The exact values of the coefficients of filtration and porosity for crushed stone and sand were separately studied and determined, but in no reference it is impossible to find the given indices for a combined gravel-sand mixture. This is important, since in real construction conditions, not the "pure" material, but its mixture, is most often used. Therefore, the research task was to study the physical and mechanical properties and mechanisms of the functioning of the combined gravel-sand mixture. As a tool for solving the problem, the Darcy instrument was chosen. The laboratory installation, by means of which the experiments were performed, was designed and assembled specially for the experiment. The description of the sequence of experiments is presented, the results obtained in the course of the experiment are presented, namely: the values of the filtration and porosity coefficients for soils with a specified percentage, and the corresponding graphical dependencies of these indices on the composition of the mixtures studied. Studies have shown that the change in the sought-for quantities is nonlinear, and, depending on the percentage of the samples in question, their values are different tens of times. These characteristics and the dependencies built on them are of practical use in the construction and operation of hydraulic structures, in geotechnics, melioration and petroleum and gas industry, since they allow selecting the necessary composition of inert materials (sand and crushed stone) to ensure a given filtration factor and predict the filtration properties combined soils with known proportions. Due to the obtained results, it is possible to artificially combine the proportions of the mixtures so that the filtration coefficient of the created material is many orders of magnitude lower than that of the original ingredients alone.