Studying the geotechnical stability of the earth’s surface during the formation of different backfill mass types in quarry cavities

The research focuses on studying the geotechnical stability of backfill mass in the mined-out spaces of inactive quarries for restoring the earth’s surface in the conditions of the Kryvyi Rih region, where large-scale complex iron ore mining is conducted. Based on the analysis, it has been determined that in the region, to fill inactive quarry cavities and failure zones, the traditional method of filling them with dump waste rock is used. Given that the city of Kryvyi Rih is a densely populated, highly indutrialised agglomeration, the restoration of additional land areas could bring significant benefits for the economic development of the region, but the rock mass is not able to provide reliable geomechanical stability of the earth’s surface. The transformation of the physical state of the backfill mass from a loose to a monolithic state is proposed. In order to select alternative backfill methods, the quantitative and qualitative structure of the accumulated wastes of the mining-metallurgical complex that can be used as backfill materials is analyzed. Taking into account the significant volumes of accumulated beneficiation tailings and their limited utilization, it is recommended to use them as part of cemented paste backfilling, which will lead to an improvement in the environmental situation. The research methodology consists of laboratory tests of physical-mechanical properties of dump hard rocks, analysis of the properties of paste backfilling, and numerical modeling of stability of various types of backfill masses. It has been found that the mixture of rocks of 0...100 mm in terms of bulk density and voidness is close to the minimum fraction of 0...5 mm, which does not require the need to select a certain fractional composition. A «stress-strain» curve of a 0...100 mm rock mixture with a logarithmic relationship, characterized by three stages of strain, has been plotted. The strain modulus value has been calculated. The similarity of the mechanical characteristics of the studied mixture to the full-scale characteristics of dump hard rocks is substantiated. The modeling results show that the maximum strains in the case of rock backfilling reach 780 mm, while the calculations of paste backfilling show a significant decrease in the subsidence value, reaching only 43 mm. The difference in the values of subsidence is due to a significant difference in physical-mechanical properties. Based on studying the hardening conditions of paste backfill mixtures and temperature dynamics in the region, a seasonal approach and the formation of a combined backfill mass are proposed to maintain the reclamation rate. The research findings are valuable for the development of the construction direction of reclamation in the Kryvyi Rih region and the rational use of restored land areas.