Development Of Surface Cracks Under Mining Conditions Of Thick Loess With Thin Bedrock Research On Laws

As a main factor in the damage inflicted by mining subsidence on ground environments,ground cracks caused by underground coal mining constitute serious safety issues.There are obvious differences in the laws of surface movement and deformation under different geological and mining conditions,especially in the loess-covered areas in Shanxi Province.Under this condition,ground fissures are more likely to occur due to mining.Therefore,it is essential to study the relationship between the main influencing factors and the movement and deformation of overlying strata and the development of ground fissures.This paper takes the surface damage caused by coal mining in many mining areas in Shanxi Province as the research background,and uses field investigation,theoretical analysis and numerical simulation methods to conduct in-depth research on the movement law of overlying strata and surface discontinuous deformation under the conditions of bedrock and loess with different thicknesses.The movement structure and mechanical characteristics of stope roof,the movement law of overburden and surface discontinuous deformation under the condition of thick loess thin bedrock and the influence factors of overburden and surface discontinuous deformation are mainly discussed.The main work is as follows :(1)Taking a mining face in Shanxi Province(serious surface discontinuous deformation)as background,the structural model of the first break and periodic weighting of the basic roof of stope was established,and the structural stability of the two states of the first break and periodic weighting of the basic roof was analyzed by theoretical analysis method.The results show that the rock block of shallow coal seam is generally more than 1.0,and the structure is easy to slide and instability,and finally the surface step subsidence is formed.The study lays a foundation for better analyzing the mechanical mechanism of overlying strata movement in coal seam mining.(2)Considering the advantages and disadvantages of many domestic numerical simulation methods and the particularity of ground fissure development,the particle flow program(PFC2D)was used to select the typical coal seam stope as the research object,and the numerical calculation model under the geological mining conditions was established.The law of overlying strata and surface discontinuous deformation during coal seam mining was studied,and the formation mechanism was revealed.It is believed that :(1)With the continuous advancement of the working face,the overburden failure successively experienced the process of the initial breakage of the main roof,the initial collapse of the main roof,and the periodic weighting of the main roof.(2)In the process of coal seam mining,the form of force chain experienced the dynamic process of force chain arch formation-force chain arch development-force chain arch failure,indicating that the force chain arch is the main bearing capacity system of stope surrounding rock,and its breaking instability is the main cause of surface discontinuous deformation.(3)PFC can accurately track the dynamic development of cracks,which is of great significance to crack prediction.The results reasonably explained the causes of surface discontinuous deformation.(3)Based on the field investigation,the numerical calculation models of bedrock and loess with different thicknesses were established by using particle flow code(PFC2D),and the relationship between the main influencing factors and the movement and deformation of overburden rock and the law of crack development was studied.The results show that RLD(ratio of the loess layer thickness to the mining depth),RRM(ratio of the overlying rock thickness to the mining thickness),and RHD(relative height difference at the surface)have a crucial influence on the ground cracks formed in underground coal mining.The change in stope stress and the distribution of the internal force chain of the overburden reflect the movement and deformation of the overburden and surface mining failure.The study establishes the relationship between the width and depth of crack caused by mining,which provides a reference for the dynamic prediction of cracks.In summary,this paper analyzes the relationship between the main influencing factors and the movement and deformation of overlying strata and the law of crack development,and reasonably explains the causes of surface discontinuous deformation.This research goes towards preventing ground hazards caused by underground coal mining and land ecological restoration.