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.

The Figure shows a raft of a building consists of two rectangular parts, which are completely connected. The raft is 50 [cm] thick and has a foundation depth of 2.50 [m] under the ground surface. It is planned to construct a tunnel diagonally to the building axis. A primary estimation expects that the tunnel will cause a settlement trough of about 9 [m] width with a maximum lowering of 3 [cm] for the building ground. The settlement trough is plotted in the Figure as contour lines, running symmetrically to the tunnel axis. The influence of the settlement trough due to construction of the tunnel is considered in the analysis of the raft. The raft carries two equal column loads, each of P = 18 000 [kN] and line loads of p = 300 [kN/m] from edge walls. The edge walls have 0.30 [m] breadth and 3 [m] height.