Study On Deformation And Failure Mechanism Of Coal And Rock Under Unloading Conditions

With the rapid development of economy and society,the construction depth of engineering infrastructure and mineral exploration are gradually increasing.Under the conditions of deep mining,the phenomenon of high in-situ stress is increasingly prominent,which caused rock bust,water inrush stope,stope instability and a series of severe accidents occurred frequently.Based on the problems above,in order to study on the deformation and failure mechanism of coal and rock under unloading conditions,the complicated loading and unloading process of deep underground engineering excavation is chosen as the projiect background.The specific research contents are as follows:(1)Based on the laboratory uniaxial compression and triaxial loading and unloading tests,the deformation and failure characteristics of coal and rock are analyzed,and the variation law of mechanical parameters of coal and rock under loading and unloading is studied.The results show that in the process of uniaxial compression,with the increase of loading rate,the uniaxial strength and peak strain of red sandstone increase gradually,while the uniaxial strength and peak strain of coal decrease gradually.The failure forms of coal and rock are axial splitting failure and shear failure.During the triaxial loading process,the triaxial strength and peak strain of red sandstone and coal increase with the increase of confining pressure,and the failure mode is mainly shear failure.During the triaxial unloading test,the peak strength and strain of red sandstone and coal sample gradually increase with the increase of unloading confining pressure.The higher the unloading rate,the more easily the red sandstone and coal samples are destroyed and the smaller the rupture angle.It is found that the novel polyaxial strength criterion is more suitable for describing the failure mechanism and strength behavior of coal sample and red sandstone triaxial loading and unloading strength.(2)The evolution law of acoustic emission characteristics during the failure process of coal and rock samples under different loading and unloading conditions was studied,and the acoustic emission signals were analyzed by using fractal theory.The results show that the acoustic emission characteristics and stress-strain curves of red sandstone and coal sample have a very good correspondence.The maximum value of acoustic emission activity occurs in the rock sample damage.The higher the confining pressure,the higher the level of acoustic emission activity of the coal and rock sample,and the acoustic emission energy rate increases.With the increase of unloading rate,the activity of acoustic emission of coal and rock samples increases obviously,and the energy rate of failure increases obviously.The series of acoustic emission energy parameters of coal and rock samples has fractal characteristics during the test.The deformation and failure of coal and rock can be predicted by the trend of fractal dimension.(3)According to the thermodynamic law,the energy evolution law of coal and rock under loading and unloading is analyzed,the strain energy characteristics at the peak strength point are compared,and the failure criterion of coal and rock strength based on energy catastrophe is established.The results show that in the process of uniaxial compression,the energy storage limit of coal sample decreases gradually with the increase of loading rate,and the energy storage limit of red sandstone increases gradually.In the triaxial compression process,the increase in confining pressure will increase the energy storage limit of red sandstone and coal samples.During the triaxial unloading test,the increase of unloading confining pressure increases the total energy,elastic energy and dissipated energy of red sandstone and coal sample at the peak intensity point,and the energy conversion inside the sample is more severe.The acceleration of the unloading rate will gradually reduce the elastic energy of the red sandstone and coal samples at the peak intensity point,and the internal energy conversion of the sample is more severe.