Study On Roof Instability Process Of Existing Goaf In Open-pit Mine Under Impact Load

The existence of existing goaf in open-pit mines restricts the safe and efficient mining of mineral resources.The instability of different forms of goaf under blasting load is similar to the failure of different forms of goaf roof under impact load.Therefore,the influence mechanism of thickness-span ratio,shape of goaf and impact height on the instability process of goaf roof under impact load is deeply analyzed,which provides a theoretical basis for seeking efficient goaf treatment technology.Firstly,drop hammer impact tests were carried out on the roof of goaf under different roof thickness-span ratios,different goaf shapes and different impact heights.The strain time history curve,ultimate strain change trend,crack evolution characteristics and failure mode of goaf roof were analyzed.The results show that the ultimate strain increases with the increase of thickness-span ratio and impact height.Compared with other shape gaps,the ultimate strain of the elliptical short-axis shape gap is the largest,reaching 4419μ-5793μ.Among the three factors,the thickness-span ratio and impact height have a great influence on the crack initiation position of the macroscopic crack in the goaf roof,while the goaf shape has little influence.The crack at the bottom plate of the goaf with tip is more likely to propagate downward along or near the tip.Additionally,the failure process the goaf roof is mainly dominated by tensile cracks or tensile-shear composite cracks,which is more dependent on the shape of the goaf.Secondly,PFC2D numerical simulation software is used to analyze the meso-dynamic failure mechanism of the goaf roof.The results show that the dynamic stress-strain curve of the goaf roof has gone through the linear elastic deformation stage,crack initiation and stable expansion stage,crack unstable expansion stage and post-peak stage.The dynamic strength is proportional to the thickness-span ratio and impact height of the roof.For elliptical short-axis and circular goaf,increasing the thickness-span ratio can greatly improve the bearing capacity of the roof,and the strengthening effect is weakened after reaching a certain value.Additionally,the crack initiation stress ratio k_ciand damage stress ratio k_cdof the roof with different shapes decrease with the increase of thickness-span ratio and impact height.In the numerical simulation,the goaf failure is mainly caused by tensile cracks,and the formation of meso-cracks is mainly caused by the fracture of the inter-particle force chain in the tensile stress concentration zone.When the thickness-span ratio and the impact height are constant,the cracks in the roof of the circular and elliptical short-axis goaf are more regular and expand vertically parallel to the load direction.Finally,according to the principle of dimensional analysis,the prediction model of instability in goaf roof is established.The energy evolution law of roof under three factors is analyzed,and the criterion of dissipation energy is constructed.The results show that the dynamic strength of roof in goaf is closely related to impact height,thickness-span ratio,cohesion and tensile strength.The elastic strain energy and dissipation energy of the roof under different influencing factors experience the change trend of’micro growth-linear growth-linear decrease’and’micro growth-slow growth-stable growth’,respectively.With the increase of thickness-span ratio,the peak dissipation energy ratio increases,while the initiation and damage dissipation energy ratio increases first and then decreases.At the same time,among the five shapes,the crack initiation and damage dissipation energy of the rectangular goaf roof is relatively small,but the peak dissipation energy ratio is large.Additionally,the safety factor of the roof is proportional to the thickness-span ratio and inversely proportional to the impact height.When the thickness-span ratio and the impact height are constant,the safety factor of the roof of the circular and elliptical goafs is greater,which is 3.25（circular）and 3.4（elliptical short-axis）respectively,and the probability of roof instability is smaller.