The Long-term Stability Evaluation Methods Of Coal Pillar For Partial Extraction Mines

Partial extraction method including room-and-pillar mining method and strip pillar mining method is an important mining method when extracting coal resources under infrastructures and water bodies in China.The coal pillars are the key structures that supporting the overburden strata,ensuring the safety of mining activities and controlling the ground surface.Current mine stability evaluation and pillar design theories for partial extraction mines seldom consider the time-dependent degradation of pillar stability,leading to a result that many pillar collapses in abandoned partial extraction mines and the infrastructures on the ground surface are damaged.As the failure of the partial extraction mine is unpredictable,and because the mine failure may occur in a very long period and be accompanied by violent strata movements,the abandoned partial extraction mines have become potential threats to the mining cities.It is of great significance to study the long-term stability evaluation methods for partial extraction mines.Considering the shortages of current pillar stability evaluation theories of partial extraction method,and using numerical simulation,theoretical analysis,similar material simulation and case study,the effects of topography and the effects of mine size on coal pillar stress were studied,and the effects of pillar peeling on long-term stability of coal pillar were studied.On this basis,the pillar peeling model and the longterm pillar stability evaluation methods for partial extraction mines were were proposed,providing a theoretical support for long-term pillar stability evaluation and pillar design of partial extraction.The research contents and achievements are shown as follow:(1)To overcome the shortage that the topography effects on roof stress and pillar stress are not considered in current mine stability evaluation methods,using numerical simulation,theoretical analysis,similar material simulation and case study,the effects of a single slope and the effects of a valley on the roof stress and pillar stress in partial extraction mines were studied.The research results show that there exist a concentrated horizontal stress BAS(Bottom Abrupt Stress)in the roof below the slope toe.The BAS may become the maximum horizontal stress in roof-pillar-floor system when the mining depth is shallow,and it is an important factor that induces the failure of partial extraction mines.The effects of multiple factors(e.g.the slope angle,the mining depth beneath slope toe,the bottom width of the valley and the slope height)on the BAS value were analyzed and the formula to calculate BAS was proposed.Based on these results,the calculation method of vertical pillar stress at different position below a mountainous topography and the failure criterions of coal pillar were proposed,providing a theoretical basis for pillar design and pillar stability evaluation in mountainous mining areas.(2)To overcome the shortages that the mine size effect is not considered and the pillar stress may be overestimated in current pillar design theories,the effects of mine size on the maximum vertical pillar stress in partial extraction mines were studied by using numerical simulation and theoretical analysis.New pillar stress formulas were proposed for the mines with arbitrary sizes,providing a theoretical basis for the pillar stress calculation of partial extraction mines with small sizes.(3)Considering the features of pillar peeling behavior,the pillar peeling models for room-and-pillar mines was proposed by using theoretical analysis.The effects of multiple factors(e.g.the pillar height,the pillar width,the bulk factor of coal,the reposed angle of coal fragments pile and the mining width)on the maximum peeling depth into the pillar were analyzed,and the long-term pillar stability evaluation and pillar design methods for room-and-pillar mines were proposed based the pillar peeling models,providing a theoretical basis for long-term pillar stability evaluation and pillar design of room-and-pillar mining.The reliability of the proposed methods was verified by around 500 coal pillar cases from China,South Africa and India.(4)Considering the features of pillar peeling and strip pillar mining,the pillar peeling models for strip pillar mining were proposed.Combining the pillar peeling models with the Wilson pillar design theory,the long-term pillar stability evaluation methods for strip pillar mining were proposed,advancing the current strip pillar design theories.(5)The failure effects of a single pillar on its neighboring pillar were analyzed.The research results indicate that the failure effect of a diagonal pillar on pillar stability is smaller than the failure effect of an abut pillar.If the initial safety factor of a coal pillar exceeds 2,the pillar can be stable even if all of its neighboring pillars collapse.