Study On The Mechanical Properties Of Coal Under Dynamic And Static Loading And The Influence Of Three-dimensional Stress Evolution On Rockburst

In the deep mining process of the mine,the loading state of coal and rock mass is characterized by’multi-dimensional dynamic and static load combination’due to the influence of in-situ stress and mining activities.Coal and rock mass are usually in three-dimensional,two-dimensional and one-dimensional dynamic and static load combination mechanical state.Studying the dynamic characteristics of coal under true triaxial static load conditions is closer to engineering practice,and has important guiding significance for revealing the mechanical characteristics of coal under complex stress states and the evolution process of disasters such as mine rockburst.At the same time,as a typical dynamic disaster in the process of coal mining,the development process of rockburst can also be regarded as the process of instability and failure of coal and rock mass under dynamic and static loads.As a key factor affecting rock burst,the change of stress affects the stability of coal rock structure and the accumulation of energy.Monitoring the stress change of coal rock mass is of great significance for the prevention and control of rockburst.Based on this,this thesis analyzes the mechanical properties and differences of coal under one-dimensional and three-dimensional dynamic and static loads by carrying out dynamic experiments of coal under true triaxial static load conditions,study the mechanism of three-dimensional stress changes on rockburst,and preliminarily explores the rock burst early warning method based on three-dimensional stress evolution.The three-dimensional stress evolution law in the mining process of the working face is measured and analyzed,and the early warning method is applied in the field.The main conclusions are as follows:（1）The dynamic strength and the degree of fragmentation of coal samples under one-dimensional dynamic and static load increases with the increase of impact velocity.The greater the impact velocity,the more serious the damage and deformation of coal samples.The dynamic strength of coal samples under three-dimensional dynamic and static loads increases first and then decreases with the increase of axial pressureσ1.The continuous increase of axial pressureσ₁ will increase the difference of three-dimensional static load prestress,thus aggravating the internal damage of coal samples and reducing the strength of coal samples.（2）The three-dimensional static load prestress has obvious constraint on coal samples,which can enhance the ability of coal samples to resist deformation and failure.The dynamic strength of coal samples under one-dimensional dynamic and static loads is smaller than that under three-dimensional dynamic and static loads.Under one-dimensional dynamic and static loads,coal samples are broken into small blocks and granular samples,and large deformation occurs in the post-peak stage of stress.Under the three-dimensional dynamic and static load,the damage degree of the coal sample is relatively light,and only the granular and powder samples fall,showing slight macroscopic fracture,and only small deformation occurs in the post-stress peak stage.（3）The three-dimensional stress of mining coal and rock changes with the excavation of working face.The decrease of horizontal stress in advance will lead to the increasing difference of three-dimensional stress,which will reduce the stability of coal and rock structure in mining space and accumulate the elastic energy inside coal and rock mass.In the process of early warning and prevention of rock burst,it is necessary to pay attention to the size of vertical stress and the sudden drop of horizontal stress,so as to avoid the excessive difference between vertical stress and horizontal stress,which will cause the structural instability of coal and rock mass or the sudden release of energy,and the dynamic load should be avoided or reduced as much as possible near the stress concentration area.（4）The three-dimensional stress of I030901 working face is monitored and analyzed.The results show that the mining advance influence range of the working face is about 120 m,and the high stress area is located in the range of 8.3～47.4 m in front of the working face.The influence range of vertical stress is the largest,followed by strike stress,and the influence range of dip stress is the smallest.The three-dimensional stress change trend of mining coal rock is basically the same.With the approaching of working face,it experiences four stages:stress stability,stress growth,high stress and stress drop.The horizontal stress is ahead of the vertical stress and the stress drop occurs.The horizontal stress is less than the vertical stress,but the horizontal stress concentration factor is greater than the vertical stress concentration factor.The research shows that the three-dimensional stress difference can reflect the stress state of coal and rock mass to a certain extent.Based on the three-way stress difference,the three-way stress difference change rate is used as the early warning parameter of rockburst,which has certain guiding significance for the prevention and control of rockburst.The thesis has 45 figures,5 tables and 85 references.