Raft foundations (sometimes known as Mat Foundations) are a large concrete slab which can support a number of columns and walls. The slab is spread out under the entire building or at least a large part of it which lowers the contact pressure compared to the traditionally used strip or trench footings.
Where we can use it?
Raft foundation may be preferred over spread footings on strata that are erratic or have low bearing capacities, or they have a heterogeneous character, or where large differential settlement is likely if one were the foundation is below the water table and there is need to eliminate water infiltration into basement-type installations.
Advantages of Raft Foundations:–
- It may be able to overcome differential settlement problems for the raft acts as a unit. Load incurred by raft foundation will be transferred to the underlying soil by reinforced concrete continuous slab by covering the entire site structure.
- Raft foundation can across the low soil bearing capacity and will distribute the loads on wide area. It spreads the loads over a larger area when the construction takes place on the soft or loose soils with low bearing capacity.
- Reduce differential settlements as the concrete slab resists differential movements between the loading positions.
- To carry loads which are too heavy to be supported by a shallow foundation. The loads are to be transferred to deeper, stronger and less compressible strata or over a larger depth of the foundation soil as in foundations of tall buildings.
- To carry horizontal loads as in bridge abutments or retaining walls and also to increase the stability of tall buildings. Inclined piles are also used to carry inclined loads with horizontal force components.
- To withstand uplift forces in foundations as in expensive soils and floating foundations
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