Raft foundation is a type of foundation formed by the reinforced concrete slab of uniform thickness (150 mm to 300 mm) spreading over the large area specifically entire footprint of the building.
It supports the columns or walls and transfer the load from structure to wide area of ground. When constructed it looks like mat of concrete spread over wide area, thus it is also recognised as mat foundation. The slab take the load of whole building and transfer it to ground, so it can be considered to float on ground same way the raft float on ground.
Conditions where Raft Foundation is preferred
When the soil at site has a low bearing capacity, thus load needed to be distributed over large area.
When it’s unpractical to provide individual or any other type of foundation. For example. when the strip foundation would cover 50% or more area of building footprint, in such cases raft foundation would be more practical.
When there is possibility of excessive settlement or differential settlement, when other type of foundation is provided. Thus settlement chances are reduced by providing raft foundation.
When the soil beneath is unpredictable and might contain pockets of easily compressible soil, which might result in individual footing failure. This risk is eliminated by providing raft foundation.
When basement has be to constructed. It’s unpractical and raises many problems if we provide other types of foundation than raft foundation when we has to provide basement. Raft foundation provides a good way to transfer load and also acts as an floor for basement.
Why do we provide raft foundation when the bearing capacity of soil is low ?
It’s is known fact that when the soil has low bearing capacity we provide raft foundation at site. So to know why ? lets know the working principle of raft foundation;
Working of raft foundation
Raft foundation transfer the total load of the building to soil under neath.
The deign of raft foundation is simple.
The total load on building (dead + live + other loads + self weight of building and foundation) is calculated.
This total load of building is divided by total area of foundation (i.e. footprint of building). This gives us stress on per sq.m of soil.
As in case of raft foundation the load transfer area of footing is much more than the other types of foundation, thus the stress on soil is much lesser, thus it can be easily resisted by soil with low bearing capacity and risk of shear failure is avoided.
In this extra beams are provided to connect the columns and walls. This beams are cast monolithically with the foundation slab. It is generally provided when the columns are located at larger distance from one another with variable loads. The beams helps to transfer the load from one column to another resulting in uniform loading over the foundation slab.
Various steps of constructing the raft foundation are given below;
Calculate the bearing capacity if soil at site and load on structure.
based upon this , decide the type of raft foundation to be provided at the site.
Calculate the depth to which the raft foundation is provided. This depend upon type of soil at site,load on structure,type of raft foundation to be provided etc.
Excavate the soil up to calculated depth.
Compact the soil and prepare a proper & uniform base for foundation.
Provide water proofing membrane at base.
Provide 3″ of plane mortar(cement + sand), to properly level the base.
Place reinforcement at designed spacing and provide proper .
Pour concrete and attain desired depth.
Cure the slab.
Disadvantage of Raft Foundation.
It tend to be less effective when the structural loads are concentrated over particular area. It can prone to erosion over the particular area.
In some cases drainage under foundation may create a big problems in raft foundation.