A raft foundation, also known as the mat foundation is a continuous concrete slab extending over the entire footprint of the building. This slab supports several columns and load-bearing walls, thereby supporting the building and transferring its weight to the ground.
For people building their first home, it is important to know that there’s more than one way of laying this type of shallow foundation. So, in this article, we will explore different types of raft foundations. Let’s start.
Types of Raft Foundations
1. Flat Plate Raft Foundation
It is the most elementary form of raft foundation and is ideal to use when the walls and columns are evenly spaced at small intervals and have to carry relatively small loads. The slab is reinforced in both directions with a steel mesh. The mesh is placed on both the top and the bottom of the slab to stabilise the downward and upward bending forces. Also, in this type of raft foundation, the maximum measurement of the concrete slab is 300mm. And for this reason, it is perfect for small projects as anything above 300mm thickness increases the budget.
2. Thickened Plate Raft Foundation
As mentioned above, a Flat plate raft foundation is not ideal for carrying heavier loads. Therefore, the thickness of the slab is increased under the walls and the columns, and hence, it is called the ‘Thickened’ Plate raft foundation.
The slab has to be thickened because the extremely heavy loads from the columns bring diagonal shear and negative bending moments onto the slab. Another way to support the heavy loads is to provide a pedestal below the column and you don’t even have to increase the thickness of the slab with this method.
3. Two-way Beam and Slab Raft Foundation
Next on the list of types of raft foundations is the Two-way Beam and Slab Raft Foundation. In this type of foundation, the beams are cast perpendicularly and monolithically with the raft slab connecting the walls and the columns. It is well suited if the columns are spread at a larger distance and carry unequal amounts of load.
4. Piled Raft Foundation
A piled raft foundation is used when the water table is high and soil found at a shallow depth is extremely compressed. Piles are placed below the raft to reduce settlement and control the buoyancy effect. The Piles raft foundation increases the load-bearing capacity of the foundation and hence, it is ideal for high-rise buildings.
5. Rigid Frame Mat Raft Foundation
This is one of the most efficient types of raft foundation because here the walls function as a deep beam. It is used when columns have to carry immensely heavy loads and the joining beams are more than 90 cm deep. In this type of foundation, box-like structures are created and the walls of each box serve as the beam and they are connected by the slabs at the top and the bottom.
The slabs and beams are connected in such a way that their cross-section looks like an L-section. This forms a cellular raft foundation and therefore, it is also called a Cellular raft foundation. It can endure extremely high bending stresses and is perfect for loose soils. Additionally, when the thickness of the slab is very high, this type of raft is the most economical.
After the different types of raft foundations, let’s move on to the advantages and disadvantages of using a raft foundation.
Advantages of Raft Foundation
The raft foundations have several advantages. Here are some of the most prominent ones:
- It is a type of shallow foundation and hence, it requires less excavation
- Great for soils with low bearing capacity
- It helps if a basement needs to be constructed
- The load coming down from the structure is distributed over a large area
- It reduces the differential settlement of soil
Disadvantages of Raft Foundation
Here are some of the disadvantages of the raft foundation:
- Oftentimes, a lot of reinforcement is required for these types of raft foundations, which can significantly increase the cost of the project
- If they have concentrated loads, they need special attention
- There’s a chance of edge corrosion if they are not taken care of properly
- For the construction of raft foundations, skilled workers are required
The Takeaway
We all know that a good foundation makes a steady structure, but choosing from different types of raft foundations can get tricky. Hence, we have made an effort to help you choose from the types of raft foundations with this guide. The raft foundations mentioned in this guide begin with the most economical to the most costly ones. Nonetheless, you must investigate the soil conditions, find out the loading conditions, and choose a suitable raft foundation for safe construction.
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