A mat foundation, also known as a raft foundation, is a reinforced concrete slab that covers a large surface area of soil and offers support for multiple columns and load-bearing walls. This foundation system is commonly used and considered a type of shallow foundation. The main purpose of a raft foundation is to distribute the weight of the building evenly across the soil to prevent any settlement or unevenness in the structure. The thick slab of concrete helps to spread the load and reduce the pressure on the soil. Due to their ability to provide support for heavy loads and resist differential settlement, raft foundations are particularly useful in areas with weak soil conditions.
Fig 1: Raft or Mat Foundation
Types of Raft Foundation
Following are different types of raft foundations,
- Flat Plate Type Raft Foundation
- Thickened Flat plate Type Raft Foundation
- Beam and Slab Type Raft Foundation
- Piled Raft Foundation
- Cellular Raft Foundation
1. Flat Plate Type Raft Foundation
A flat plate type raft foundation is a type of foundation that consists of a reinforced concrete slab covering the entire bearing area. This type of foundation is appropriate when the columns are evenly spaced and carrying small, uniform loads. The slab is reinforced with steel mesh in both directions, with two layers of mesh at the top and bottom of the slab to balance upward and downward bending forces. To ensure that the slab is not too thick, a maximum thickness of 300 mm is provided for the concrete slab in the case of a raft foundation.
Fig 1: Flat Plate Type Raft Foundation
2. Thickened Flat plate Type Raft Foundation
When column loads are excessively heavy, a flat plate type structure may not be suitable. In such cases, the slab thickness must be increased to make it viable. This is because heavy column loads introduce negative bending moments and diagonal shear into the slab. To counter this, a portion of the slab under the column should be thickened to resist the negative bending moments and diagonal shear. Alternatively, the provision of a pedestal under the column can help receive heavy loads, without increasing the slab thickness. By employing these measures, it is possible to make the flat plate type structure suitable for heavy column loads.
Fig 2: Casting Pedestals on Raft Foundation
3. Beam and Slab Type Raft Foundation
A raft foundation consisting of beams and a slab is used when the columns that support a building carry unequal loads and are situated far apart from each other. The foundation is constructed by laying beams in perpendicular directions and connecting them with a slab, which forms the base of the foundation. The columns are located exactly where the beams of the foundation intersect with each other. This type of foundation is ideal for situations where there is a significant distance between the supporting columns and where the loads that they carry are uneven.
Fig 3: Beam and Slab Type Raft Foundation
4. Piled Raft Foundation
A piled raft foundation is a type of foundation where a raft slab is supported by piles embedded in the soil. This type of foundation is particularly suitable for soils that exhibit high compressibility and have a high water table. It is commonly used for constructing high-rise buildings. The inclusion of piles underneath the raft slab helps to minimize settlement and effectively counteracts buoyancy. As a result, the ultimate load capacity of the foundation is enhanced.
Piled raft foundations are particularly advantageous when dealing with soils that have a tendency to experience significant settling. By providing additional support through piles, the foundation is able to withstand the weight of the structure and the loadings placed upon it. Additionally, the presence of the piles also helps to improve the overall stiffness of the foundation.
Another benefit of using a piled raft foundation is that it helps to mitigate the impact of buoyancy. In areas where there is a high water table, the upward force of buoyancy can cause significant problems for traditional foundations. However, by incorporating piles into the design, the foundation is able to resist the upward force of buoyancy, thereby improving the overall stability of the structure.
Overall, a piled raft foundation is an excellent choice for high-rise buildings or other structures that require a strong, stable foundation. By utilizing the combined strength of a raft slab and piles, this type of foundation is able to provide enhanced load capacity and improved resistance to settling and buoyancy.
Fig 4: Piled Raft Foundation
5. Cellular Raft Foundation
Cellular raft foundation, also referred to as box type or rigid frame raft foundation, is a type of foundation that utilizes box-like structures. These boxes have walls that act as beams and are connected by slabs at the top and bottom. The cross-section of the beams and slabs resemble an I-section.
The design of this foundation allows it to withstand high bending stresses and is particularly useful in loose soils where settling can occur unevenly. The strength of the foundation comes from the way the beams and slabs are integrated, creating a rigid structure that can distribute the weight of the building evenly.
The use of cellular raft foundation is a viable solution in construction projects where a conventional foundation may not be suitable due to soil conditions. By providing a stable base, this type of foundation ensures the longevity and stability of the building structure. Overall, cellular raft foundation is a reliable option for builders looking to construct in areas with weak or unstable soil.
Fig 5: Cellular Raft Foundation
Advantages of Raft Foundation
Raft foundations offer several advantages in construction. First, they require less excavation as they are constructed for shallow depths. This means that the process of building a raft foundation is generally faster and more cost-effective than other types of foundation that require deeper excavation.
Another advantage of raft foundations is that they are well-suited for use in soils with low bearing capacity. This is because the foundation spreads the load from the superstructure over a larger area, reducing the stress on any one point of the soil.
Furthermore, the distribution of the load over a larger area can help to reduce the risk of differential settlement. This is important because differential settlement can cause damage to the superstructure and result in expensive repairs.
Overall, the advantages of raft foundations make them a popular choice for construction projects where shallow depths and low bearing capacity soils are a concern.
Disadvantages of Raft Foundation
Raft foundations, while offering certain advantages, also come with several disadvantages that need to be considered. One such disadvantage is the potential need for a large amount of reinforcement, which can drive up the overall cost of the project. Additionally, when it comes to concentrated loads, special attention is required to ensure the raft foundation can adequately support them. Neglecting this consideration could result in damage to the structure or foundation.
Another potential issue with raft foundations is the risk of edge erosion. This is an area where the edge of the foundation can become weakened and eroded over time, compromising the overall stability of the structure. To prevent this from happening, it’s important to take appropriate measures to protect the edges of the foundation and monitor them over time.
Finally, constructing a raft foundation requires skilled workers who are knowledgeable about the construction process. This can be a challenge, especially in areas where such expertise is in short supply. It’s important to work with experienced professionals who can ensure the foundation is built to the appropriate standards and specifications, minimizing the risks associated with this type of foundation.