Introduction. In complex geotechnical conditions during construction and operation of engineering structures, polyurethane compositions are used to strengthen loose soils. Analysis of current research has shown that the effect of polymers on sandy soil filtration properties is poorly studied. The purpose of this study is to determine the dependence of permeability and microstructure of chemically strengthened sand on the treatment method, polyurethane resin consumption and external compressive loads. Methods and Materials. Experiments are carried out with fine and medium-grained sands. Two-component highly elastic and one-component polyurethane resins are used for strengthening. Their main purposes are soil stabilization, waterproofing, and formation of cutoff curtains. The effect of the resins on rock permeability and microstructure is assessed based on the experimental results. The experiments include formation of the polymer-sand mixtures using one-solution and two-solution resin treatments with resin/rock volume ratios of 0.05-0.25, microstructure study and filtration tests under various sample loading conditions. Results and discussion. The dependence of the sand permeability on the method of strengthening with polyurethanes has been determined. In the case of one-solution treatment with highly elastic resin with composition/rock volume ratio of 0.2, the permeability of the samples is 7-13 -10^-3 pm². A two-fold decrease of the resin content causes an increase in permeability by 1-2 orders of magnitude. Such behavior is explained by the structure in which open intergranular pores predominate and form connected pore channels. The addition of a small volume of a one-component rigid polyurethane composition reduces sand permeability by 1.5-3.2 times and improves the stability of samples under compressive loads. Conclusion. The practical significance of the results consists in the increasing efficiency of sand filtration properties reduction with polyurethane resins strengthening. The proper choice of a chemical treatment method with consider of the geotechnical problem provides both effective rock permeability reduction and decrease in the consumption of expensive polymer compositions for the construction of cutoff curtains and screens in the rock mass.