Overburden Failure Evolution And Risk Evaluation On Water And Sand Inrush Due To Paste Backfilling Mining

This paper focuses on the scientific problem of overburden failure evolution and the risk evaluation on water and sand inrush during the process of backfill mining under loose aquifers.According to the scale model test,the evolution of the overburden fracture induced by paste backfill mining was studied combining with the fractal geometry theory,and then the inhibition effect of the paste backfill on the overburden failure was analyzed,which involves a case study of backfill mining in the Taiping Coal Mine,Shandong province.A qualitative evaluation model of “SICI-3E” was constructed based on the key influencing factors on water and sand inrush due to the paste backfill mining under loose aquifers.A quantitative evaluation model for the risk assessment of water and sand inrush was established based on AHP-entropy.The main results are as follows.(1)The evolution characteristics of the fracture network during overburden failure due tothe paste backfill coal mining were investigated.Based on the scale model test,the evolution characteristics of fractal dimension of overburden fracture network under the conditions of paste backfill and caving mining were comparatively analyzed combining with the fractal theory.The results show that the fractal dimension of the overburden fractures increased rapidly,as well as the height of the water-conducting fractures during the caving mining process.While the fractal dimension of the overburden fractures developed steady with a small varation during the paste backfill mining process,and the development of the water-conducting fractured zone was inhibited.It indicated that the paste backfill has effectively supported the overburden load,controlled the subsidence and mitigated the stress concentration of the overlying strata,and the height of fractured zone decreased obviously and the development of the overburden fracture network was relatively stable.(2)A key-factor-chart based qualitative risk evaluation model of “SICI-3E” for the water and sand inrush due to paste backfill mining under loose aquifer was established based on the key factors which influence the safety of mining under loose aquifers.The water abundance of loose aquifer,the thickness of bottom clay,the geological structure and the undulation of bedrock surface,the thickness of overburden and the height of overburden failure were selected as the key factors for the risk evaluation of water and sand inrush in coal mines after a comprehensive analysis for key factors which influence the risk of water and sand inrush due to backfill mining under aquifers.The “SICI-3E” evaluation model for the risk of water and sand inrush was used to determine the critical conditions and gradation for different factors.The specific method is based on the four key factor charts,including the specific capacity of the aquifer in the Quaternary or Neogene,isopach of the bottom clay of the Quaternary or Neogene,the contour contour line of the and isopath of the overburden thickness.The comprehensive risk evaluation includes hydrogeological evaluation,engineering geological evaluation,and the safety and suitability evaluation.Consequently,the safety mining guarantee measures can be put forward based on the evaluation results for water and sand inrush risk.(3)A case study based on the Panel S03 in the Taiping Coal Mine was carried out using the “SICI-3E” evaluation model to assess the risk of water and sand inrush due to paste backfill mining.Results indicated that the Quaternary aquifer has a moderate water abundance with a critical hydraulic gradient varies between 0.903 and 1.145.The allowable water head of sand and clay layers in the aquifer ranges between 32 m and 38 m,which indicated that there would be no risk of water and sand inrush if the initial water level falls below this value.Calculation results indicated that the maximum heights of water-conducting fractured zone with a mining thickness of 2.2 m in the superimposed and enlarged zone are 23.88 m and 13.86 m,respectively.In order to avoid water and sand inrush,the waterproof coal pillars with a height of 27 m and 17 m,resectivelly,for the superimposed and enlarged zone should be reserved according to the evaluation results.The mining confirmed the panel was safe and there was no water and sand inrush.(4)The quantitative evaluation index system of water and sand inrush due to paste backfill mining under loose aquifer was constructed,and then a quantitative evaluation model based on AHP and entropy theory was established.Based on the analysis of the key influencing factors for the risk of water and sand inrush,the evaluation criteria layer of geological structure,aquifuge,water abundance of aquifer and coal seam mining and its sub-criteira layer were constructed.A case study based on the 6th district in the Taiping Coal Mine was carried out to quantify the evaluation index.The weight of the evaluation index was determined by the improved analytic hierarchy process(AHP)combined with the information entropy theory.The quantitative evaluation model for the risk of water and sand inrush due to paste backfill minin under loose aquifer was established according to the risk index RI,and the risk zones were divided.Mining activities in the study area confirmed the zoning and the evaluation model.This paper has 59 pictures,35 tables,and 189 references.