Most of the available solutions used to determine the flexibility coefficient, or the modulus of subgrade reaction, assume that the subsoil consists of a homogeneous layer. In reality, the soil consists of different material features in vertical and horizontal directions. In practice, a number of vertical soil profiles defines the soil under the foundation. Each one has multi-layers with different soil materials. Therefore, three-dimensional coefficient of flexibility, or variable modulus of subgrade reaction, must be taken into consideration. Kany (1972) determined the two-dimensional flexibility coefficient for beam foundation by determining flexibility coefficients for the existing boring logs first. Then, by interpolation can obtain the other coefficients outside the boring logs. The following paragraph describes the methods that are available in program ELPLA to determine the three-dimensional coefficient of flexibility or variable modulus of subgrade reaction.

A general example is carried out to show the applicability of the different mathematical models for analysis of irregular rafts on irregular subsoil.

In one case the raft carries many types of external loads: concentrated loads [kN], uniform load [kN/m²], line load [kN/m] and moments [kN.m] in both x-and y-directions as shown in the Figure.