| As people's demand for resources and the degree of dependence on public transportation facilities increase,a large number of geotechnical projects are or will be undergoing large-scale excavation and construction,such as deep mining of mineral resources and urban underground space construction.Fissured rock is the common construction object in geotechnical engineering.The stress environment is often complex and changeable,such as cyclic load and impulsive load,due to the combined action of in situ rock stress and mining disturbance.A large number of engineering practices have shown that the causes of geological disasters are often closely related to the deterioration of the mechanical properties of fissured rock masses.Therefore,it is urgent to carry out to research the fracture and instability mechanism of fissured rock masses under complex stress conditions for avoiding the occurrence of such disasters and realizing disaster early-warning and control.In order to address this issue,this paper assumes the stress state in geotechnical engineering as multi-level cyclic loading and conducts indoor experiments.Furthermore,numerical simulation experiments were performed to explore its mechanical properties and energy dissipation laws and mechanism of microcrack growth under multi-level cyclic loading on fissured sandstone.The results show that:(1)Whether in the uniaxial compression test or in the multi-level cyclic loading test,the peak strength and deformation modulus increases from the increase in the pre-crack angle.Compared with the constant lower limit multi-level cyclic load,the variable lower limit multi-level cyclic load can strengthen the deformation modulus of the fissured sandstone earlier,and the strengthening effect will be more obvious.(2)Based on the characteristic parameter RA-AF value of the acoustic emission signal,the failure mode of the fissured sandstone sample is analyzed.The failure mode of the sample gradually changed from mainly tensile cracks to mainly shear cracks with the increase in the pre-crack angle.However,the development of tensile cracks will be inhibited,and the performance is more obvious in the variable lower limit multi-level cyclic loading test.(3)The destruction process of rocks is accompanied by the transformation of energy,and energy dissipation can better reflect the destruction process of rocks.Based on this,the evolution law of the dissipated energy of the fissured sandstone in the multi-level cyclic loading test is analyzed.Furthermore,we defined the damage variable D based on the density of the dissipated energy.The results show that the damage variable D increases in the form of a cubic polynomial function with the increase of the number of cycles in the variable lower limit multi-level cyclic loading test and increases in an exponential form in the constant lower limit multi-level cyclic loading test.(4)The spatial distribution of micro-cracks,and the change of inter-particle contact force field,the fracture and instability process of fissured sandstone was studied based on the evolution of the number of micro-cracks.The results found that the initiation of the micro-crack is near the tip of the pre-crack,and the rupture mode of the sample is closely related to the pre-crack for the pre-crack small angle(??60°).On the contrary,for the large pre-crack angle(75°???90°),the prefabricated crack has little effect on the micro-crack initiation position and the macroscopic failure mode of the sample.(5)The mechanism of the macro cracks in fissured sandstone specimen is analyzed,and the method to distinguish tension cracks and shear cracks is proposed base on the velocity field.These results show that the increase of the prefabricated crack angle result in significant decrease in the macroscopic tensile cracks and increase in shear cracks.This is basically consistent with the conclusions obtained from the laboratory rock mechanics test,which explains the macroscopic test phenomenon from the perspective of microcrack propagation. |
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