Study On Mechanical Properties And Damage Mechanism Of Coal And Rocks With Hole Defects Under Eccentric Load

Due to the influence of topographic relief or engineering excavation,the coal and rock mass around tunnels and underground mining roadways are often subjected to eccentric loads in varying degrees.Especially for the coal and rock mass with certain defects after excavation,it is more prone to deformation and damage under eccentric load,which brings great difficulties to the support and maintenance of roadway(tunnel)engineering.In view of this,this paper takes raw coal and briquette samples as the research object,with the help of laboratory tests,theoretical analysis,numerical simulation and other research means,on the basis of analyzing the mechanical properties and damage laws of hard brittle raw coal and soft plastic briquette under uniformly distributed load,the crack propagation and strain field localization evolution process of coal and rock surface under uniaxial compression with different degrees of eccentric load were observed in real time.The relationship between the evolution law of crack propagation on coal rock surface and the evolution law of strain field localization zone under eccentric load was discussed,and the influence of the coupling action of load eccentricity and hole defects on the shape of strain field localization zone in coal rock was analyzed.The damage variable was defined based on the statistical index characterizing the localization zone of strain field,the damage constitutive model considering the influence of eccentric load was established,and the rationality of the model was verified.Finally,the damage and fracture mechanism of coal and rock under the coupling action of eccentric load and hole defects was revealed.(1)The starting stress of strain localization of raw coal under uniform load compression is about 94.4% of the peak stress,and the starting stress of strain localization of briquette sample is about 85.1% of the peak stress.The soft plastic briquette is easier to show the three-stage characteristics of strain localization than the hard brittle raw coal sample.(2)The eccentric load does not change the periodic characteristics of the stress-strain curves of raw coal and briquette coal samples.The peak stress and localized starting stress of raw coal and briquette coal gradually increase with the increase of the eccentric coefficient of load,which shows a linear function law.The greater the load eccentricity,the more likely the raw coal and briquette coal samples are to be damaged,and the existence of shear stress action zone at the interface between the direct and indirect action zone of load is the main reason for the macro shear failure of raw coal and briquette coal.(3)Under eccentric load,the multi peak phenomenon of AE parameters of coal samples at crack initiation and propagation stage is more obvious than that under conventional uniaxial load,and the multi peak phenomenon of AE parameters at crack unstable development and penetration stage is less obvious than that under uniaxial load.Under the action of eccentric load,the b value of acoustic emission of coal and rock fluctuates and decreases with the increase of stress curve.The overall fluctuation is greater than that of uniformly distributed load,and there is a process of rapid decline of multiple b values.(4)The localization band of the strain field can well reflect the location of the surface dominant crack.The localization band of the strain field and the shape of the surface crack are affected by the relative position of the loading zone and the hole defect.When the hole defect is in the non loading area,the evolution direction of the two is affected by the shear stress concentration zone caused by eccentric load and the hole defect,and the formed strain field localization zone connects the end of the loading plate with the hole defect.When the hole defect is in the non loading area,the hole defect only guides the evolution direction of the two,and its guiding degree depends on the strength of coal and rock.The greater the strength of coal and rock,the smaller the influence of hole defect on their morphology.(5)Through parameter calibration,a set of micromechanical parameters which can reflect the mechanical properties and damage law of raw coal in laboratory test are obtained.The simulation of meso damage process of coal and rock during uniaxial and triaxial compression under eccentric load is carried out.It is found that the peak stress,initiation stress and expansion stress of coal and rock decrease linearly with the increase of load eccentricity coefficient.The evolution of meso crack number has four stages: calm stage,stable crack propagation stage,unstable crack propagation and penetration stage,and post failure stage.(6)The distribution characteristics of principal stress difference of coal and rock mass under eccentric load and the distribution characteristics of principal stress difference around the hole defect are solved,and the distribution law of main control cracks in laboratory test is explained theoretically.Based on the statistical index proposed to quantitatively characterize the localization characteristics of strain field,the damage variable is defined,and a coal rock damage constitutive model which can reflect the influence of eccentric load is established.The model also considers the influence of hole defects on the deformation characteristics of coal and rock.Through the verification of test data,the model can well reflect the phased characteristics of stress-strain curves of raw coal and briquette samples.