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.

For the explanation of the influence of a neighboring building, the influence of a new building on an existing old one is examined in this example.

The Figure shows plan and section of new building (II) beside similar old one (I). The building (I) was constructed since long time, while the building (II) will be constructed close to the first one. The two buildings have the same construction geometry and loads. Also, every building is symmetrical about both x- and y-axes.