Abstract
In structures carrying heavy superimposed loads involving closely spaced columns or other supports, a raft foundation is normally adopted especially when the soil bearing capacity is low. This type of foundation is generally economical if the area under a group of independent footings exceeds sixty percent of the total covered area and offers the additional advantage of simplifying the water proofing of basements. Furthermore, a raft reduces the differential settlement of supports. Two methods of analysis are available for dealing with rafts. In one method the earth-pressure is assumed uniform and the raft considered as a reversed floor slab or beam and slab system, the elastic properties of the soil being ignored. Design methods on this basis are empirical and are thought to be unrealistic. An alternative approach treats the raft as a plate on elastic support, the subgrade properties being assessed from tests on the soil. Methods in this group are involved and only a limited number of mathematical solutions of practical importance are available. The moire technique presents a means of solving plate problems and has been extensively used for dealing with plates of various shapes with different boundary conditions and for a variety of loading conditions. The work reported in this thesis was directed towards applying the moire technique for solving the particular problems relevant to the design of rafts. Rafts models were prepared from black Perspex and sponge rubber was used to simulate an elastic subgrade. It was found that subgrade reaction at low stress was a linear function of deflection, an assumption inherent in the governing differential equation. It was also established that results obtained on a model can be applied to a prototype providing the model dimensions and grade of rubber are appropriately chosen. Simple rafts of square and rectangular configuration were tested and theoretically analysed by the finite difference technique using a thirteen point net. The comparison of results enables conclusions to be established regarding the utility and also the limitations of the experimental technique.