It provides support to the columns or walls and transmits the load from structure to large area of ground. Once it is built up, it seems mat of concrete extends over large area, as a result it is also known as mat foundation.
The slab bears the load of entire building and delivers it to ground, so it is assessed to float on ground similar way the raft floats on ground.
Raft Foundation is effective under the following situations :-
When the soil at site contains a low bearing strength, thus load required for being allocated over big area.
When it is ineffective to arrange individual or any other type of foundation. As for instance, when the strip foundation would encompass 50% or more area of building footprint, in such conditions raft foundation is very useful.
When there are chances for unnecessary settlement or differential settlement, when other type of foundation is arranged. In this way, settlement chances are minimized by setting up raft foundation.
When the soil situated below is unstable and might comprise of pockets of easily compressible soil, which may lead to individual footing failure. By arranging raft foundation, this risk is reduced.
If it is required to build up basement, raft foundation is necessary. It’s unreasonable and creates various issues if other types of foundation are arranged except for raft foundation at the time of providing basement. Raft foundation is very suitable for transmitting load and also functions as a floor for basement.
Why raft foundation is necessary if the bearing strength of soil is low?
Generally, it is found that if the soil contains low bearing strength, the raft foundation is arranged at site.
Benefits of raft foundation.
Raft foundation transmits the total load of the building to the soil below.
The deign of raft foundation is simple.
The total load on building (dead + live + other loads + self weight of building and foundation) is estimated.
This total load of building is divided with total area of foundation (i.e. footprint of building). It provides us stress on per sq.m of soil.
For raft foundation the load transfer area of footing is much more as compared to the other types of foundation, therefore, the stress on soil is much lesser, thus it can be avoided by soil having low bearing capacity. The risk of shear failure can also be resisted.
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