| Coal mine water damage is one of the important disaster-causing factors in the mining process,which seriously threatens the life and property safety of coal mines and workers.In recent years,water inrush accidents have occurred frequently in old air,and water-proof coal pillars are an important safety precaution.Most of the old empty water is acidic,and its continuous damage to the water-proof coal pillar and weakening of its strength is one of the important reasons for the disaster.In this study,acid mine water eroded coal and rock as the research object,in order to characterize the characteristic law of eroded coal and rock,reveal its damage mechanism,and determine its long-term strength as the research purpose.The macroscopic mechanical characteristics of the etched coal rock were obtained by triaxial mechanical test,and the mesoscopic characteristics of mechanical damage were obtained by PFC numerical simulation and inversion.Scanning electron microscopy（SEM）,computed tomography（CT）and 1H nuclear magnetic resonance（NMR）Microstructure change characteristics;using X-ray diffraction（XRD）,infrared spectroscopy（FIRT）test and cavitation theory,the damage mechanism of coal rock is revealed from two aspects of chemical damage and physical damage;through triaxial creep test and long-term strength The theory determines the long-term strength of the etched coal and has been applied in engineering in Sitai Coal Mine.The following research results and conclusions are obtained.（1）The mapping relationship between the macro-mechanical characteristics of etched coal and the two influencing factors of etching acidity（pH）and etching time（t）was established.The strength of the eroded coal rock is negatively correlated with the strengthening of the influencing factors.The maximum stress in the compaction stage continues to decrease,the effective compression range in the linear elastic stage continues to be small,the threshold for the generation of micro-cracks gradually decreases,and the strength of the coal rock continues to weaken.The largest reduction is 58.51%.In contrast,the weakening of coal rock strength by acidic influence factors is more significant.Based on the law of conservation of energy,a constitutive model of chemical energy erosion damage to coal and rock was established for the first time.（2）The failure mechanism and meso-grain characteristics of eroded coal are revealed.With the strengthening of the two influencing factors of etching acidity（pH）and etching time（t）,the cumulative damage of cohesion and contact tensile strength is gradually increasing,but the increase is gradually decreasing;And the box in the low stress area shows a decreasing trend;the particle moving speed shows a decreasing trend,and the box interval is getting smaller and smaller;the development of micro-cracks shows an increasing trend;the particle displacement tends to decrease,and different etching acidity The magnitude of the reduction varies,and the time factor has a slightly weaker effect on particle displacement.（3）The micro structure change characteristics of the etched coal rock were characterized.With the strengthening of the influence factor,the surface porosity tends to increase.In the early stage of etching,the acid factor has a greater impact on the surface porosity.With the increase of time factor,the increase of surface porosity shows the stage rhythm of accelerated dissolution,stable dissolution and deceleration dissolution.The fractal dimension D increases,and the planar pore structure tends to be chaotic,showing an obvious boundary at pH 3.8.The three-dimensional distribution characteristics of pores show that when the acidity is weak,there is obvious pore expansion in the places where the larger pores are easy to penetrate.With the increase of acidity,the surface porosity increases as a whole.If the surface porosity is below 0.12%,the increase is very small.If it is above 0.3%,the surface porosity has almost doubled after etching,and some surface porosity has decreased.The phenomenon.The porosity,permeability,T2 cut-off value,and flowable body saturation of eroded coal showed a decreasing trend,and the proportion of micro-pore size decreased,and the proportion of transition pores,mesopores,and macropores increased.（4）The damage mechanism of eroded coal and rock is revealed.The interlayer spacing（d002）,the average height Lc,and the number of layers N of the coal microcrystals all showed a decreasing trend;the average diameter La of the coal microcrystals showed an increasing trend.The important demarcation point of the influence of（d002）value on the macroscopic strength of eroded coal rock is 0.35nm,and the influence factor is at pH3.8.The aromatic hydrogen ratio（farH）,aromatic carbon ratio（farc）and relative abundance index（I）in the aromatic structure of coal organic matter all decreased gradually,and the degree of condensation（DOC）of aromatic nucleus showed an increasing trend;A（CH2）/A（CH3）decreases,the detachment of branched alkyl groups causes the shortening of the aliphatic chain;the ’C’ in the oxygen-containing structure increases,and the alcohol,phenol and ether groups rich in C=O groups in coal are converted into carbonyl groups,Aldol and carboxyl.（5）The long-term strength of the etched coal rock was determined.With the increase of the acid factor,the long-term strength of coal and rock shows a decreasing trend,and the maximum weakening is 38.40%of the long-term strength of raw coal.The weakening process of the time factor to the long-term strength of coal is the process of time accumulation,accelerated weakening and re-accumulation.The relationship between long-term strength and compressive strength is divided into two sections,the rapid decrease section decreases from 87.35%to 80.42%;the other section has a relatively stable decreasing trend,around 80%.pH4 is the clear cut-off point.The analysis shows that the water-proof coal pillar of Sitai Coal Mine reaches the long-term strength of eroding coal rock at 0.19L2,and the whole coal pillar will be unstable at 0.56 L2. |
