the heavy loads and prevent negative bending moments.

Cellular Raft Foundation

This is also known as the box-type raft foundation in which box-like structures are created. The box walls act as the beams which are connected by slabs. In the cellular raft foundation, the slabs and beams are connected in a way as this is looks like an I-section. High bending stress can be carried by this type of raft foundation.

To learn more, watch the following video tutorial.

Video Source: Shefden Academy

Piled Raft Foundation

In this case, the raft foundation has been supported by the piles in the ground. The soil with a high water table or high compressibility, this foundation type is preferred. The use of piles helps to reduce the amount of settlement and the ultimate load-bearing capacity of the foundation has been increased.

Beam and Slab Type Raft Foundation

In this type of raft foundation, the beams are located in perpendicular directions which are connected by the raft slab. In the case of there being a large space between the column and the column carrying unequal loads, beams and slab-type raft foundations are appropriate.