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Raft foundation is a thick reinforced concrete slab which spreads over a large area of soil and provides support for several columns and load bearing walls. It is also called as mat foundation which are widely used foundation system. Raft foundation is a type of shallow foundation.
Fig 1: Raft or Mat Foundation
Contents:
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
Flat plate type raft foundations consists a reinforced concrete slab of uniform thickness covering the whole bearing area. This is suitable when the columns are placed with uniform space between them and carrying equal and small loads. Reinforcement in slab is provided in both directions in the form of steel mesh. Two mesh are reinforced at top and bottom of the slab to balance upward and downward bending forces. Maximum thickness of 300 mm is provided for concrete slab in case of raft foundation.
Fig 1: Flat Plate Type Raft Foundation
2. Thickened Flat plate Type Raft Foundation
Flat plate type is not suitable when the column loads are very heavy. To make it suitable, slab thickness must be increased. The heavy loads from column introduces negative bending moments and diagonal shear in to the slab. So, to resist this a portion of slab under the column should be thickened. Provision of pedestal under the column without increasing slab thickness also helps to receive heavy loads.
Fig 2: Casting Pedestals on Raft Foundation
3. Beam and Slab Type Raft Foundation
In case of Beam and slab type raft foundation, beams are laid in perpendicular directions and all of these beams connected by raft slab. Columns are situated exactly on intersections of beams of raft foundation. This type raft foundation is suitable when the columns are carrying unequal loads and there is large space between them.
Fig 3: Beam and Slab Type Raft Foundation
4. Piled Raft Foundation
In case of piled raft foundation, raft slab is supported by piles in soil. This type of foundation is preferred for soils of high compressibility and high water table. These are well suitable for high rise buildings. Provision of piles under the raft reduces the amount of settlement and also controls the effect of buoyancy. Ultimate load capacity of foundation also increases.
Fig 4: Piled Raft Foundation
5. Cellular Raft Foundation
Cellular raft foundation is also known as box type raft foundation or rigid frame raft foundation. In this type of foundations, boxes like structures are formed where the walls of each box acts as beams and these are connected by slabs at top and bottom. The beams and slabs are united in such a way that their cross section look like I-section. This type of foundations can resists very high bending stresses and suitable for loose soils where settlement is uneven.
Fig 5: Cellular Raft Foundation
Advantages of Raft Foundation
The advantages of raft foundation are as follows:- Raft foundations are constructed for shallow depths hence, it requires less excavation
- Well suitable in soils of low bearing capacity.
- Loads coming from superstructure are distributed over a larger area.
- Differential settlement of soil can be reduced.
Disadvantages of Raft Foundation
The disadvantages of raft foundation are as follows:- In some cases, large amount of reinforcement is required for raft foundation which increases the cost of project.
- Special attention on raft foundations is required in case of concentrated loads.
- If they are not treated properly, there is a chance of edge erosion.
- Skilled workers are required to construct the raft foundations.
