Raft foundation is generally employed in swampy or marshy areas where the soil contains moisture and it's not possible to erect structures by the usual way. Depending on the wetness of the soil on which you wanna build your structure,you need to vary the amount of hard materials like granite or rock,silica or sand and concrete bed. The lower most layer should be stuffed with granite stones(Which forms the base of the foundation and they should be placed as compact as possible) and there should be no chance left for the structure to go further down into the soil.Above that you should have a layer of water proof material and above that sand layer that could absorb any further moisture.Above the sand layer you need to build your concrete structure and the concrete bed should be think enough not to allow seepage of water. Above the concrete layer,you need to have another layer of water resistance material and your foundation is ready. Caution:The above design would last only if,your walls are percolation or seepage proof.Hence the materials you use for the walls should be water resistant so that they don't allow water to get into the foundation beyond the permissible limits. Hope that helps.ETW.