Raft Foundation is also known as Mat Foundation is a footing spread over the entire area of the foundation transferring the load of the entire structure to the ground by taking the load from the number of columns.
Raft Foundation as spread over the entire foundation transfers the equal load to soil hence causing the even settlement of the footing adding to its one of the best advantages.
What Is A Raft Foundation or Mat Foundation?
A Raft or Mat Foundation firstly is a continuous slab on 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.
Working Theory of Raft Foundation:
The following is the working principle of raft or mat foundation,
- As the definition of Raft or mat Foundation suggests, the footing is spread over a wider area taking load from numbers 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.
The Necessity Of Adopting Raft Foundation:
Following are the the necessities of Raft or Mat Foundation:
- Raft or mat foundation is beneficial for the poor quality of soils on which foundation is to be laid.
- One of the necessary principles of adoption Raft or mat foundation is when the columns are closely placed and the load imposed is high by the structure.
- If the water level is high in some subsequent areas where construction is to be carried out raft foundation is generally suggested by engineers.
- If found to be constructed covers more than 60% of the total area of construction raft foundation is the most suitable one.
- In the constructions where the task of waterproofing of substructure is to be rendered, mat foundation is adopted.
- The stress of soil will be another factor where raft foundation is taken into consideration(As I reduce the stress on the soil.)
- After the examination, if the soil is showing the signs of Shear failure under loading raft foundation is one of an option.
Types of Raft Foundation:
Following are the different types of raft foundation:
- Flat Plate type Raft Foundation
- Plate thickened under the column
- Piled Raft Foundation
- Beam and Slab Type
- Cellular Raft Footing
1. Flat Plate type Raft Foundation:
- Apart from all the types of Raft or mat Foundation, 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 are provided for stability purpose.
- 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 to 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 providing 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.
Read More: 8+ Foundation Construction Steps
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 structure shallow and deep foundation is preferred for construction.
- Wherever the bearing capacity of soil is acceptable for the load imposed by structure Shallow Foundation is taken into account.
- In the case where the bearing capacity of soil is acceptable for the load imposed by structure deep foundation is generally used for prober stability of the structure.
- In their normal form, raft foundations (sometimes referred to as mat foundations) are shallow foundations formed by a reinforced concrete slab of uniform thickness (typically 150-300 mm) covering a wide area, often the entire footprint of a building.
- This ‘raft’ spreads the load imposed by a number of columns or walls over the area of foundation and can be considered to ‘float’ on the ground as a raft floats on water.
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 the 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.
- In the remaining area forming the central panels are given with slabs supported on these grid beams or walls.
- In Beam and Slab type Raft functions are served the same function as in the thicken flat slab raft foundation but providing more benefit over excessive bending movement caused due to uneven loading.
5. Cellular Raft Foundation:
- Cellular Raft foundation is also mentioned as Rigid Frame Mat on a 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 the structure leading to damage, in such cases 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 uneven manner.
- Sites where such foundations are expected to be found is heavy mining areas or poor soil fields where large bending moments need to be resisted due to uneven settlement and upheaval.
- While being economical for such purposes it is costly in general sense.
Raft or Mat Foundation Construction Process:
The following is construction process of raft or mat foundation,
- The site on which the foundation is to be laid is first surveyed and excavated to a depth up to which foundation is to be constructed. The base of the excavation is leveled to avoid uneven settlement.
- If after excavation soil is wet or the water table is reached then pumping is carried out to remove water and to start further excavation. The waterproofing membrane is provided at the base to avoid water entering the slab.
- Cement-sand paste is poured to ensure the binding with soil once the concrete will be poured.
- This process is followed by lying reinforcement which is to be completed before concreting the base so that concrete can be performed in one go.
- After finalizing the depth of the slab concrete is poured accordingly to the required height letting it for curing for several ways.
Q.1 What is Raft Foundation?
A raft foundation firstly is a continuous slab on soil that extends over the whole footprint of the building to be constructed by supporting the building and transferring its weight to the ground.
Q.2 When should a Raft Foundation be used?
1. Raft Foundation is constructed on weaker soils that are poor with respect to their bearing capacity.
2. Raft foundation is used when columns are closely placed.
3. It is used when the load imposed by the structure is higher in magnitude.
4. When the water table is higher on the construction site Raft Foundation is adopted.
5. Similarly it can be preferred to avoid the shear failure of the structure due to uneven settlement.
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