Potash mining is usually implemented under water bodies both on ground surface and underground. Potentiality of hydraulic connection to form between these bodies and mine roadways because of natural (jointing) or induced (aquifer undermining) reasons can cause flooding of some roadways or total closure of mines. An obligatory flooding protection structure in potash mine construction and mining is a waterproof wall. Such walls are constructed in case of hazards of water inrushes in mines. Designs of the water retaining walls take into account the engineering, geomechanical and hydrogeological factors, including the critical factor of permeability. Aimed at evaluation of permeability, the Saint-Petersburg Mining University developed an appropriate procedure and accomplished in-situ experimental activities in roadways on some mine of EuroChem Group AG, at a depth of 1000 m below ground surface. The geological structure and great depth of occurrence of the test roadways govern a high hydrostatic pressure. The in-situ test data processing shows that the permeability of rocks composing a safety pillar is 2 orders of magnitude lower than the preset threshold. The results imply feasibility of construction of water-retaining walls within the limits of the safety pillar. The authorial procedure can be used by research and production agencies when selecting correct construction sites and parameters of waterproof walls aimed at mine flooding protection.