Comprehensive geomechanical assessment of pillar and roof stability during secondary extraction by the room-and-pillar method

Purpose. This study aims to substantiate the rational parameters of the room-and-pillar mining system for the Zhaman-Aibat copper sandstone deposit (Zhomart mine), taking into account the geomechanical characteristics of the rock mass. Methods. The research employed analytical calculations based on Shevyakov, Tsern, and Chernikov methods, numerical modeling using CPS 2005, Pillars 3, and Examine 2D software packages, as well as field observations and pilot-industrial testing. The stability of inter-room and barrier pillars was assessed iteratively, considering pillar shape factor, loading conditions, and long-term strength. The effectiveness of warning (yield) pillars and different rock bolts was experimentally verified. Findings. The optimal system parameters were determined as follows: panel span of approximately 70 m, extraction room width of 7 m, and inter-room pillar spacing of 16×16 m with a diameter of 9 m (pillar area about 64 m2). Experimental results confirmed the necessity of leaving 18-20 m2 warning pillars, functioning in an over-stressed deformation regime. The calculated width of barrier pillars was 30 m, with a stability factor of n ≈ 2.1. Structural drawbacks of steel-polymer bolts with metric threads were identified, as they can trigger progressive failure; instead, using A20V rope-thread bolts is recommended. Numerical modeling demonstrated that camouflet blasting of boreholes above barrier pillars helps unload the rock mass and reduce stress concentration. Originality. For the first time under the geological conditions of the Zhomart mine, a comprehensive stability assessment of inter-room, warning, and barrier pillars was conducted using back-analysis data and in-situ observations. A quantitative relationship was established for the parameters of warning pillars operating under over-stressed deformation, substantiating the necessity of their application. The effectiveness of camouflet blasting for controlled caving of the overlying strata was also demonstrated. Practical implications. The developed recommendations allow for the reduction of ore losses in pillars, the improvement of the overall stability of the mining system, and the minimization of geotechnical risks. The obtained results can be applied in designing and operating mines with similar geological and structural conditions.