Mechanism Of Disaster Induced By Dynamic Strata Pressure While Mining In Beneath An Interval Gob Of Shallow Close Distance Seams

In order to address the dynamic load problems caused by extracting coal under an adjacent interval gob in Yulin-Shenmu-Fugu mining area,a comprehensive investigation is conducted on the the formation,evolution and instability mechanism of the spatial structure of the overlying strata of the shallow close distance seams interval gob and the instability characteristics of the middle strata structure during the mining of the lower coal seam through field survy,similar simulation,theoretical analysis,numerical simulation and field test.The outcome of this study provides a theoretical guidance for roof dynamic pressure prevention and control of repeated mining under shallow close distance insufficient mining conditions in Yulin-Shenmu-Fugu mining area.The main contents and results of this paper are presented as follows:(1)Based on the mechanical properties of coal and rocks derived from labrotory experiments,the characteristics of stress distribution of interval coal pillar and temporary coal pillar in interval gobs are analyzed,and the evolution law of plastic zone of interval coal pillar and temporary coal pillar under the load of overlying strata is studied.The research shows that the temporary coal pillar cannot maintain long-term stability,and the isolated coal pillar can maintain long-term stability.(2)In the early stage of the formation of interval gob,the overlying strata of the gob roof forms a stress release zone.With the redistribution of overburden stress,the fractures in the roof strata gradually expand upward to form a semi-circular arch fracture damage zone.In the interval gob,with the sudden instability of the temporary coal pillar,a larger range of stress release zone and a larger arch fracture damage zone are formed above the gob,and the"W-shaped voussoir beam" structure is formed after the failure and instability of the main roof of the gob,and the "trapezoidal-semicircular arch" fracture zone is formed after the collapse and backwardness of the overlying strata and soil layers.(3)Based on the creep theory of coal and rock,the collaborative bearing structure model of "W-shaped voussoir beam+double arch bridge" in the process of roof structure instability of interval gob is established.The conditions for the formation of "W-shaped voussoir beam"structure in interval gob and the formation of "double arch bridge" structure in interval coal column side are determined.This paper analyzed the stress distribution characteristics of"double arch bridge"structure,revealed the stress transfer law of pier column in the bottom plate,and divided the dynamic pressure disaster prevention and control area under an adjacent interval gob.(4)The concept of effective bearing layer between adjacent coal seams is put forward,and the loading characteristics of effective bearing layer structure in different mining areas are analyzed.The dynamic load mechanical model of "double arch bridge structure-combined key layer-immediate roof-support" is established.It is revealed that the linkage between the structural instability of interlayer effective bearing layer and the structural instability of"double arch bridge" in upper coal seam is the source of dynamic pressure during the extraction of lower coal seam.(5)When mining under the structure of "W-shaped voussoir beam",the ground pressure of working face appears obviously,showing the phenomenon of large and small periodic weighting.When mining under "double arch bridge" structure,the ground pressure of working face appears violently,the dynamic load coefficient reaches 1.83 when dynamic pressure occurs,and the transient stepped roof cutting subsidence of roof strata reaches 0.69 m.According to the characteristics of strata pressure in different areas of working face and the relationship between the support strength of working face and the roof subsidence,the reasonable value of support resistance is determined.(6)In view of the problem that dynamic mine pressure is easy to occur in the process of advancing through the "double arch bridge" structure of the roof of the interval gob in the lower coal seam working face,a comprehensive prevention and control technology is proposed.It releases the stress of the roof overlying the interval coal pillar in the gob of the upper coal seam,strengthens the support strength of the working face and increases the advancing speed.The field test is carried out in the 30103 working face,which successfully prevents the occurrence of dynamic pressure and ensures the safe production of the working face.