Study On The Stability Of Pillar In Goaf With Hard Surrounding Rock

There are a large number of mined out areas left after the end of mining.The stability of mined out areas determines the stability and availability of the surface site.Therefore,the study on the stability of mined out areas has been widely concerned by scholars at home and abroad.In hard rock mines,because the surrounding rock and ore body are relatively hard,the open field method is often used.The key to maintain the stability of the mined out area in the open stope method is the stability of the pillar.Because of the blasting disturbance in the mining process and the influence of the factors such as the long-term immersion and softening of the underground water in the later stage,the strength of the pillar is constantly reduced.These factors are mostly taken as reduction coefficient or directly ignored in the existing calculation formula of the pillar stability,resulting in large errors in the calculation results.Based on this,this paper takes the limestone pillar of Xinpu Phosphate Mine in Lianyungang as the research object,using the methods of field investigation and analysis,field drilling,laboratory test and demonstration,and numerical simulation analysis,deeply studies the pillar failure mode in Xinpu Phosphate Mine Area,and discusses in detail the influence of blasting vibration,immersion softening and aspect ratio on the stability of the hard rock pillar.The main research results are as follows:(1)Based on the existing design data and field survey,this paper analyzes the geological characteristics of the study area,such as topography,geological structure,stratigraphic structure,hydrogeology and engineering geology,and investigates and analyzes the surface collapse of the site in the goaf.(2)Through the analysis of the failure mode of roof and pillar in the process of open stope mining,the key role of pillar stability is determined.Based on the theoretical analysis of the factors affecting the stability of the pillar,combined with the actual situation of the study area and the existing calculation formula of the stability of the pillar,the factors such as the upper load,the strength of the pillar itself and the development of the structural plane of the pillar are excluded,and it is determined that the main factors affecting the stability of the pillar in the study area are the blasting vibration,the ratio of width to height and the softening effect of immersion.(3)The numerical simulation software is used to analyze the blasting vibration.The HJC(holmlquist Johnson cook)material model and fluid structure coupling calculation method are used to explore the change of pillar strength under different charges during the room method mining in the stage of deep hole violent transportation.According to the analysis of damage nephogram of rock mass,the strength of the pillar gradually decreases from the outside to the inside.According to this,the pillar is divided into complete damage area,partial damage area and no damage area.The width fitting curve of each area under different charge quantity is used to explore the impact of blast wave on the strength of the pillar.Combined with the actual charge quantity in the mining process of the study area,the results are obtained The damage range of pillars in Xinpu Phosphate Mine under the influence of blasting disturbance.(4)The numerical simulation software is used to explore the influence of different aspect ratio design on the pillar strength.The control variable method is used to study the peak strength of the pillar under different meso strength.The input meso strength in the simulation process is determined by indoor physical and mechanical experiments.Through the research on the peak strength of rock mass under different aspect ratio,the numerical analysis software is used to fit the simulation data curve,and the peak strength formula of the pillar under different aspect ratio is obtained.(5)Based on the field sampling and indoor physical and mechanical experiments,the influence of the long-term immersion of groundwater on the strength of pillars is explored,and the formula of the strength of pillars changing with the immersion time is obtained.Combined with the actual immersion time in the study area,the reduction coefficient of the strength of pillars under the long-term immersion softening effect of groundwater in the study area is obtained.(6)Based on the analysis and summary of the above research results,combined with the specific time of water immersion softening,blasting vibration charge and width height ratio design in Xinpu Phosphate Mine Area,the middle section of-200 m in the mine area is selected to analyze and calculate the stability of the pillar in the area,and the calculation results are divided into stable,basically stable and unstable zones,combined with the field survey The results show that the area of unstable pillar is consistent with the actual collapse area,which verifies the accuracy of the above conclusions.