Experimental Study On Dynamic Response Characteristics Of Coal Rock Under Disturbance Load Of Medium And Low Strain Rate

The stability of the surrounding rock at the working face of a relict coal mining operation is affected by the combination of disturbance loads from mining action,blasting and concentrated local energy release,and static loads caused by multiple prior mining operations.This paper focuses on the scientific problem of the mechanical response characteristics and destabilization mechanism of coal rocks under the coupling of low and medium strain rate disturbance loads and static loads,and is based on rock mechanics tests,field research,theoretical analysis and numerical simulation.By means of mechanical tests on coal rock under coupled low to medium strain rate disturbance loads and static loads,supplemented by acoustic emission monitoring,DIC monitoring,etc.Using theories from the interdisciplinary disciplines of rock mechanics,material mechanics,structural mechanics,mine rock mechanics,elasticity mechanics and rock dynamics,we have analysed the synergistic evolution of acoustic emission signals,local deformation and full-field strain during the dynamic response of coal rocks,explored the effects of disturbance load magnitude and pre-static load level on the dynamic mechanical properties of coal rocks,revealed the damage mechanism of coal rocks under the coupling effect of dynamic and static loads,clarified the information on the precursors of dynamic destabilization damage of coal rocks,and investigated the strengthening mechanism of the legacy coal pillar mechanical properties and resistance to disturbance.The study investigated the influence of column-side filling on the mechanical properties of the legacy coal columns and their resistance to disturbance.The main research elements of the thesis and the innovations achieved are as follows:(1)The static load characteristics of interbedded rock seams and legacy coal pillars during the upward mining process of the column mining hollow zone and the basic characteristics of the disturbance load during the mining process are investigated.Based on numerical simulations,field monitoring and literature summary,the dynamic evolution characteristics of the stress concentration and static loads on the remaining coal pillars and interbed rock layers at different locations from the working face during the formation of the pillar mining hollow zone and during the advance of the upstream mining face are clarified.At the same time,the waveforms and spectral characteristics of the stress waves of various disturbing loads were obtained through on-site monitoring,and the characteristics of the disturbing loads generated by blasting,coal miner cutting,tunnel disturbance and surrounding rock breakage during the mining process were studied.Combined with the propagation and attenuation characteristics of stress waves,it is pointed out that the disturbed load on the surrounding rock at the far field of the disturbance source is dominated by the attenuated low frequency disturbance.The engineering problem of the stability of interbed rock and coal pillar under the influence of upward mining disturbance in the column mining area is condensed into a scientific problem of the dynamic response of coal rock under the action of medium and low strain rate disturbance load,and the disturbance load for the laboratory rock mechanics test is clearly a 5Hz sine wave.(2)The tensile mechanical properties of sandstone under the coupling of dynamic and static loads were investigated using typical sandstone from coal-bearing strata as the research object.The effects of pre-static load level and disturbance load amplitude on the dynamic tensile mechanical properties of sandstone were investigated.The degree of damage caused by the disturbance load was positively correlated with the pre-static load level,with the pre-static load increasing30%?_t from80%?_t,the acoustic emission ringing count rate and AE energy level increasing by a factor of about 11,and the tensile strength of the disturbed damaged sandstone was reduced macroscopically.The existence of the threshold value of the disturbance load amplitude during the dynamic damage of sandstone in high stress state is verified,and the quantitative relationship between the strain evolution of sandstone and the disturbance load amplitude in high stress state is clarified.Based on the dynamic fracture model and the sandstone dynamic damage morphology,the damage mechanism of sandstone during the disturbing load action is investigated,and it is proposed that the local strain information of sandstone can be used to characterize the disturbing damage,and the inflection point of exponential jump in tensile strain can be used as the dynamic destabilization precursor information of sandstone.(3)The effects of pre-static load level and disturbance load amplitude on the mechanical response characteristics of coal samples under coupled dynamic and static loads are investigated.The effects of pre-static load level on the mechanical properties,acoustic emission response,surface deformation evolution and damage morphology of coal samples are systematically analysed,and the special mechanical properties of coal samples that are different from those of more homogeneous rocks during the coupling of dynamic and static loads are explained from a detailed viewpoint.Based on the surface deformation evolution of intact coal samples and the dynamic crack expansion of prefabricated fractured coal samples,the dominance of the"mesh crack"formed by the progressive expansion and penetration of primary fractures on the dynamic response of coal samples is clarified,and the damage induced cracking mechanism of low and medium strain rate disturbance loads is revealed.(4)The effect of disturbance load amplitude on the dynamic response characteristics of high-stress coal samples is investigated,and the quantitative relationship between the number of cycles and amplitude of disturbance load before dynamic damage of high-stress coal samples is quantified with the aid of multiparameter characterisation.From the energy point of view,it is suggested that the energy in the"turbulent"state during the disturbing load weakens the orderliness of crack extension and induces secondary crack extension and penetration within the coal sample,which explains the phenomenon of large-scale crack extension and increased fragmentation at the moment of damage of the high-stress coal sample.(5)The influence of the enclosing pressure on the strengthening of the coal sample against disturbance was analysed,and the differential response characteristics of the coal samples at different levels of enclosing pressure were obtained.The minimum enclosing pressure value for the strengthening of the coal sample against disturbance was proposed,and the strengthening mechanism of the enclosing pressure on the coal sample against disturbance was explained from the perspective of stress wave theory.Based on the mechanical tests of cement mortar-filled body-confined coal samples,the feasibility of column-side filling to strengthen the disturbance resistance of coal columns is explored.