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| Experimental study on deformation，permeability and AE characteristics of sandstone under multi-stage cyclic loading with a constant amplitude |
| CHEN Xu1，2，XIAO Yi2，TANG Minggao1，2，JIAO Tong2，ZHANG Fan3 |
| (1. State Key Laboratory of Geohazard Prevention and Geoenvironment Protection，Chengdu University of Technology，Chengdu，Sichuan 610059，China；2. College of Environment and Civil Engineering，Chengdu University of Technology，Chengdu，Sichuan 610059，China；3. Geohazard Prevention and Control Center of Henan Geological Bureau，Zhengzhou，Henan 450012，China) |
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Abstract In order to explore the influence of multi-stage cyclic loading with a constant amplitude on rock deformation，damage and permeability，a four-stage cyclic loading and unloading triaxial test was carried out on sandstone samples under different confining pressures. The permeability was measured and acoustic emission (AE) signals were monitored in real time during the test. The effects of confining pressure and cyclic loading on the characteristic stresses，permeability，b-value and RA-AF (risetime/amplitude - average frequency) value of AE signals during multi-stage cyclic loading are analyzed. The results show that：(1) Compared with the conventional triaxial compressive test，the volume strain of the sandstone crack under each stage of the multi-stage cyclic loading test is expanded，and the peak stress decreases. With the increase of confining pressure，the difference between the two peak stresses decreases，and the cyclic loading effect is weakened. (2) Under both the conventional triaxial loading and multi-stage cyclic loading conditions，the permeability decrease first and then increase. After cyclic loading，the permeability strain-based loss rate(PSL) of the rock sample is smaller，and the macroscopic deformation of the rock is aggravated. When the confining pressure increases，the PSL increases and the macroscopic deformation decreases. Under low confining pressure，the stress-based irrecoverable permeability coefficient(EIP) in each cycle is negative or small，and the permeability increases. Under high confining pressure，EIP value in the fourth cycle is positive，and the formation of seepage channel is affected by confining pressure constraints. (3) Compared with the conventional triaxial test，the b-value of AE signals under multi-stage cyclic loading fluctuates more. Under the two loading modes，the rock samples are dominated by tensile cracks. Under the condition of multi-stage cyclic loading，there are more shear cracks in the rock sample，and with the increase of confining pressure，the proportion of rock tensile cracks gradually decreases，and the proportion of shear cracks increases.
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