The paper presents analytical and numerical approaches to the evaluation of the depth of influence zone; the term influence zone is assigned to the region where the load has substantial influence on the deformation of the soil skeleton. The approach relies on the analytical solution based on the analysis of Westergard subspace assuming elastic material response. The depth of the influence zone must be also taken into account in the back analysis of the soil parameters, e.g. the deformation modulus. The necessity of that is shown by field tests. The standard back analysis based on the Boussinesq half space theory provides unreal data when the load of the plate is not sufficient and the depth of influence zone is small.