Homogenization of Layered Subsoil
If the soil below the footing bottom is inhomogeneous (or if there is ground water present) then the input profile is transformed into a homogeneous soil based on the Prandtl slip surface (see Fig.), which represents the type and location of failure of the foundation.
The Prandtl slip surface
Determination of equivalent values of φ (angle of internal friction), c (cohesion of soil) γ (unit weight of soil below footing bottom) is evident from the following formulas. The unit weight of soil above foundation is derived in the same way.
Procedure for computation of auxiliary values