Details of Raft Foundations
Raft foundations alias Mat Foundations belong to a large concrete slab which provide support to various columns and walls.
The slab is expanded under the whole building or relatively a large part of it that reduces the contact pressure with regards to the conventionally applied strip or trench footings.
The raft foundation is inexpensive, simpler to install and most significantly, adequate excavation is not essential as compared to the usual strip foundations.
In order to apply a raft foundation, it should be designed and sanctioned by Building Control. It makes the whole operation complicated and laborious. So raft foundations are not always recognized.
The proper conditions for using Raft Foundations:
Rafts are mostly recommended when the strata is inconstant or (due to this) a normal strip foundation usually cover over 50% of the ground area below the building. There are other conditions (generally in areas where mining is happened) where there exist areas of movement in the strata.
They are generally used in developing commercial building as well as domestic homes in the UK.
How Do Raft Foundations Work?
A raft foundation transmits the weight of the building above the entire ground floor area of that building. The raft is positioned on a hardcore, or scalping bed and generally condensed at the edges, particularly in very feeble ground. Rafts are mostly recommended when the ground contains excellent load bearing capacity and minimal endeavor is needed to obtain a solid foundation.
Raft Foundations are developed under following conditions::
The soil removed down to exact depth
The foundation bed is then condensed with ramming
Place reinforcement on spacers over the foundation bed
Pour the concrete over the reinforcement
The foundation is solidified by ribs or beams which are developed throughout construction to provide additional strength and firmness.
Ref.: onlinecivilforum.com
When Raft Foundations Are Used?
A raft foundation can be used under following conditions :-
It is applied for greater loads. So, they are mostly found in commercial building that is much larger as well as heavier as compared to domestic homes.
The soil contains a low bearing capacity consequently the weight of the building should be expanded over a big area for the development of a solid foundation.
The proportion of individual footings to total floor space is high. Normally, when the footings cover over half of the construction area as compared to the raft foundation is applied.
If the walls of the building are very nearer that it would induce the individual footings to overlap, then raft foundations are recommended.
