In many practical cases, it becomes important to study the interaction of elastic or rigid foundations, which are constructed simultaneously. In this case, there will be interaction of foundations due to the overlapping of stresses through the soil medium, however the structures are not statically connected. The interaction of foundations will cause additional settlements under all foundations.

The conventional solution of this problem assumes that the contact pressure of the foundation is known and distributed linearly on the bottom of it. Accordingly, the soil settlements due to the system of foundations can be easily determined. This assumption may be correct for small foundations, but for big foundations, it is preferred to analysis the foundation as a plate resting on either elastic springs (Winkler’s model) or continuum model. In spite of the simplicity of the first model in application, one cannot consider the effect of neighboring foundations or the influence of additional exterior loads. Thus, because Winkler’s model is based on the contact pressure at any point on the bottom of the foundation is proportional to the deflection at that point, independent of the deflections at the other points. Representation of soil as Continuum model (methodes 4, 5, 6, 7 and 8) enables one to consider the effect of external loads.

The study of interaction between a foundation and another neighboring foundation or an external load has been considered by several authors. Stark (1990) presented an example for the interaction between two rafts. Kany (1972) presented an analysis of a system of rigid foundations. In addition, he presented a solution of system of foundations considering the rigidity of the superstructure using a direct method (Kany 1977). Recently, Kany/ El Gendy (1997) and (1999) presented an analysis of system of elastic or rigid foundations on irregular subsoil model using an iterative procedure.

This section presents a general solution for the analysis of system of foundations, elastic or rigid, using the iterative procedure of Kany/ El Gendy (1997) and (1999).

Besides, the possibility of analysis of large foundation system with many elements by the procedure of Kany/ El Gendy (1997), the mesh of the rigid foundation can be generated in analog mode to the finite element mesh of the elastic foundation in one program. Comparing results from analysis of system of rigid rafts with those of elastic or flexible rafts with the same input data is possible. Subsequently the results of the three analyses are compared in an example.

In this example, the settlements of structures due to interactive analysis of system of rigid, elastic and flexible rafts are studied. This example is chosen from the reference Graßhoff/ Kany (1997). System of two large rafts and additional two external footings are constructed near each other. The dimensions are shown in the Figures.