The Broken And Instability Law Of Hard Roof Of Fully-Mechanized Top Coal Caving Mining In Extra-Thick Coal Seam

In the past,we used "beam theory" to study the broken and instability law of hard roof..For the thin hard roof,the result is accurate,while for the thick hard roof,the result is not enough accurate.And the hard roof deformation will lead to fracture fields and stress fields change in overburden rock. But In the past we did not study the relationship between roof deformation and fracture fields and stress fields in overburden rock.Therefore, this paper focuses on different thickness hard roof of 8212 face in datong tashan mine,using related theory to get the breaking span of different thicknesses hard roof, explaining the stratum caving with different weighting,,using 3DEC numerical simulation to verify the theoretical calculation. By similar simulation experiment,we know the change law of abutment pressure peak of coal wall, and the deformation and deflection of fissure in overburden strata along with hard immediate roof. By numerical simulation we know the change law of stress fields and plastic zone in different hard roof breaking span,which can provide theoretical support for similar situation.The results are as follows:(1) Based on the key stratum theory, Vlasov thick plate theory and thin plate theory, this paper deduces the breaking span discriminant of thin and thick hard roof. According to the discriminant this paper gets the breaking span of immediate roof is 35.8m and the breaking span of main roof is 69.4m.When the immediate roof firstly breaks,it will lead to weak weighting,when the immediate roof breaks again,the main roof breaks too,which will lead to strong weighting,and the effect scope and intensity are larger. Theoretical calculations is consistent with field data.(2) This paper gets the breaking structure of hard roof by simulation experiment. The results are as follows: before the 20 m main roof breaks,the structure of mining stope keeps stable generally,while when the 20 m main roof breaks, the structure of mining stope begin to damage in large-scale. Mining-induced fracture form“O”ring.The Fissures in height is approximately 91 m, Showing a trapezoidal,the caving angle in front and back are between 60° and 70°.(3) This paper gets the relationship between the peak abutment pressure and the deflection of hard roof, the relationship between the mining fracture fractal dimension and the deflection of hard roof. The results are as follows:the peak changes of abutment pressure and the fractal dimension changes of fissure corresponding to the hard immediate roof deformation show staggered transformation, in which the former changes quickly in the beginning, then slowly, and steadily at last, while the latter changes slowly at first but quickly later and slowly at last; in addition, the fractal dimension of fissure shows a mutated increase when the hard immediate roof is caved.(4) This paper gets change law of mining stress field and plastic zone when hard roof breaks periodically by numerical simulation. The results are as follows:the shape of support pressure measuring lines gradually change from line to opening upward parabolas,and with the face advance distance increases, the shape of the opening of the parabola increases too.When the change becomes gentle,the mining full extraction.The extended upward angle of plastic zone is roughly the same, the plastic zone is mainly due to shear failure, and the yield area is increasing,the shape of the entire stope is similar to trapezoid.