In many situations, it becomes important to assess the behavior of a foundation due to its interaction with another neighboring foundation or external load. First, It must be distinguished between two types of problems concerning neighboring foundations:

- The first problem occurs when new building is constructed beside existing one. In this case, the new building will cause an additional settlement under the existing structure due to the increase of stress in soil.

- The second problem occurs when structures 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 study of interaction between a foundation and another neighboring foundation or an external load has been considered by several authors. Mikhaiel (1978) presented an application on the use of the elastic half space model in the determination of the effect of neighboring loads on the existing building. Selvadurai (1983) examined the interaction between a rigid circular foundation and an external load. The additional settlement due to neighboring foundation, external loads and buried structures can be considered as follows indicated in the next paragraphs.

Besides the possibility of studying the influence of neighboring structures on the foundation by the program ELPLA, the described algorithm of ELPLA can be used also for the calculation of settlements outside the foundation. This can be carried out through one of the following two ways:

i) Using a net for the foundation and the unloaded areas outside the foundation. Then, the rigidity of the unloaded areas can be eliminated by assuming very small thickness.

ii) Using two independently nets one for the foundation and the other for the unloaded areas outside the foundation as considered in this example. The Figure shows an irregular raft has the contact area (I) with opening inside it. It is required to determine the settlements at the area (II) around the raft and at the opening of area (III).