Study On Short-term Creep Characteristics And Failure Precursor Identification Of Oil Share Under Uniaxial Compression

With the depletion of shallow resources,the depth of coal mining is increasing.The complex environment of deep high stress leads to large deformation and creep of surrounding rock in soft rock roadway.In particular,the rock mass with certain initial damage caused by excavation disturbance is more prone to time-dependent deformation and failure under long-term load,which brings great challenges to the stability control of roadway surrounding rock and poses a great threat to the safety of coal miners and production equipment.How to monitor and warn rock creep disaster has become an urgent problem to be solved in deep mining,and the identification of rock instability and fracture precursor information is the key scientific problem of rock creep monitoring and early warning.Based on the above situation,this paper takes Xiaokang Coal Mine in Liaoning Province as the background,combined with previous research,indoor uniaxial compression test and graded loading short-term creep test are carried out on oil shale.The mechanical properties and acoustic emission b value,acoustic emission characteristic parameters and acoustic emission parameter variance and autocorrelation coefficient variation based on critical slowing theory of uniaxial compression test and graded loading short-term creep test are studied.The criterion of precursor information of rock instability and fracture is established.The results show that :(1)In the acoustic emission test of oil shale under uniaxial compression,the acoustic emission characteristic parameters(energy calculation,ringing count,radiation value)can characterize the development of rock cracks.In the short-term creep test of graded loading,the first to fourth stress loading stages show an active period,and the dead load stage gradually tends to be stable,that is,the overall performance is ’ active period-stationary period’.At the fifth stress level,the acoustic emission signal is more intense in the loading stage.When entering the dead load stage,the acoustic emission signal gradually tends to be stable.When the rock sample enters the accelerated creep stage,the acoustic emission activity begins to become active,and the overall characteristic is ’ active period-stationary period-active period ’.The phenomenon that the acoustic emission signal is transformed from the ’ active period-stable period ’ feature to the ’ active period-stable period-active period ’ feature can be used as the precursor signal of rock creep rupture.(2)Through the short-term creep test of oil shale under graded loading,it is concluded that the strain rate of the sample shows a similar ’ L ’ rule under the first four stress levels,that is,when the stress is loaded,the loading rate increases and the dead load stage decreases rapidly.At the fifth stress level,the strain rate shows a ’ U ’ type law,that is,the strain rate decreases first and then increases uniformly,and finally increases rapidly in the accelerated creep stage.The strain rate law of the sample is similar to that of the cumulative characteristic parameters of acoustic emission.(3)In the short-term creep test of uniaxial compression and graded loading,the types of cracks produced in the process of compression failure of oil shale are tensile cracks and shear cracks.Due to the difference of internal primary cracks,different samples will appear different degrees of shear cracks at the beginning of the test,but still mainly tensile cracks.When the sample fails,the shear cracks increase.(4)The b value of acoustic emission is obtained by calculation,and the variation law of b value with time is analyzed in depth.The results show that the b value of acoustic emission has different degrees of sudden drop before the instability failure of the sample,which indicates the instability failure of rock earlier and can be used as an effective precursor information of rock instability failure.(5)Based on the critical slowing down theory,by calculating the variance and autocorrelation coefficient of acoustic emission parameters generated by short-term creep tests of rock under uniaxial compression and graded loading,the results show that there are obvious critical slowing down characteristics before rock fracture,which can effectively identify and extract the precursor information of rock instability and fracture.