Long-term Pillar Stability Enhancement Using Fly Ash As A Backfill Material After Room And Pillar Coal Mining

Room and pillar mining method is an important coal mining method for the "three under" coal mining in coal mines.It mainly consists of excavating a series of rooms with different sizes in the coal seam,and the coal rooms are connected by connecting lanes,leaving coal pillars to support and maintain the stability of the overlying strata.In the process of room mining,coal pillar is the key supporting structure to support the overlying strata,ensuring the safety of coal mine production and controlling the surface subsidence.Many coal pillars are left in goaf due to room mining,which not only causes the loss and waste of high-quality coal resources,but also easily causes geological disasters such as coal pillar instability,mine earthquake and spontaneous combustion of coal pillars in goaf.The main reason is that the coal pillar will creep under the long-term load of overlying strata,and the strength of coal pillar will gradually decrease with time.When the load of the overlying strata reaches the ultimate strength of the coal pillar,some coal pillars will be damaged,which will lead to stress transfer,leading to the "domino" instability of the coal pillar group,and induce large-area surface subsidence and mine earthquake disaster.Unlike longwall mining method,room and pillar mining is used for shallow coal seams,it requires less capital and it can be used when the roof does not cave safely.Nearly all pillar design formulae are based on geometrical parameters of pillars resulting from empirical investigations,with primary focus on the pillar width-to-height(W/H)ratio.The objective of coal pillar-design research has,and continues to ensure that catastrophic events of pillar instability do not occur.In recent years,filling mining technology has also made rapid development,among which solid backfill mining,ultra-high water filling mining and cemented filling mining are the most representative.Rich research results have been obtained in the research of surrounding rock stability control,and a relatively perfect theoretical system has been formed.Because of the long-term,sudden and severe instability of the room type goaf,it has become a potential major security threat to the mining area.Therefore,it has important theoretical and practical value to study the long-term stability guarantee of the left coal pillar for room and pillar safe mining and environmental protection.This paper mainly carries out the theoretical research on the long-term stability of the coal pillar reinforced by fly ash cemented filling in room and pillar mining.By using the methods of theoretical analysis and experimental test,the paper simulates the instability and failure characteristics of coal pillar in different maintenance periods under laboratory conditions,and tests the mechanical properties of cemented filling materials with different proportions of fly ash,the results show that the cemented filling with fly ash can improve the residual strength of coal pillar in room and pillar mining.The results are as follows:(1)Using gangue powder(crusher dust),fly ash,cement and other similar simulation materials,the failure characteristics of coal pillar left over from room and pillar mining were studied.The test results show that the uniaxial compressive strength of the simulated coal pillar is 6.92 MPa ? 25.2 MPa under different curing time of 3days,7days,14 days and 28days;The uniaxial compressive strength of cemented filling materials with different proportions of fly ash is 2.6 MPa ? 5.45 MPa.(2)With the decrease of width to height ratio(1.0 ? 0.67)and degradation attenuation of coal pillar left over from room mining,the uniaxial compressive strength of coal pillar gradually decreases(curing period: 28 days: 18.5 MPa ? 13.7MPa);Compared with the condition of no filling reinforcement,the fly ash cemented filling material can limit the destruction of the coal pillar,and improve the overall compressive strength of the coal pillar(increase 2.0 MPa ? 3.0 MPa),and is conducive to its long-term stability.(3)Based on the theoretical analysis of coal pillar stability,coal pillar design,coal pillar instability and coal pillar reinforcement,the use of mine solid waste materials(such as fly ash,gangue,etc.)as the filling and reinforcement materials of the left coal pillar in room and pillar mining can improve the long-term stability of the surrounding rock of the left coal pillar of room and pillar mining,and at the same time dealing with the solid waste products such as fly ash and gangue,protecting the surface subsidence as well as environmental protection.The application prospect is broad.There are 26 figures,11 tables and 71 references in this paper.