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).

To verify the iterative procedure of Kany/ El Gendy (1997) and evaluate its accuracy for interactive large system of rigid rafts, consider the example 2 in the User’s Guide of program STAPLA (Kany (1976)). The computed settlements obtained from the iterative procedure are compared with those of program STAPLA (Kany (1976)).

For a sewerage station, two isolated containers A and B were constructed simultaneously. Then, lately to extend the station another two isolated containers C and D would be constructed at the same area. Those two external C and D containers would provide an additional settlement on containers A and B.

It is required to assess the tilting of each container and the settlement considering the interaction between the containers through the subsoil at the end of construction. The tilting and settlement of the containers are main factors for designing of the pipe connections. The Figure shows the location of containers to each other and the soil layers. The moduli of compressibility of the soil layers for loading and reloading parts are taken as the same E_s. W_s.