To maintain moisture and best spread it in the horizontal direction under in-soil irrigation of agricultural plants there is a necessity of laying moisturizers together with an impermeable screen. For this purpose, share mole plow can be used. Substantiation of tool's geometrical parameters and modes of operation is an actual scientific problem. Solution of this problem requires formalizing the soil as the medium on which the tool is directed, as well as formalizing the interaction of the tool with the soil. This article discusses the dynamics of the ploughshare forming a horizontal gap into which the anti-filter screen extends. For this, the interaction of the plowshare of the mole with the soil in the contact zone during compaction of the walls of the formed cavity is under considered. Herewith, the degree of compaction of the walls affects the stability of the arch of the formed cavity. During the study, the soil is presented in the form of an elastic viscous deformable medium, and the ploughshare - in the form of an absolutely solid. The surface of this solid is described by a three-dimensional equation in an implicit form, the coefficients of which characterize the geometric parameters of the tool. In a previous article, the authors obtained functions of the rates of soil deformations from the action of the tool. Based on the strains functions with use the physical equations of the connection between stress and strains, the functions of the stress components on the mechanical properties of the soil and the parameters of the tool are obtained. The stress components obtained in the contact zone of the plowshare with the soil made it possible to determine the function of changing the specific volume of the soil. This function is inversely proportional to soil density. The use of equilibrium equations on the contact surface made it possible to determine the components of the forces that act from the soil side on the ploughshare. Integration of the obtained components of the distributed forces over the ploughshare surfaces made it possible to obtain the sum of the resistance forces to the ploughshare movement, depending on the mechanical properties of the soil and tool parameters. As a result of the study, rational parameters of the equation coefficients of the ploughshare surface that characterize its geometric parameters are determined.