Research On Crack Propagation Laws And Mechanical Properties Of Pre-cracked Rock

With the rapid development of China's economy,the demand for energy and mineral resources continues to grow.Due to the huge coal reserves in China,the dominant position of coal in China's energy structure will not change in the coming decades.However,as the mining depth and intensity continue to increase,engineering disasters are also increasing,such as rock-bursts,intensified mining pressure,and large deformation of surrounding rock in roadways,which pose a huge threat to the safe and efficient mining of resources.The emergence of the technology of automatic coal-pillar-free mining with cut-top pressure-relief and automatic tunneling effectively solves the problem of disasters in the mining process.At present,this technology has been widely used in the coal industry across the country.Pre-splitting roof rock layer is one of the key processes for forming a tunnel.When pre-splitting a single thick rock layer,if the rock layer cannot be completely pre-splitting,the crack propagation law of the incompletely pre-splitting roof and its mechanical characteristics determine whether it can be successfully collapsed.Falling and affecting the design scheme of roadway support,currently there is still no thorough discussion and research on this issue.Therefore,the study of the crack propagation law and mechanical characteristics of pre-cracked rock has important academic value and research significance for the popularization and application of the technology of automatic coal-pillar-free mining and the improvement of the theoretical system.The support design of the tunnel provides an important theoretical basis.In this paper,the methods of theoretical analysis,model experiments and numerical experiments are used to study the crack propagation law and mechanical properties of pre-cracked rocks.The main content of the study is divided into the following aspects:?1?Analysis of crack propagation mechanism and mechanical model of pre-cracked rock:Through in-depth analysis of the technological process and structural mechanics model of cut-top pressure relief automatic tunnel formation technology,simplify its boundary conditions,and establish a mechanical model of pre-cracked rock.?2?Experimental research on crack propagation and mechanical characteristics of pre-cracked rock:Building plaster was used to make prefabricated crack specimens with different heights,the water-paste ratio was 0.6:1,and the specimen size was450mm×150mm×150mm.The indoor direct shear experiments revealed the crack propagation rules and mechanical properties of precast cracked gypsum specimens with different heights under shear load.The results show that:?1?When the specimen cut rate is 0,the final crack morphology is jagged when the specimen is broken,and the main crack surface is rough;when the specimen cut rate is 13.3%to53.3%,the final specimen is broken when the specimen is broken.The shape of the crack is annular,and the main crack surface is relatively smooth.When the specimen cut rate is 73.3%,the final crack shape is arc-shaped when the specimen fails,and the main crack surface is approximately a straight line,which is the smoothest.?2?The slitting rate has a significant effect on the strength of the specimen.As the slitting rate increases,the strength of the specimen gradually decreases.?3?he in-depth analysis of the relationship between the slitting rate and the peak strength of the test piece yields the calculation formula for roadway support strength:?_q=?x+?_c??<0?,?is affected by the cut rate.?3?Numerical experimental study on the micro-crack evolution law and mechanical characteristics of pre-cracked rock:In order to study the micro-crack evolution law and mechanical characteristics of pre-cracked rock specimen,the particle flow numerical software PFC^3D was used to perform numerical shearing on the pre-cracked rock specimen.The experiment results show that:?1?When the shear rate of the specimen is 0 under the shear load,micro-cracks of the specimen first appear at one end of the applied shear load,and then at the other end of the specimen.Then,the micro-cracks at both ends expand toward the center of the specimen.Micro-cracks propagate and eventually form a macroscopic zigzag fracture surface;when the specimen cut rate is 13.3%to 73.3%,the micro-cracks of the specimen first appear at the crack tip and expand along the shear stress direction,and when they expand to a certain position When the crack growth rate slows down,micro-cracks appear in the upper part of the test specimen and accelerate the growth,which is opposite to the crack growth direction at the tip.The micro-cracks extend through and eventually form a macroscopic annular fracture surface.When the slit ratio is 93.3%,the initial micro-crack.The crack is generated at the tip of the crack.As the shear stress increases,the crack continues to propagate,forming a nearly straight fracture surface.?2?The slitting rate has a significant effect on the strength of the specimen.As the slitting rate increases,the strength of the specimen gradually decreases.?3?The slitting rate has a significant effect on the number of micro-cracks on the specimen.As the slitting rate increases,the number of micro-cracks on the specimen gradually decreases.Comparing and analyzing the numerical experimental results with the laboratory experimental results,it is found that the numerical experimental results and the laboratory experimental results are in high agreement.?4?According to the above conclusions,it can be concluded that in the process of cutting the roof to form the roadway,with the increase of the cut rate,the roof of the goaf is more likely to collapse,and the mining stress has a small effect on the stability of the roadway along the goaf.The amount of deformation is small,and the required support strength in the roadway is relatively low,which is conducive to maintaining the stability of the roadway.