Study On Evolution Characteristics Of Mining Response And Slope Angle Effect From Open Pit To Underground Mining

After the transition from open pit to underground mining,the stress field,displacement field and deformation and failure field of the open pit slope,surrounding rock and overlying rock mass constitute a complex and changeable dynamic evolution system.Based on the engineering background of pit no.2 of Jinning phosphate mine,the mining effect factor K is introduced through field investigation,similar physical model test,numerical simulation and theoretical analysis,the mechanical analysis model of overlying rock and the fracture evolution equation of underground stope are established,and the criterion of mechanical instability under the influence of mining action is put forward,the main research results are as follows:(1)Based on the similarity theory,the similarity models of open-pit final slope angles of45°,55° and 65° are set up,and the deformation and fracture characteristics and slope angle effect of surrounding rock and overlying rock in goaf after the open-pit turning to underground mining are analyzed,the results show that the stress unloading area is formed at the top of the mined-out area,and the unloading extent is the largest at the center of the top,and the unloading area increases with the increase of the exposed area of the top,the unloading degree shows an increasing trend.The movement direction of the overlying strata of the open slope and the underground stope all points to the mined-out area,and the maximum horizontal and vertical displacement appears at the direct roof of the mined-out area.The maximum displacement and the maximum fracture height of the three models are Quantitative analysis,it shows that the greater the slope angle is,the more serious the damage degree of underground stope is.(2)The fracture evolution of the overlying strata in the stope is a layered dynamic process which develops upward in turn.The outer outline of the fracture zone of the overlying strata is similar to an elliptic paraboloid,so it is based on the elliptic paraboloid equation,the relationship between the open-pit slope angle A and the maximum development height of the cracks in the overlying strata above the stope is established.(3)MATDEM software is used to analyze the characteristics of rock mass stress field,displacement field and fracture evolution of overlying strata after the transition from open-pit to underground mining.The results show that the collapse of rock stratum is more obvious with the increase of slope angle,and the fracture pattern of overlying rock stratum has obvious layered structure in the process of mining,and the caving zone appears from bottom to top,fracture through zone and micro-fracture loose zone.(4)Based on the factors of open-pit slope angle a and disturbed stress in underground stope,the change of original rock stress calculated from the change of rock mass displacement is accurately expressed by introducing the factor K of slope mining effect Based on the Reissner theory,the failure modes of roof in different excavation stages are divided into four-sided fixed support stage and four-sided simply supported stage.(5)After the phosphorus ore body was transferred from open pit to underground mining,the direct roof of underground stope was destroyed in the form of an “Elliptical arch”,and when the internal stress value of the pressure arch exceeded the ultimate strength of the rock stratum,there are four failure modes: compression failure,shear failure,tensile failure or combined failure.