Study On Failure Behavior Of Coal-rock Combined Body Under Mining-induced Stress And Differential Energy Instability Model

In recent years,coal resources in Mid-western China have been continuously exploited.The coal-measure rock masses in some western mining areas have been found to be mainly weakly cemented soft rock,and the mining practices have revealed that a variety of coal mine disasters frequently occur,such as coal bumps,roof collapses,water and sand inrushes,and so on.In fact,these disasters have been determined to be closely related to the mechanical behaviors of the entire structure composed of soft rock and coal.However,the current research results on the deformation and failure of coal-rock combination samples have mainly been based on combination models composed of hard rock and coal,in which the uniaxial compressive strength of rock has been much higher than that of coal.Therefore,it is important to determine whether or not the mechanical properties obtained by the hard rock-coal combination models in the past are suitable for combination models composed of soft rock and relatively hard coal.In addition,with the increase of coal mining depth,the frequency and intensity of rockburst disasters have also been raised.In the deep mining circumstance,many rockburst disasters show the overall instability of the "surrounding rock-coal seam" composite structure.Consequently,it is necessary to study the burst liability of coal-rock composite structure in order to accurately reflect the actual burst tendency of coal seam.In this thesis,taking the coal and rock samples of Bayangaole Coal Mine as the research object,the conventional uniaxial and triaxial compression tests of coal samples,rock samples and coal-rock combination samples and the mining-induced stress test of coal-rock combination samples are carried out respectively,and the mechanical properties,failure modes and strain energy evolution laws of these samples under different stress conditions are analyzed.Based on the evolution characteristics of acoustic emission during loading,the progressive failure process of coal-rock combination samples is analyzed.Finally,based on the difference of energy accumulation characteristics between coal and rock masses in combination samples,the instability process of coal-rock combination model is analyzed,and the burst liability index for coal-rock combination model is proposed.The main conclusions are summarized as follows:(1)Based on the conventional uniaxial and triaxial compression tests of coal samples,rock samples and coal-rock combination samples,the effects of confining pressure on the stress-strain curve characteristics,peak axial strain,peak strength,failure mode,proportion of strain energy and peak elastic energy density of these three kinds of samples are compared and analyzed.Based on the analysis of the stress state of the elements at different positions in a coal-rock combination model under uniaxial compression,the influence of the bond constraint effects at the coal-rock interface on the strength characteristics of the combination sample is discussed.The results show that the uniaxial compressive strength of coal-rock combination samples is affected not only by the size effect of rock and coal,but also by the bond constraint effects at the coal-rock interface.(2)Based on the analysis of post-peak progressive failure process of coal-rock combination samples under different confining pressures,a post-peak nonlinear model considering crack axial strain is presented,which can better describe the strain softening process and residual strength of combination samples.Based on the shape of post-peak stress-strain curve and the pre-and post-peak energy accumulation and release characteristics,two indexes to evaluate the brittleness of coal and rock mass,namely stress softening coefficient and revised energy drop coefficient,are respectively established.The results show that with increasing confining pressures,the brittleness of the coal-rock combination samples decreases,and the range of confining pressure at which the brittleness-ductility transition occurs is 10 to 20 MPa.(3)The compression tests of coal-rock combination samples under different mining-induced stress conditions(different initial confining pressures and different mining layouts)are carried out,and it is found that the coal-rock combination samples show typical brittle failure characteristics.Based on the double circumferential strain measurement system,the evolution characteristics of the circumferential deformation and circumferential strain change rate of the coal and rock masses in the combination sample are compared.At the same time,the differences between the peak circumferential deformation of coal and rock masses under mining-induced stress and that of coal and rock samples under conventional loading are also compared,respectively.The effects of initial confining pressure and mining layout on the peak strength,peak axial strain,failure mode,proportion of strain energy and peak elastic energy density of combination samples are analyzed and compared with the experimental results under conventional triaxial loading.The evolution characteristics of strain energy change rate of coal-rock combination samples under different mining-induced stress condition are investigated.It is discovered that the change rate of elastic energy density will have a quiet period before the failure of the samples(except when the initial confining pressure is high),which can be regarded as a precursory characteristic of combination sample failure under low and medium initial confining pressure.Moreover,the evolution law of elastic energy between coal and rock in combination sample is compared,and the result shows that the coal mass is the main area of elastic energy accumulation and release in combination sample.(4)The crack axial strain method for determining the crack initiation and damage stress thresholds of coal and rock mass is proposed,and compared with the commonly used acoustic emission characteristic parameter method,the rationality of this method is verified.Based on the evolution characteristics of acoustic emission energy,the acoustic emission activities in the pre-peak stage are distinguished into three periods,which correspond to different crack evolution stages of coal-rock combination sample.The effects of initial confining pressure and mining layout on crack initiation and damage stress are analyzed,and it is found that the ratios of these two crack characteristic stresses to peak strength are maintained at a relatively stable level.The variations of tension and shear cracks in coal-rock combination sample under different mining-induced stresses are analysed based on the evolution characteristics of RA value and mean frequency,showing that tension cracks are dominant in the early loading phase,while shear cracks dominate at the moment of complete failure.Based on the evolution characteristic of Ib value,the fracture states of coal and rock masses in coal-rock combination sample under different mining-induced stress are analyzed.It is found that the Ib values of both coal and rock masses fluctuate violently up and down near the failure stage,approximately showing a "W" shape,indicating that the internal damage of coal and rock masses is intensified at this phase.Therefore,this phenomenon can be regarded as the precursory information of the impending complete failure of coal and rock masses.By comparing the peak frequency evolution characteristics of coal,rock and coal-rock combination samples under the same loading condition,the progressive failure mechanism of such soft rock-coal combination sample under uniaxial compression is analysed.(5)Based on the difference of energy accumulation characteristics of different types of rocks,the evolution laws of elastic energy density of coal and rock masses in coal-rock combination samples are analyzed.It is found that the peak elastic energy density difference between coal and rock masses is closely related to the severity of failure of combination sample,that is,the greater the peak elastic energy density difference,the more violent the failure of the sample tends to be.By retrieving a large number of test data,the factors affecting the peak elastic energy density difference of coal-rock combination samples are further discussed.After that,based on the viewpoint of nonequilibrium thermodynamics and dissipative structure,the deformation instability process of coal-rock combination model is analyzed from the perspective of elastic energy density difference.When it is in the critical state,the elastic energy density difference of the system is the largest,and the system is in the most unstable state.Accordingly,the ratio of peak elastic energy density difference of coal-rock system to instability duration is proposed as an index to evaluate the bursting liability of coal-rock combination model.The index essentially represents the release rate of stored elastic energy during the instability process of coal-rock system,and the rationality of the index is verified by experimental and simulated data.