Creep Acoustic Emission Characteristics Of Granite With Persistent Joints

The rock with joint development is one of the most active areas of ground pressure activity in the underground mining process of metal mines.Its creep property is one of the important factors that induce typical disasters such as roadway spalling,stope roof fall and goaf collapse.Reliable and accurate prediction and prediction of creep failure of jointed rock is a technical problem that metal mines need to solve urgently.In this paper,the creep test of granite with through joint is carried out by step loading.The long-term strength of the specimen is estimated by steady-state creep method.The activity characteristics of acoustic emission event rate and cumulative event number of granite with through joint during creep are analyzed.The relationship between acoustic emission activity characteristics and joint inclination angle is studied.The source space coordinates of the creep failure process of each specimen are obtained by the simplex positioning algorithm,and the source rupture type is identified by the moment tensor analysis method.On this basis,the evolution law of creep acoustic emission source and the space-time fractal dimension characteristics of acoustic emission source are analyzed,and the prediction method of creep failure of granite with penetrating joints is proposed.Through the debugging of mesoscopic parameters,a PFC model which can reflect the creep characteristics of granite with penetrating joints is established.The variation characteristics of elastic strain energy and dissipation energy during creep process are studied,and the relationship between energy evolution and joint dip angle in creep failure process is analyzed.The main conclusions are as follows:(1)The uniaxial compressive strength and elastic modulus of granite with penetrating joints decrease with the increase of joint dip angle,and the strain in the compaction stage increases with the increase of joint dip angle.(2)The long-term strength of granite with joint dip angles of 15°,30°,45°and 60°is52.3 MPa,46.4 MPa,42.3 MPa and 22.7 MPa,respectively.The long-term strength of the specimen decreases with the increase of the joint inclination angle and can be expressed by a function:y=-0.016x~2+0.6x+46.3.The overall trend of the instantaneous strain of each specimen decreases with the increase of creep stress,and the instantaneous strain of the specimen is the largest under the first stage creep stress,accounting for about 66%-79%of the total strain.When the creep stress is less than the long-term strength,the average acoustic emission event rate of each specimen in the constant speed creep stage increases with the increase of the joint inclination angle,decreases and increases.When the creep stress is greater than the long-term strength,the average acoustic emission event rate of each specimen in the constant creep stage decreases and then increases with the increase of the joint inclination angle.(3)The shear-type source accounts for about 65.5%～69.9%of the total source number.The tension-type source accounts for about 18.9%-22.8%of the total number of sources;the mixed source accounts for about 9.1%～12.2%of the total number of sources.When the creep stress is less than the long-term strength,the shear source increases with the increase of joint dip angle.The tensile source increases first and then decreases with the increase of joint dip angle.The mixed source increases with the increase of joint dip angle.When the creep stress is greater than the long-term strength,the shear source increases greatly with the increase of joint dip angle.The tensile source increases with the increase of joint dip angle;the mixed source increases with the increase of joint dip angle.(4)In the deceleration creep stage,the source z_i value and r_i value fractal dimension of the specimen decrease.In the constant creep stage,thez_i value fractal dimension of the source and the r_i value specimen is basically stable;in the accelerated creep stage,the z_i value and the r_i value fractal dimension remain basically unchanged.To a certain extent,the value can be increased at the same time as the value,as a precursory feature of creep failure of granite with through joint.(5)When the joint dip angle is less than or equal to 45°,the elastic strain energy increases with the increase of the joint dip angle,and the elastic strain energy decreases when the joint dip angle is greater than 45°.The variation of the dissipated energy with the joint inclination angle is as follows:when the inclination angle is less than or equal to 30°,it remains relatively stable,decreases with the increase of the inclination angle between 30°and 45°,and increases with the increase of the inclination angle when it is greater than 45°.For the creep failure process of jointed rock,the dissipation energy increases with the increase of creep duration,but the elastic strain energy decreases suddenly at the starting point of accelerated creep.