In underground mining of potash and rock salts, as is known, room-and-pillar mining systems are widely used, providing high labor productivity and low cost of mineral extraction. However, losses in inter-chamber pillars in these systems reach 50% at domestic potash and salt mines. Mineral losses can be significantly reduced by reducing the width of inter-chamber pillars by filling the mined-out chambers with backfill. The most economically effective is hydraulic filling with waste from concentrating plants, as in this case the issue of dump placement is solved at the same time. It is also expedient to fill the mined-out space with backfill material specially extracted for this purpose (sand, gravel) ...