The Distribution And Evolution Of Failure And Stress During Mining Under Thin Bedrock

Failure of roof strata during mining can form caving zone,water flowing fractured zone and bending subsidence zone.When the bedrock thickness is large,the bedrock has good bearing capacity,but when the coal seam under the thin bedrock is mined,the roof fracture fracture caused by stress redistribution may connect the aquifer or sand layer in the loose layer,bringing the risk of water inrush and sand inrush.The coal-bearing strata in Yuzhou coalfield,Henan Province are covered by thick Neogene,and the upper strata are Quaternary strata.Due to the semi-diagenetic characteristics of Neogene system,the study of mining stress change and mining failure under thick loose layer and thin bedrock has important guiding significance for the mining decision of Neogene and thin bedrock.In this paper,a thin bedrock working face in Quandian Coal Mine is taken as the research background.The failure characteristics,stress distribution and evolution of thin bedrock are analyzed in depth by using the methods of field measurement,theoretical analysis and numerical simulation.The influence of different factors on the stress change of thin bedrock is studied.The formation mechanism of stress arch shell is analyzed and the mechanical model of thin bedrock stress arch shell is established to study its mechanical properties and stability.As follows :Firstly,based on the geological background of thin bedrock in the study area,the mining conditions and major influencing factors of thin bedrock were analyzed,and the variation characteristics of failure stress in thin bedrock mining are studied by numerical simulation combined with field measurement.The stress types of thin bedrock are divided into bidirectional compressive stress zone,bidirectional tensile stress zone,unidirectional tensile stress zone and stress transition zone.The bidirectional compressive stress zone will form a stress arch shell with constant height and increased span and range in the process of propulsion.At the same time,the arch shell structure will also be formed in the loose layer,but its bearing capacity is very weak.The bidirectional tensile stress zone inside the thin bedrock stress arch shell causes roof failure,and the unidirectional tensile stress on both sides of stress arch shell in thin bedrock forms an inverted triangle failure zone,which is easily connected to the roof failure zone.After obtaining the stress evolution law of thin bedrock,the influence of coal seam mining thickness,bedrock thickness,advancing speed and coal seam dip angle on the stress change of thin bedrock was studied.The stress concentration of stress arch shell structure in bedrock increases with the increase of mining thickness,which is not conducive to safe mining.The larger the thickness of bedrock is,the larger the volume of stress arch shell is,and the overall stress concentration coefficient is reduced,which is conducive to the stability of surrounding rock.The faster the advancing speed of the working face,the greater the peak stress of the coal wall,the more obvious the stress concentration,which is not conducive to safe mining;The larger the dip angle of coal seam,the more difficult it is to form a closed stress arch shell,the water diversion channel is easy to form on the working surface,and the bedrock may collapse as a whole,which will increase the risk of safe mining.Finally,the formation mechanism of stress arch structure was analyzed,and the mechanical model of stress arch shell was established to study the shape and internal force equation of stress arch shell,and to analyze the stability of stress arch shell.It is concluded that the arch stress structure with extremely weak bearing capacity will be formed in the loose layer,and the stress arch shell with equal height to the thickness of bedrock will be formed in the bedrock and semi-diagenetic neogene,which can play a strong bearing role,and its internal force is affected by the thickness of bedrock,arch span,lateral pressure coefficient and loose layer load.With the increase of the thickness of bedrock,the bending moment,shear force and axial force of thin bedrock stress arch shell decrease first and then increase.It is concluded that the optimal working face length of thin bedrock coal seam is twice the thickness of bedrock,and the stability of surrounding rock is the best in this case.The vault and waist of thin bedrock stress arch shell are more likely to be damaged at the initial stage of mining and after a certain distance of mining respectively.There are 45 figures,5 tables and 85 references in this paper.