Anything to reduce cost and still maximize benefits goes in today’s world. Sometimes we find ourselves with only a few options but we can still explore them. Unlike short bored piles, constructing deep piles are normally expensive compared to raft foundations because not only would the piles be installed using various machines or equipment they usually will have large caps and beams constructed over and connecting them respectively. Therefore when cost becomes an issue you can find an acceptable way to run the race and win the prize. Where practically possible and technically okay, a combination of two or more foundation types can be adopted. In practice both raft and pile foundation can be used together on the same structure to achieve a foundation that performs excellently under load and still reduce overall cost and when you choose this option, there are several factors to take into consideration.
For instance you intend to put up a block of 8 flats on four floors; 2 flats per floor and your soil test report allows you to combine pile and raft foundations dependently or independently without necessarily specifying where either of the two should be used, you can in agreement with your engineer/builder opt to use pile on one wing and raft on the other. In other cases, project owners who intends to build semi-detached duplex and bungalow together or semi-detached duplexes or terrace houses or even two separate buildings lying side by side to each other can use a combination of pile and raft foundations perfectly. The type of foundation arrangement discussed here is not the same as combined Pile Raft Foundations (CPRF) which is achieved by constructing raft over existing piles instead it is about the use of two different foundation types in different locations on the same building or buildings sitting close to each other on the same land. However in any case, it is important to follow the lead of the soil investigation before venturing.
To get the procedure right, basically obtain a geotechnical report (soil test) to know the soil strata and bearing capacities at various depths. Pre-inform the geologist or structural engineer handling your soil test of the type of structure and your intention to combine raft and pile foundation so they can compute various strength characteristics in line with your option. A lot of technicalities are involved in the combination of these two types of foundation especially for the same building but engaging a professional structural engineer will make the work look very easy and the resultant cost reduction is amazing! For instance the interaction between both foundation elements and the subsoil to achieve maximum functionality are primarily hinged on the characteristic pile and base resistance to load and against settlement hence the foundation must achieve resistance that is sufficient enough not to fail or settle excessively or inordinately at any given time. Also, the internal and external bearing capacities of both foundation elements which border largely on strength of materials used and bearing pressures exerted on, by and around the foundation respectively should be properly analyzed. If you get your procedure right you would have cut some 20% off your foundation cost and that’s huge savings especially on a large scale.