Study On Evolution Law Of Surrounding Rock Force Chain In Shallow Coal Seam

Evolution law of surrounding rock force chain in shallow coal seam and mechanical relationship between support and surrounding rock is deeply studied by the comprehensive research methods of numerical simulation and theoretical analysis.The main research results are as follows:The structural characteristics and dynamic characteristics of overburden bedrock and unconsolidated layers in shallow coal seam are analyzed.The strata pressure behavior of shallow coal seam is summarized,and it is pointed out that the thickness of the rock layer and the granular layer,the height of mining and the working resistance of the supports are the main factors affecting the pressure behavior of shallow coal seam.Evolution law of surrounding rock force chain is studied by the PFC2D software based on the particle flow method.Before excavation,the internal force chain network of the surrounding rock are disorganized.After excavation,the force chain breaks and reorganizes.In the bedrock,the force chain is bunched up to form a strong force chain arch.With the advance of the working face,in a certain range,the height and span of the strong force chain arch are increasing.When the arch develops to the top of the bedrock,the height of the chain arch cannot be increased because there is little cohesive force between the particles in the granular layer.When the excavation space further increases,the strength of the chain arch will be broken because of the load exceeding the strength limit.Due to the lack of the bearing of the strong chain arch,the collapse of the roof strata,the granular layer and the base rock synchronous subsidence,resulting in severe strata behaviors.The results show that the failure of the strong chain arch is the basic reason of the strong dynamic phenomenon in the shallow coal seam.The kind of strong chain arch is the microscopic form of stress shellThe mechanical model of the support and surrounding rock in shallow coal seam is established based on the evolution law of the force chain,and the trace equation of the stress shell and the support resistance before and after stress shell instability are obtained.The results show that the resistance of the working face is greatly increased after the failure of the shell.Therefore,the imbalance of stress shell is the basic reason for the support break-off in the shallow coal seam.At the same time,the instability of the shell is analyzed,and the conditions for the occurrence of several unstable forms are given.