The scope of this paper is the investigation of the variation of the soil resistivity and the grounding system resistance during the year. Measurements of soil resistivity variation as a function of time, at certain characteristic locations, have been carried out. The annual change of multi-layer earth structure parameters has also been calculated by the analysis of data of measurements. The calculation of the earth layer structure parameters is in fact an optimization problem. On this purpose, data treatment has been based on a methodology which uses a genetic algorithm model. The proposed methodology provides evidence of highly accurate results in the calculation of multi-layer soil parameters. These results are used as input data for the simulation of grounding systems behavior for this particular soil type. In this way, an optimized design of the grounding system under consideration becomes possible, eventually resulting in minimized step-voltage and touchvoltage built-up.