Study On The Creep Damage Mechanism Of Coal Petrography And Gob-side Coal Pillar Stability

The gob-side coal pillar will occur damage accumulation effect affected by roadway excavation,working face mining disturbance.Meanwhile,The gob-side coal pillar which has remarkable rheological characteristics under the influence of long-term stress from the thick sandstone hanging structure,induce hidden dangers such as large deformation,serious side spalling and difficult support,which brings severe challenges to mine roadway support and high-yield and high-efficiency production.Based on the stability of gob-side coal pillars in Xiaojihan coal mine,the creep characteristics of damaged coal samples,fractional-order nonlinear creep model considering initial damage,creep damage evolution mechanism of gob-side coal pillar under thick sandstone roof are analyzed by means of experimental test,theoretical analysis,field monitoring and numerical simulation.The following innovative results are obtained:(1)Triaxial compression tests of damaged coal samples were tested,and triaxial stress-strain curves with initial damage variables,loading rate and confining pressure were obtained.The effects of three factors on the deformation,peak stress,volume expansion and failure characteristics of damaged coal samples were analyzed.The functional relations between the three factors and the peak stress and dilatancy stress of damaged coal samples were determined.(2)Triaxial creep tests of damaged coal samples were tested,and the creep characteristic curves with initial damage variable,loading rate and confining pressure were obtained.The characteristics of axial deformation,creep failure stress,deformation rate,volume expansion stress and dilatancy stress during creep process of damaged coal samples with three factors were analyzed.The functional relations between the three factors and creep failure stress and dilatancy stress of damaged coal samples were determined.(3)Based on the basic concepts of fractional derivatives,basic rheological elements and basic properties of rheological composite elements,the integer-order nonlinear viscoelastic-plastic creep model and the fractional-order nonlinear viscoelastic-plastic creep model considering initial damage are established.The integer-order and fractional-order nonlinear model parameters are identified.The accuracy and correlation of two model are evaluated,which shows that the fractional creep model has more advantages.(4)Based on the occurrence of overlying strata and the roof characteristics of near 11215 working face of Xiaojihan coal mine,the “O-X” type breaking and movement law of overlying strata in "thick sandstone roof working face" is analyzed.Based on the measured data of the strong pressure in working face,the working resistance,periodic pressure step,dynamic factor and number of safety valve opening times of hydraulic support in different stages are studied.The stress,displacement and damage and fracture evolution characteristics of coal pillar are also studied.The external dynamic source and internal damage mechanism of gob-side coal pillar is revealed.(5)The mutational instability criterion of gob-side coal pillar is put forward.The nonlinear fractal three-dimensional creep numerical model is established.The evolution characteristics of stress,displacement and damage of coal pillar under different initial damage degree,different width of coal pillar and no hanging roof structure are analyzed.The stability of coal pillar under low initial damage,wide coal pillar and no hanging roof is determined.The key technology of coal pillar stability control is put forward.