Experimental Study On Macro-meso Mechanical Properties Of Limestone Under Multi-level Cyclic Loading And Unloading

The rock mass is essentially subjected to the mechanical action of cyclic loading and unloading under the strong disturbance of large-scale mining in deep mines.This kind of mining cycle disturbance leads to fatigue damage of rock mass,which easily induces deep mine dynamic disasters such as large-scale instability and collapse of stope,seriously threatening the safe production of deep mine.Taking the typical limestone in Guangxi area as the research object,a series of multi-level cyclic loading and unloading tests with ramp signal characteristics were carried out by comprehensively using theoretical analysis,laboratory tests and numerical simulations.Combined with the test results of the acoustic emission（AE）of macroscopic failure and nuclear magnetic resonance（NMR）of microscopic crack propagation,the influence of different amplitudes and different number of inter-stage cycles on the mechanical properties of limestone was analyzed,and the damage mechanism of limestone under multi-level cyclic loading and unloading was explored from particle flow code（PFC）numerical simulations.The main research results are as follows:（1）The mechanical test of multi-level cyclic loading and unloading with ramp signal characteristics was carried out,the strength and deformation characteristics of limestone under the load were obtained,and the fractal characteristic function of limestone fragments after failure was established.When each stage is cycled for 10 and 20 times,the strength of limestone decreases with the increase of amplitude,which has typical strength deterioration characteristics.With the increase of cycle times,the cumulative residual strain of limestone increases step by step.The broken fragments of limestone under the action of multi-level cyclic loading have good fractal characteristics,and the fractal dimension increases with the increase of the amplitude and decreases with the increase of the number of inter-stage cycles.（2）The evolution characteristics and failure modes of internal meso-fractures in limestone under the action of multi-level cyclic loading and unloading were revealed.Acoustic emission（AE）method was used to monitor the whole process of crack evolution during multi-level cyclic loading and unloading,and the three-stage characteristics of crack evolution during limestone fracture were proposed.The increase of inter-stage cycles would induce limestone to enter the failure stage more quickly.As the increase of the number of inter-stage cycles,the failure mode of limestone changes from tensile failure to shear failure.（3）The distribution characteristics of T₂ spectrum of limestone meso-pore structure were analyzed,and the quantitative relationship between the proportion of pores of different scales and their variation was determined.The nuclear magnetic resonance detection tests were carried out on the limestone before and after multi-level cyclic loading and unloading by using the nuclear magnetic resonance（NMR）method.The effect of multi-level cyclic loading will increase the radius of primary pores of limestone and stimulate new pores,resulting in an increase in overall porosity.With the increase of the amplitude,the proportion of large pores gradually increases,and the proportion of small pores gradually decreases.The large pores dominate in limestone,which induces rock damage.（4）The PFC numerical model of limestone specimens with multi-level cyclic loading and unloading was established,the spatio-temporal evolution behavior and displacement field characteristics of micro-cracks of limestone were obtained,and the rock failure mechanism under the condition of multi-level cyclic loading and unloading was explored.The simulation results show that the evolution curve of the total number of micro-cracks in the rock specimen changes in stages.When the number of inter-stage cycles is constant,with the increase of the amplitude,the more the number of total micro-cracks in the rock specimen,the shorter the quiet period and the greater the rate of step growth period.The damage and fracture of the rock specimen develop gradually,and the initial damage development time is affected by the amplitude.When the rock specimen is damaged,the main fracture that penetrates is"Y"-shaped,and the main displacement field modes are direct tension（mode I）,relative tension（mode II）and mixed tension-shear（mode V）.（5）Taking the T612 stope of Tongkeng Mine as the engineering background,combined with the results of the laboratory test and the field application,the failure modes of the ore pillars were summarized,and the corresponding safety control measures of the ore pillars were put forward.The failure modes of the ore pillars include tensile cracking-peeling damage and shear damage.It is suggested that the grouting bolt method can be used to strengthen the ore pillars,the active pressure relief pre-cracking method to transfer the ore pillars stress,or the millisecond blasting method to reduce the mining disturbance.