RAFT FOUNDATION TO ISOLATED BUILDING

The raft foundation is define as a foundation are design to cover a bigger area beneath the building.The raft foundation can act also the foundation,the ground floor slab and beam and can reduced the overall construction cost.There  are 5 or 6 type of raft foundation depending on the total resultant load from the column or wall.There are 3 method to design the raft foundation and before designing process the below factor must taking consideration.

The six condition that the raft foundation is very useful or more economic to the other type of foundation.

1. The soil has a low bearing capacity.The low bearing capacity is a soil that has a soil bearing reading between 100 kn/m2 and 50 kn/m2.And the type of soil are very loose,soft and fine soil(Clay and silt).
2. Load of the structure has to be distributed over a large are.Not suitable for a small area but have a bigger column load.
3. Individual or any other foundation area would approx cover 50% of the total ground area beneath the structure.Because of the low bearing pressure reading,the size of individual pad footing are bigger between one footing to one footing and the distance between them are smaller and total are of footing are cover more than 50% of the total area of the building.
4. The columns or walls are placed so closely that the individual footings would overlaps.These always happen for renovation works.With small area,and point load and cantilever beam every were to avoid existing footing,the footings always overlap.
5. Stress on soil needs to be reduced.With larger footing the stress beneath the footing are reduced.
6. There is a possibility of differential settlement in case of individual footing is used.
7. When soil strata are unpredictable and contain pockets of compressible soil.Only can detemine when all the soil test are carried out whether Mackintosh or SPT or other type of soil test.
8. Basement is to be constructed
9. Any other type of footing cannot be used advantegeously

The above factor must be consider first when selecting raft as a suitable foundation.For the high concentration load from the individual column,the thickness  and the numbers of bar beneath the column are thicker and crowded and increase the cost of construction.The raft foundation is most suitable for UDL load such as shear wall,IBS wall,water tank and etc,The weaker side of raft foundation are prone to erosion.The main factor that the owner of the building used raft foundation is to reduced the construction cost .No need  to construct a isolated foundation,slab,beam but for the designer engineer these factor not a main factor to be considered may be after doing a calculation,the cost may be increased and not profitable. The selection of the raft foundation must do at the beginning and cooperation with the architect an d owner are need.If the conventional method are going to be used,the column position must be equal and not to far from each other to avoid higher concentration load.And the most important are the SI test result before architect design the layout of the building.

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