Research on Laws of Surface Movement under Thick Loose Layer Based on Overlying Strata Movement during Coal Mining
Author:Shi Wei Ping
Supervisor:yu sheng wen
During the coal seam mining process,when the loose layer is thicker,the surface subsidence coefficient is close to 1.0,and even the subsidence coeficient is greater than 1.0.The large surface subsidence will cause greater damage to the ecological environment.In order to achieve the purpose of reducing mining subsidence and protecting the ecological environment,this paper plans to use the method of measured data analysis,similar material simulation test,theoretical analysis and numerical simulation to carry out research on the law of overlying strata movement under thick loose layer.This paper explores the subsidence laws of bedrock movement and the law of thick loose layer during coal seam mining,and analyzes the subsidence proportions of bedrock and thick loose layer in total surface subsidence quantitatively.Through analyzing the crack propagation law and stress evolution law in bedrock movement,this paper reveals the relationship between bedrock movement and fracture propagation,between bedrock movement and stress evolution.In this paper,the effect of size parameters,such as ratio of loose layer thickness to bedrock thickness and ratio of buried depth to thickness,on surface subsidence is analyzed.At the same time,the paper analyzes the role of mechanical parameters,such as tensile strength,cohesion and internal friction angle of bedrock,on surface subsidence.In addition,this paper studies the influence of different charge ratio on surface subsidence,and provides a basis for on-site surface subsidence control.Through research,the main conclusions obtained in this paper are:(1)Through similar simulation,it is gained that the proportion of bedrock sinking in surface subsidence is 84.6%.Furthermore,the differences of parameters,such as the height of fallen strata,in with and without thick loose layer models are analyzed.It is determined that the height of fallen strata in two models is less affected by thickness loose layer,while the crack expansion angle in two models is difference from each other.(2)Considering the settlement of bedrock,dewatering settlement model of soil layers is established.Considering the change of seepage and soil weight,settlement calculation method of soil layers is given respectively.Combined with the field data,it is determined that in the mining process,the proportion of the thick loose layer dewatering settlement of 1305 working face is about 15.4%of the total surface subsidence.(3)When the mining length is 220m,through numerical simulation,it is gained that the stress concentration occurs on coal pillars at both sides of goaf,and the stress concentration decreases linearly as the increase of distance from the roof of the coal seam.The stress above the goaf increases linearly with increasing distance from the roof of the coal seam.During the mining process,the crack propagation law,the sinking law of point at the top of model,as well as the stress evolution law of the measuring point above the coal seam all can be divided into four stages.(4)Through numerical simulation,it is obtained that,in size parameters,when r(the thickness of loose layer/the thickness of bedrock)<2,the sinking coefficient increases rapidly with the increasing of r,and the sinking coefficient tends to be stable when r≥2.The sinking coefficient decreases linearly with the increasing of buried depth/coal thickness.In the mechanical parameters,the sinking coefficient decreases with the increasing of tensile strength.When the bedrock thickness is constant,there are critical cohesion value and critical internal friction angle value.When the cohesion and the internal friction angle are increased within a range smaller than the critical value respectively,the sinking coefficient is gradually decreased.When the critical value is exceeded,although the cohesion value and the internal friction angle value are further increased,the subsidence coefficient is substantially unchanged.In addition,by numerical simulation,it is obtained that when the thickness of loose layer/thickness of bedrock is larger,the control effect of the filling on the surface subsidence is more obvious.In view of the prevalence of thick loose layer in Xinjiang and Inner Mongolia,the ecological environment in where is relatively fragile,the importance of ecological environment protection is more prominent.This study can provide guidance for mining in similar conditions in the above-mentioned areas,and has broad application prospects.