A raft foundation, also known as a mat foundation, is a type of foundation used to support structures that have a large footprint or require uniform support across the entire area.
It is a solid reinforced concrete slab that sits directly on the soil and spreads the load of the structure over a large area. The foundation is designed to distribute the weight of the building evenly, reducing the risk of settlement or uneven settling. A raft foundation is typically used when the soil has a low bearing capacity, or when the structure is built on top of fill or other soft, unstable soils.
The foundation is also useful for structures that require a uniform, stable base, such as large industrial buildings, multi-story buildings, or high-rise buildings.
What Is Raft Foundation?
A Raft or Mat Foundation firstly is a continuous slab of soil that extends over the whole footprint of the building to be constructed by supporting the building and transferring its weight to the ground.
When determining which foundation is the most economical (foundation), the engineer must consider superstructure load, subsoil conditions, as well as desirable tolerable settlement.
Raft Foundation Detail
- As the definition of Raft or mat Foundation suggests, the footing is spread over a wider area taking load from a number of columns and in turn, transferring the load evenly over the entire span of footing
- Structure obtains high stability through this type of load transformation.
- The same is the reason for the even settlement of the soil hence ultimately the reason for the non-uneven settlement of the structure.
- In mat foundation, the wider area of footing is in the proximity of the soil as compared to other types of footing hence transferring the load over a larger area hence less stress is developed.
- Raft or mat foundation is suitable for weak soils hence if lesser stress is developed in the soil chances of shear failure decrease making soil suitable for bearing the structural load to an extent.
Types of Raft Foundation
Following are the different raft foundation types,
- Flat Plate type Raft Foundation
- The plate thickened under the column
- Piled Raft Foundation
- Beam and Slab Type
- Cellular Raft Footing
1. Flat Plate Raft Foundation
Apart from all the types of Raft or mat Foundations, this is the simplest of all. Once the load distributed by the column is almost equal Flat Plate type Raft Foundation is preferred. Similarly when the columns are well equally placed this type of raft foundation is used.
In the Flat Plate type Raft Foundation, bending movement is in both directions. In it, both side mesh reinforcement is provided, adding to it the more reinforcement at joints is provided for stability purposes. It provides a thickness of about 300 mm to the slab hence used when the loads are relatively small. This factor provides the economical status of this Raft type.
Since reinforcement is provided in both directions foundation can provide resistance to moments in the respective directions.
2. Plate Thickened Under The Column
In cases of columns imposing more loads on the slab underneath, Plate Thickened under the Column Raft Foundation is used. To counter the problem of high load slab under the column is thickened in order to bear the load and ultimately provide safety.
In order to do so more reinforcement is provided in the thickened slab to bear the extra load and in turn, transfer the load to the soil in proportion. Diagonal shear imposed could be taken care of in this type hence taking negative reinforcement into account.
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3. Piled Raft Foundation
The main function of the foundation is to transfer a load of the structure to the soil beneath it which is capable of bearing that load. Depending on the type of soil beneath and the massiveness of the structure shallow and deep foundation is preferred for construction.
Wherever the bearing capacity of soil is acceptable for the load imposed by the structure Shallow Foundation is taken into account. In the case where the bearing capacity of soil is acceptable for the load imposed by the structure deep foundation is generally used for proper stability of the structure.
Advantages of Pile Raft Foundation
The following are the major advantages of piled raft foundation,
- Where a conventional raft foundation does not provide adequate support, it can be enhanced by the addition of piles, creating what is known as a piled raft foundation.
- Foundations provide support for structures, transferring their loads to layers of soil or rock that have sufficient bearing capacity and suitable disposal characteristics.
- Very broadly, foundations can be classified as shallow foundations or deep foundations Shallow foundations are commonly used where the loads imposed by the structure are low relative to the bearing capacity of the surface soil.
- Deep foundations are necessary where the bearing capacity of the surface soil is not sufficient to support the load exerted by a structure and therefore those loads need to be transferred to deeper layers with higher bearing capacity.
- In its general form, raft foundations (sometimes referred to as mat foundations) are shallow foundations formed by reinforced concrete slabs of uniform thickness (typically 150–300 mm) covering an extensive area, often of a building.
4. Beam and Slab Type
When the excessive bending moment is to be resisted by column strips other types of Raft Foundation do not appear to be beneficial and economical. In such cases, beam and slab raft is adopted which seems to be promising in resisting unwanted bending movement.
In simple language when the load imposed by columns is high and variable with the length of footing Bean and Slab type of foundation is constructed. Appropriate in-dimension beams are provided with each and every row of the column in the longitudinal as well as in the transverse direction. .
5. Cellular Raft Foundation
A Cellular Raft foundation is also mentioned as Rigid Frame Mat on the property of bearing heavy loads. Beams in circular raft foundations are deeper than usual and carry heavy loads. In a Cellular Raft or mat foundation two concrete slabs are laid upon, which lock together via ground beams.
The upward movement of the ground due to the swelling of clay soils that expand when wet could cause affect the stability of the structure leading to damage, in such cases, the Cellular Raft foundation is laid. Cellular raft foundations are exceptionally rigid in characteristics, and so they are most suited for ground that is likely to settle in an uneven manner.
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