Mining Deformation In The Weak Cemented Cover Layer Of The Neogene System And Primitive/secondary Sources Of Sand Burst Debris Generation Characteristics

The water inrush and sand burst disasters caused by coal mining under weakly cemented overburden are common problems that threaten coal mining under weakly cemented strata in Henan,Shandong,Anhui,Shaanxi,Inner Mongolia,and Xinjiang.Previous studies have shown that the occurrence of sand collapse requires the presence of material sources,channels,and hydrodynamic conditions.Among them,multiple water and sand burst events in Henan,Shandong,and Shaanxi originate from the Neogene cover.The Neogene has a short formation time,weak bonding between particles,and a high content of silt and clay particles in the formation,making it a special rock and soil layer with poor hydraulic properties and low mechanical strength.Some mine surface and underground boreholes can reveal the distribution of natural loose particles,and local rock cores are quickly weathered into loose debris after being drilled.Under mining conditions,the unfavorable combination at the bottom of the Neogene,combined with mining and groundwater effects,may form a loose water sand mixture distribution area within the Neogene,forming the material source conditions for water inrush and sand collapse.Studying the primary and secondary evolution characteristics of the source of such sand breaks is of great significance for identifying the source of sand breaks.Quandian Coal Mine is located in Xuchang City,Henan Province.The main mining area of No.21 coal is under the thick Quaternary and Neogene weakly cemented layers.The 11080 working face is located in the southeast of mining area 11 on the east wing of the mine field,close to the coal seam outcrop zone,with an average coal thickness of 5.8 meters.The bedrock thickness in some areas near the outcrop zone is below 50 meters,and it is under the pressure of high water pressure and weak water rich strata at the bottom of the Neogene system.It is a typical thick coal seam mining face under thin bedrock and thick weak cementation cover layer.The distribution and formation characteristics of sand burst sources have a direct impact on mining safety.On the basis of analyzing the engineering geological properties and hydrogeological structure of the overlying strata on the working face,this article adopts methods such as indoor experiments,theoretical analysis,and numerical simulation to study the deformation characteristics of the weak cemented cover layer of the Neogene system induced by mining,as well as the generation characteristics of primary and secondary sand bursts.The main conclusions are as follows:(1)Based on the geological information and logging data obtained through ground drilling,and taking the current standards as the baseline,this study investigates the characteristics of the Neogene weakly cemented overburden layer in the study area,including its structure,material composition,particle distribution,and factors such as coal mining disturbance,based on on-site measurement results.The possibility of sand burst is determined from the perspectives of material sources,channels,and hydraulic gradients.(2)Experimental research was conducted on undisturbed samples obtained from ground drilling in the research area,and it was found that the weakly cemented strata of the Neogene system exhibit obvious disintegration characteristics.As the clay content increases,the disintegration performance of the samples gradually weakens.Gravel and calcareous nodules can weaken the anti disintegration performance of the cover layer.After the mechanical test,the disintegration speed of the broken sample is significantly improved compared to the complete sample.(3)Analyze the deformation characteristics and distribution characteristics of weakly cemented overlying layers and mining fractures under the influence of mining,taking into account factors such as gravel layers,high-strength rock layers,bedrock thickness,and aquifer characteristics.The results show that under the influence of mining,the deformation of the weakly cemented cover layer is mainly caused by creep,and the gravel layer can cause local abnormalities in mining deformation,with more dense distribution of cracks around the gravel.The hard rock becomes a rigid barrier,causing the overlying strata to be almost unaffected by mining,while the deformation degree and the number of cracks generated in the lower strata significantly increase.As the thickness of the bedrock increases,the impact of mining on the overlying layer gradually decreases.Under hard bedrock conditions,when the bedrock thickness reaches 12 times the mining thickness,the impact of Neogene mining is almost completely shielded by the bedrock.The presence of aquifers increases the degree of deformation of weakly cemented overlying layers.(4)A method for evaluating the possibility of sand collapse in the weakly cemented cover layer of the Neogene based on the conditions of material source formation has been proposed.The formation of sand burst sources within the weakly cemented cover of the Neogene requires the following conditions: the presence of natural loose particles near the bedrock surface or weakly cemented formations near the bedrock surface,aquifers near weakly cemented formations,and mining disturbances.Analyze whether water inrush and sand collapse will occur by distinguishing the existence or formation conditions of the material source.This thesis has 68 figures,11 tables,and 82 references.