What are the advantages of raft foundation for large buildings?
Answer Hi Keris, The advantages of a raft foundation are that it distributes the loading from a building relatively evenly over the footprint of the building and also minimises differential settlement of the columns. For your building, you will first need to work out the maximum pressure and ensure that the clay can safely take this without undue settlement. Then you dig down to the formation (if there are no basements, this should be at least 1.5-2 m below ground level depending on local conditions), place a 75mm of blinding/weak concrete, over which you may need to apply waterproofing, over which you put some protection boards, over which you place the rebar, then put up formwork around the perimeter and pour in the concrete. For thick rafts you will need to control the heat of hydration for about 5-7 days. For a 5-storey building the raft will not be very thick (say 600-900mm), but this depends on punching shear from the heaviest loaded columns.
