Example (2.3): System of footings on irregular subsoil



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.


Description of the problem

The influence of irregularity of the subsoil material on the behavior of foundations is illustrated through the study of the differential settlements for system of 9 footings. Consider the group of footings shown in the Figure.

Thickness of footings is d = 0.5 [m]. Unit weight of the footing is γf = 25 [kN/m]. Arrangement of footings and footing loads are shown in the Figure (a).



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