Study On Instability Mechanism And Prevention Measures Of Extraction Structure In Block Caving Mine

As a large-scale,low-cost and high-efficiency mining method,block caving method is the first choice for large-scale and efficient mining of deep mineral resources if conditions permit,which has attracted more and more attention from the international mining industry.The extraction structure of the block caving method is responsible for the ore drawing task of the stope.All the mined ores must be transported out of the stope through the extraction structure.Therefore,it is very important to ensure its safety and stability of the extraction structure,which is one of the key factors for the successful application of the block caving method.Due to the particularity of block caving mining,the extraction structure is located in a complex and variable high stress environment,and the service life of the production level is long,so the maintenance of the extraction structure is difficult and the risk of instability is high.Therefore,it is of great theoretical significance and engineering value to reveal the occurrence and evolution mechanism of instability of extraction structure in block caving method and put forward the prevention and conrtol measures.Based on Tongkuangyu mine,a typical block caving mine in China,the occurrence and evolution mechanism of three common instability types of mine extraction structure in block caving method were comprehensively studied by means of field investigation,laboratory test,numerical simulation and mechanical theory analysis,and the prevention and control measures were proposed for different mechanisms of instability formation.The main contents and conclusions are as follows:1)Based on the investigation of the instability characteristics of the extraction structure in Tongkuangyu mine and the analysis of the spatial-temporal evolution process of the instability,the general law of the formation and evolution of the instability of the extraction structure in the 530 pannel of Tongkuangyu mine was revealed.The main conclusions are as follows:A large proportion of the instability of the extraction structure occurred in the area 20-30m ahead of the advancing undercut front.With the advance of undercutting,some of the instability areas of would show repeated instability after restoration.The ground pressure behavior in the vice production level is seriously affected by the mining in the main production level,especially the extraction structure in the vicinity of an advancing undercut front became prone to instability.In the process of undercutting,residual pillars were easy to form above the tip of major apexes.Ground pressure damage of local ore-loading roadways would be serious,and there were less ores or no ores available in the drawbells near the residual pillar.2)Based on the pressure arch theory,thin plate theory and granular stress arch theory,the mechanical analysis of the whole life cycle stress process of the extraction structure was carried out.By establishing the numerical simulation model of the whole life cycle stress of the extraction structure,the evolution law of the stress and displacement of the extraction structure in the whole life cycle from the beginning of roadways excavation to the end of ore drawing is studied,and the instability mechanism of the extraction structure in Tongkuangyu mine is revealed.The main conclusions are as follows:The exraction structure in the vicinity of an advancing undercut front was easy to produce compressive stress concentration due to the space stress arch of stope.With the increase of undercutting area,the concentration degree of compressive stress was gradually strong,When the shear failure condition of the extraction structure rock mass was reached,the phenomenon of ground pressure damage would occur.With the advance of undercuting,the extraction structure in the vicinity of an advancing undercut front gradually transferred to position under the undercut space.At this time,the compressive stress concentration of the extraction structure was released.However,due to the influence of high-level tectonic stress and vertical stress,the extraction structure was subject to upward bending deformation,and the tensile stress concentration gradually appearred at the side wall of ore-loading roadways and the tip of major apexes.With the increase of undercutting area,the concentration degree of tensile stress was gradually strong,while the tensile failure condition of extraction structure rock mass was reached,the phenomenon of ground pressure damage would occur again.Therefore,a phenomenon of repeated ground pressure damage would show in the extraction structure.It was helpful to release the concentrated compressive stress of the ore drawing roadways in the vicinity of an advancing undercut front and the tensile stress at the tip of major apexes and the two sides of ore drawing roadways under the undercut space,so as to reduce the probability of instability of the extraction structure.3)The numerical simulation model of the stress on the extraction structure of the main and vice sublevel combined mining is constructed,and the evolution characteristics and laws of the stress and displacement of the extraction structure of the vice sublevel under the disturbance of the main sublevel mining were studied,and the instability mechanism of the exraction structure of in the vice sublevel was revealed.The main conclusions are as follows:The pressure stress concentration of the extraction structure in the vicinity of an advancing undercut front in the vice sublevel was aggravated,which make it easier to reach the shear failure condition of rock mass,therefore,the instability probability of the extraction structure in the vicinity of an advancing undercut front in the vice sublevel was increased.The vertical upward bending deformation and tensile stress concentration of the extraction structure under the undercut space in the vice sublevel were reduced by the advace of undercutting in the main sublevel and the caving of the overlying ores,which make it not easy to exceed the tensile strength of rock mass,and the probability of repeated instability of the extraction structure in the vice sublevel was reduced.4)The numerical simulation model of the stress on the extraction structure with poor undercutting is constructed,and the evolution characteristics and laws of the stress and displacement of the extraction structure under the disturbance of the residual pillars were studied,and the instability mechanism of the extraction structure induced by the poor undercutting was revealed.The main conclusions are as follows:High pressure stress concentration occurred in major apexes under residual pillar,with the continuous increase of the undercutting area,the compressive stress at the tip of the major apexes would continue to increase.If the shear failure condition of the rock mass was exceeded,the major apexes would lose its stability.The ground pressure behavior under the residual pillar in production level was more likely to occur in the roof of ore-loading roadways,rather than the normal undercutting area,which showed the damage in the side wall of ore-loading roadways.With the advance of undercutting,the special stress state in the extraction level excavations was gradually formed,which was that the compressive stress concentrates on the upper part and the tensile stress concentrates on the lower part.The release area of tensile stress in overlying ore body near the residual pillar was formed,and it was not conducive to the collapse of overlying ore body around ore pillar left by poor undercutting,as a result,there were no broken ores in the ore-drawbells.5)The layout scheme of microseismic monitoring system in Tongkuangyu mine was studied,and the sensor position coordinates of the 410 pannel and 530 pannel were determined.Through the simulation and analysis of indoor positioning accuracy,the scheme met the requirements of positioning error and system sensitivity,and could realize the monitoring and early warning tasks of the instability of the extraction structure.6)According to the formation and evolution mechanism of three common instability types of extraction structure,the prevention and control measures were studied.The main conclusions are as follows:A new combined support form of bolt mesh cable shotcreting and floor concrete reverse arch was put forward,which could effectively control the loose deformation of the surrounding rock of the ore drawing roadway,and improve the overall strength of the extraction structure,and increase the stability of the extraction structure.The undercutting method combining post-undercutting in main sublevel and pre-undercutting in vice sublevel was put forward.It not only ensured that the main sublevel was put into operation as soon as possible,but also improved the stress state of the extraction structure in the vicesublevel.The under height above the tip of major apexes should be appropriately increased,at the same time,the undercuting area in the middle of the undercut roadways on both sides of the major apexes should be blasted as a unit,so as to reduce the residual pillars caused by blasting clamping effect.