8 Types of Raft Foundations Used in Construction
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.
What is Raft Foundation?
A raft foundation, also known as a mat foundation, is a flat, reinforced concrete slab that covers the entire footprint of a building. It is designed to evenly distribute the structural loads over a large area of soil, preventing unequal settlement and ensuring stability. Raft foundations are commonly used in areas with weak or inconsistent soil conditions and are effective for supporting heavy structures, minimizing the risk of differential settlement and structural damage. After looking at what is raft foundation, let’s look at its types.
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.
6. Solid Slab Raft Foundations
Flat Raft Mat
Flat raft mats are commonly employed in smaller constructions where columns can be strategically placed at regular intervals across the building’s footprint. This arrangement ensures an even distribution of stress on the foundation. Steel mesh is typically utilized at both the bottom (near the column-foundation connection) and the top (mid-span between columns) of the concrete mat to provide resistance in both directions.
Wide Toe Raft
In situations where ground conditions necessitate substantial load support, using a conventional flat raft concrete slab would be cost-prohibitive due to its required thickness. Instead, a wide toe raft is employed, featuring a reinforced ‘toe’ that extends deeper to support the external side of a cavity wall.
Blanket Raft
For construction sites with small areas of weaker or diverse soil types, a blanket raft may be the optimal solution. In this design, a stone ‘blanket’ is layered on the prepared ground, gradually building up to the required level before pouring the concrete raft. The combination of the raft footing and the stone blanket collaborates to offer uniform load support, mitigating weaknesses within the build’s footprint.
Slip Plane Raft
Slip plane rafts involve laying a preliminary layer of sand over a slightly larger surface area than the required raft foundation. The gap around the raft is then filled with packed material, creating a cohesive system for even load distribution.
7. Cellular Raft
This foundation style consists of two interlocking concrete slabs connected by ground beams. Cellular raft fooding exhibit exceptional rigidity, making them particularly suitable for areas prone to uneven settling or when anticipating hefty loads. They are also a viable choice in situations where substantial ground heave is expected to pose challenges.
8. Balancing Rafts
Balancing rafts, also known as floating foundations, are increasingly employed in scenarios where minimizing soil settlements is paramount, especially in projects with exceptionally low soil bearing capacity. These foundations are crafted by excavating the soil in a manner that the weight of the removed soil, including water weight, counterbalances either entirely or partially the combined weight of the superstructure and its foundation. While this solution may not always be cost-effective due to substantial earthworks and the construction of retaining structures (permanent or temporary) before and during excavation, it becomes particularly relevant when dealing with a high water table near the ground surface.
Floating rafts find significant use in urban environments with stringent regulations on minimizing settlements in adjacent buildings. An illustrative example is the construction of large underground car parks in urban settings, where adherence to strict specifications is crucial.
After the different types of raft foundations, let’s move on to the advantages and disadvantages of using a raft foundation.
Advantages & Disadvantages of Raft Foundation
The raft foundations have several advantages & disadvantages. Here are some of the most prominent ones:
Advantages
- 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
- 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.
