Experimental Study On Mechanical Properties And Meso-damage Characteristics Of In-situ Tectonic Coal

Due to the structural characteristics of tectonic fragmentation,briquette is often used instead of in-situ tectonic coal when studying its mechanical and seepage characteristics in the laboratory.In this paper,on the basis of measuring the basic physical properties of coal,by improving the preparation method of briquette,reconstructing in-situ tectonic coal,and carrying out triaxial compression tests under different stress paths,the mechanical properties and damage of tectonic coal and primary coal were compared and measured.The damage characteristics,the strength and deformation characteristics of the coal body,the characteristics of the constitutive relationship and the damage failure mechanism are analyzed,and the damage evolution process of the coal body is analyzed according to the acoustic emission monitoring data,and a statistical damage model based on acoustic emission is constructed.The strain curve was inverted and the following conclusions were obtained:(1)After the tectonic coal has experienced special geological tectonic movement,the microscopic physical structure is severely damaged,and the pore and fracture system is significantly developed.Compared with primary coal,tectonic coal has stronger adsorption/desorption capacity.The limit gas adsorption capacity of tectonic coal is 41.58g/cm~3,which is higher than that of primary coal,which is 35.75 g/cm~3.1.44 times that of coal.The firmness coefficient of tectonic coal is only 0.18,and the firmness coefficient of primary coal is 7.33 times.Tectonic coal seams are more likely to accumulate a large amount of gas energy,and they have lower mechanical strength and higher initial gas desorption and release capacity,and are more prone to outburst disasters.(2)According to the mechanics principle of powder forming,the traditional one-way pressing method of briquette is improved,and the two-way pressing method is adopted,and a large-scale press and a high-pressure briquette processing die are used to prepare the briquette with density and porosity similar to the underground primary tectonic coal.briquette standard sample.The increase of the pressing pressure increases the density of the tectonic coal standard sample from 1.1831 g/cm~3 to 1.3756 g/cm~3;the porosity decreases from 17.58%to11.45%,a decrease of 34.86%;the uniaxial compressive strength increases from 0.99 MPa to1.72 MPa;the permeability decreased by nearly 7 times under the condition of hydrostatic pressure of 24 MPa.The tectonic coal standard sample prepared under the pressure of 280MPa is close to the in-situ tectonic coal in terms of density and porosity,which can meet the needs of mechanical experiments.(3)In the conventional triaxial compression experiment,the tectonic coal exhibits ideal plastic characteristics when the confining pressure is high,and there is a curve in which the stress remains unchanged and the strain increases continuously;while the primary coal is always elastic and brittle under different stress paths leading.The strength and deformation resistance of coal are positively correlated with the confining pressure.The elastic modulus of the tectonic coal in the conventional triaxial test and the pre-peak unloading confining pressure test is increased by 2.14 times and 2.93 times,respectively,and the primary coal is increased by 1.08 times and 1.09 times,respectively.Confining pressure unloading has a significant effect on the deterioration of coal strength,the cohesion of tectonic coal is reduced by 44.3%,and the cohesion of primary coal is reduced by 47.0%.(4)Under the condition of triaxial compression,tectonic coal has obvious expansion and expansion characteristics,and the coal body undergoes multiple shear failures,accompanied by a large amount of pulverized coal.Coal pulverized coal,and the higher the confining pressure,the higher the degree of pulverization of the coal body;the primary coal is mainly shear failure,mainly with a single shear plane,and the broken section is accompanied by fragmented coal.The higher the confining pressure,the lower the degree of coal body damage,and the failure angle shows an evolution trend that decreases with the increase of confining pressure.(5)Confining pressure has obvious inhibitory effect on acoustic emission activity,which is mainly manifested in that the greater the confining pressure,the longer the quiet phase of acoustic emission activity.In the conventional triaxial compression test,the cumulative number of acoustic emission events and energy accumulation of tectonic coal and primary coal decreased with the increase of confining pressure,and the acoustic emission activity of tectonic coal was significantly weaker than that of primary coal.The theoretical curve obtained by the statistical damage model based on acoustic emission is in good agreement with the experimental curve,and the model parameters can well reflect the controlling effect of confining pressure on the evolution of coal damage.