Study On Catastrophe Mechanism For Roof Strata In Shallow Seam Longwall Mining

In the northwestern region of China,a large number of shallow seams are present,which are typically characterized by shallow depth,thin bedrock and overlying thick loose sand layers.The practice of longwall mining shows that it is not easy to form a stable structure after the key stratum of roof is broken,and the movement of the key stratum directly affects the surface,and the strata pressure in the stope appears intense,which brings serious challenges to the safe mining of the mine.Therefore,it is of great significance to study the disast.er mechanism of key strata of shallow seam roof.In accordance with the occurrence behavior of roof strata and the breaking convergence characteristics of key roof strata in flat dipping shallow seam longwall mining,by applying the initial post-buckling theory and the cusp catastrophic model,the post-buckling behaviors and instabilities behaviors of gentle dipping key roof stratum during the first weighting are studied.which obtained the critical load of the gentle dipping key roof stratum,derived the formulae of main roof a break:ing span and breaking convergence,determined the final subsidence and themaximum rotation angle for broken key stratum,built the stability criterion of step convergence and lost stability due to rotation,determined the step subsidence of roof with or without support.Boundary value method based on research of surface subsidence prediction,the rule of surface subsidence caused by breakage of key roof strata in gently inclin,ed coal seam mining is discussed.some computation formulas for soil displacement and surface,and the range of the surface subsidence was determined.Finally,taking the C202 and 1203 working faces of Daliuta in Shendong Mining Area as engineering examples,the paper carries out engineering verification and analysis.The study found that the stability for bifurcation point equilibrium configuration and post-buckling equilibrium path of roof strata can be revealed by using initial post-buckling theory;the stability of equilibrium path at the bifurcation was affected by coal seam occurrence conditions,rock slope,the material of rock mass and structural section geometry;either step convergence or lost stability due to rotation for rock blocks must occer after the main roof breaking;the step convergence was caused by the reverse rotation of single rock block and was determinately related with the breaking convergence;the lost stability due to rotation is caused by two rock blocks in opposite rotation directons,an instantaneous system formed in the roof structure is the sufficient condition for it,deformation characteristics of surface subsidence and maximum surface subsidence,which are not only related to the thickness,lithology and dip angle of the strata and surface soil above the coal seam,but also to the movement curve of rock blocks after fracture,and the latter has greater influence.In accordance with the occurrence behaviors of roof structures,rock materials and thick unconsolidated layers above the roof,the integrated application of the initial post-buckling theory and the catastrophe theory can be adopted to determine catastrophic forms of the main roof structures during the first weighting.