Research On The Failure Mechanism Of Coal-Backfilling Structure And Its Effect On The Stability Of Surrounding Rock In The Pillar-Side Backfilling

Due to the influence of private mining from small coal kilns in the earlier period,pillarresidual mining area are widely distributed in the central and western China.Thus,a large number of densely distributed coal pillar groups have also been formed in the underground mining-out area,the long-term stability of which is essential to ensure safe,green and high recovery mining of the adjacent residual coal resources.The stability of coal pillar groups can be degraded by the effects of overburden loads,disturbance stresses,waterlogging and natural weathering.When instability occurs in a local area of the coal pillar,it may not only induce a chain of catastrophic instability in a group of adjacent coal pillars in the lateral space,but also lead to linked catastrophic instability in the overlying rock in the vertical space.Pillar-side backfilling can restrain the lateral deformation of coal pillar groups,generate lateral forces,and form a synergistic load-bearing structure in combination with coal pillar groups,which is an important method to prevent and control the chain instability of coal pillar groups left in pillar residual mining area.Pillar-side backfilling can transform a single"coal pillar"into a composite"coal pillar+backfilling body"in the residual mining area,which can form a coordinated load-bearing structure(hereafter referred to as"coal-backfilling structure").Its instability response characteristics and instability mechanism deserve in-depth attention,providing theoretical guidance for the good implementation of pillar-side backfilling in pillar residual mining areas,and further ensuring the low-carbon development and healthy operation and maintenance of pillar residual mining spaces.This paper takes the coal-backfilling structure as the research object and adopts a combination of data research,indoor tests,numerical simulation and theoretical analysis to investigate the multiparameter response characteristics of the coal-backfilling structure damage,study the non-equilibrium bearing characteristics of the coal-backfilling structure,reveal the instability mechanism of the coal-backfilling structure,and explore the influence of the coal-backfilling structure on the stability of the surrounding rock.The main research achievement of the thesis are as follows:(1)The types of coal pillar groups in the pillar residual mining area were analyzed,based on their relative position to the residual coal to be mined,the pillar groups were divided into:pillar residual mining area coal pillar groups parallel to the remnant coal to be mined,pillar residual mining area coal pillar groups located below the remnant coal to be mined,pillar residual mining area coal pillar groups located above the remnant coal to be mined,and composite pillar residual mining area coal pillar groups located above and below the remnant coal to be mined.The characteristics of the lateral chain and longitudinal linkage disaster instability of the coal pillar groups in the pillar residual mining area are summarized.A method for the prevention and control of catastrophic instability in pillar-side backfilling coal pillar groups is proposed.The synergistic load bearing capacity of the pillar-side backfilling coal pillar is solved and the equations for the compressive strength,lateral restraint force and critical width of the pillar-side backfilling are obtained.Based on this,the concept of coal-backfilling structure is proposed:A load-bearing structure with a synergistic relationship between the coal pillar and the backfilling body.The coal-backfilling structure are classified as BP coal-backfilling structure and BPB coal-backfilling structure based on the variability of unilateral backfilling next to the pillar and bilateral backfilling next to the pillar.(2)Mechanical tests on BP and BPB coal-backfilling structure under uniaxial compression conditions were carried out to understand the mechanical behaviour,surface deformation field evolution and acoustic emission response characteristics of coal-backfilling structure damage.The effects of backfilling strength,coal-backfilling width ratio and loading rate on the multiparameter response characteristics of coal-backfilling structure damage are revealed.The uniaxial compressive strength of BP coal-backfilling structure was found to be positively correlated with the strength and loading rate of the backfilling and negatively correlated with the coal-backfilling width ratio.The modulus of elasticity was also positively correlated with the strength and loading rate of the backfilling,but showed a trend of"decreasing and then increasing"with the increase of the coal-backfilling width ratio.The peak strain was positively correlated with the strength of the backfilling and negatively correlated with the loading rate,and showed a trend of"increasing and then decreasing"with the increase of the coal-backfilling width ratio.The uniaxial compressive strength of the BPB coal-backfilling structure increases with the increase of the backfilling strength,decreases with the increase of the coal-backfilling width ratio,and shows an"increasing then decreasing"trend with the increase of the loading rate;the modulus of elasticity is positively correlated with the filler strength and loading rate,and shows a"decreasing-increasing-decreasing"trend with the increase of the coal-backfilling width ratio;the peak strain shows an"increasing then decreasing"trend with the increase of the backfilling strength and coal-backfilling width ratio,and shows a"negative-positive-negative"correlation with the loading rate.Correspondingly,the surface deformation field evolution,acoustic emission response characteristics and crack distribution of the coal-backfilling structure reflect the above mechanical behaviour well.(3)The stress sensing system for coal-backfilling structure elements was developed,the stress sensing test for coal-backfilling structure elements was carried out,the distribution pattern of stresses in BP and BPB coal-backfilling structure elements was mastered,the non-equilibrium load-bearing characteristics of coal-backfilling structure elements were discovered,and the calculation equations for the loads borne by coal-backfilling structure elements and backfilling elements were derived respectively.The evolution of force chains and crack expansion laws for the progressive destruction of coal-backfilling structure have been mastered,and the progressive damage process of uniaxial compression of BP and BPB coal-backfilling structure is clarified:early damage at the coal-backfilling interface→joint damage of coal elements and backfilling elements→overall damage of the coal-backfilling structure,the alternating load-bearing characteristics of coal elements and backfilling elements during the destruction of coal-backfilling structure have been discovered.An"II"spring mechanics model for the destabilization of BP coal-backfilling structure and a"III"spring mechanics model for the destabilization of BPB coal-backfilling structure were constructed to determine the stability conditions of the coal element,the backfilling element and the coal-backfilling interface in coal-backfilling structure respectively.On this basis,the concept of"key domain"of coal-backfilling structure is defined:The most vulnerable region of the coal-backfilling structure,and the"key domain"induction mode of the overall coal-backfilling structure damage is condensed,i.e.when the"key domain"does not occur,the coal-backfilling structure must not be destabilized;on the contrary,when the overall coal-backfilling structure damage occurs,the"key domain"must have occurred early damage.(4)Taking the upward re-mining of the 6~#residual coal in the pillar residual mining area ofYuanbaowan coal mine as an example,the stress distribution,movement deformation and breakage collapse characteristics of coal rock under the scenarios of not pillar-side backfilling,unilateral pillar-side backfilling and bilateral pillar-side backfilling were compared and analyzed to reveal the influence of pillar-side backfilling on the stability of the surrounding rock.The results show that when not pillar-side backfilling,under the influence of the over-supporting pressure of the 6~#residual coal upstream re-mining,the group of residual coal pillars in the pillar residual mining area will be destabilized by chain disaster as the working face advances,leading to inter-seam rock linkage damage and affecting the safe green high recovery rate mining of residual coal resources.After coal-backfilling structure,as the width of the backfilling increases,the stress concentration of the residual coal pillar groups in the pillar residual mining area gradually decreases,the movement and deformation of the interlayer rock gradually decreases,and the overall stability of the"residual coal pillar group-interlayer rock"is continuously strengthened,which can ensure the safe upward re-mining of the 6~#residual coal.At the same time,the control effect of bilateral pillar-side backfilling is significantly better than unilateral pillar-side backfilling.