| The unclassified tailings downward drift cemented fill mining method can not only improve the ecological environment,ensure the safety of human life and property,but also control the stability of surrounding rock in underground mining and improve the utilization rate of mineral resources,so it has been widely used in the mining of metal mines in China.Due to the influence of filling capacity and slurry flow characteristics,the final solidified cemented tailings backfill has an inclined layered structure.The key to the success of the application of the downward drift cemented fill mining method is the stability of the backfill,which is directly related to its strength characteristics.Therefore,it is necessary to study the mechanical characteristics and stability of the inclined layered cemented backfill.In this paper,the effects of the number of layers,the angle of layers,the mass concentration of filling slurry and the ratio of cement to tailings on the bending mechanical properties of layered cemented backfill are studied through laboratory experiments;At the same time,the influence of the above factors on the macroscopic failure mode of the layered cemented backfill after the loading failure is analyzed;PFC3D was used to simulate the crack initiation mechanism and microscopic failure characteristics of layered cemented backfill under the action of three-point bending;Through FLAC 3D,the effects of the number of layers,the angle of layers,the thickness of roof and the ratio of span to height of goaf on the stability of layered cemented backfill are studied;Finally,the influence of the above factors on the safety factor of the layered cemented backfill is calculated by using the cusp catastrophe theory based on the strength reduction method,and the distribution of the plastic zone before and after the instability is analyzed.The results show that:(1)The load-deflection curve of layered backfill has obvious "double peak" characteristics.The flexural strength and flexural modulus of the backfill are positively correlated with the mass concentration and the ratio of cement to tailings,but negatively correlated with the number of layers and the angle of layers.(2)Under the action of three-point bending,each group of backfill first occurs tensile failure at the lower part to produce the main crack,and then gradually expands to the upper part,and finally the vertical connection causes the backfill to have macroscopic fracture.The existence of layered contact surface makes the main crack have obvious inflection point at the layered surface,and the inflection point direction is highly consistent with the layered angle.(3)The evolution curve of the total number of microcracks in the non-layered backfill is mainly divided into slow growth stage,fast growth stage and stable stage.The change trend of the total number of microcracks in the layered backfill has an obvious step jump phenomenon.(4)After excavation,each working condition has the characteristics of roof subsidence and floor heave,and the maximum displacement of the stope is located in the middle of the exposed backfill roof above the goaf.The more the number of layers,the greater the angle of layers,the smaller the thickness of the roof,and the greater the ratio of the span to height of the goaf,the greater the roof subsidence displacement and the worse the stability of the backfill.(5)The number of layers,the angle of layers,and the ratio of the span to height of the goaf are negatively correlated with the safety factor of the roof of the layered backfill,while the thickness of the roof of the backfill is positively correlated with the safety factor.After excavation,the floor and two sides of the stope mainly produce tensile plastic zone,while the roof is dominated by shear plastic zone.With the constant increase of the reduction coefficient,the plastic zone of the backfill roof gradually connects from bottom to top,resulting in instability. |
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