Study On Roof Fracture Mechanism Of Long-wall Pitching Oblique Stope In Steeply Dipping Coal Seam With Medium Thick

The occurrence range of steeply dipping coal seam in China is wide and most of them are scarce coal.The mining of this kind of coal seam plays an important supporting role in China’s regional economic and social development.The pitching oblique longwall fully mechanized mining method is a commonly used mining method for steeply dipping coal seam.In the special-shaped stope space,the characteristics of roof deformation,failure,migration and caving filling are more complicated than those of general steeply dipping coal seam.It is of great theoretical significance to study the roof fracture mechanism of pitching oblique working face in steeply dipping coal seam with medium thick.Taking the 3222-working face of Lvshuidong coal mine in Sichuan Province as the research object,using the comprehensive research means of numerical simulation experiment,theoretical analysis,three-dimensional physical similarity simulation experiment and field measurement,the spatial shape evolution law of pitching oblique stope and roof spatial fracture mechanism in steeply dipping medium thick coal seam were analyzed and studied.The main conclusions are as follows:(1)When the pseudo-inclined working face is mined,the stress profile of surrounding rock shows an asymmetric arch shell along the inclined direction.The stress distribution pattern of roof is large small in the lower direction.Stress concentration occurs in the coal wall and boundary coal pillar area in front of working face,and the stress profile of working face roof decreases with the increase of roof layer under the same vertical stress.When the working face advance degree is constant,the deformation area of the main roof changes from approximate rectangle to approximate parallelogram with the increase of pseudo bevel angle.The main roof in front of the work tends to form an asymmetrical plastic destruction zone with a small large size.When the pseudo slope angle is constant,with the mining of coal seam,the influence area of the main roof stress and the deformation failure range increase.(2)In the mining of long-wall pitching oblique stope in steeply dipping coal seam,the low-level immediate roof falls with mining,and the relatively high-level immediate roof generally breaks and collapses about 6m behind the working face.The first fracture sequence of the main roof is "inclined middle→inclined upper→inclined lower",and the first weighting distance is about 30 meters.The periodic fracture sequence of the main roof is"inclined upper part→inclined middle part→inclined lower part",and the periodic weighting distance is about 9～15 meters.The overall fracture profile of the main roof is in the shape of an oblique "O",and fractures in the fracture area are in the shape of an oblique "X" along the pseudo-oblique direction.During periodic fracture,the overhang length of main roof is inclined lower part>inclined upper part>inclined middle part.(3)By establishing the mechanical models of the parallelogram plate and the strike beam,the deflection,bending moment and internal force formulas of the initial and periodic fracture of the main roof are deduced respectively,and the criterion of the main roof fracture is given.When the roof first breaks,the two long sides of the plate break first,followed by the middle area of the roof,the lower surface of the middle part and the upper surface of the two long sides are tensile damaged.In the middle of the roof plate,an "O" ring along the pseudo-oblique direction is generated,and an asymmetric "X" crack is formed locally.The principal stress and fracture morphology of the top plate are asymmetrically inclined "O-X" shape.When the parallelogram plate first break,the maximum deflection is inversely proportional to the pseudo bevel angle.During periodic fracture,the fracture position of the main roof rock beam is about 3 meters in front of the coal wall of the working face.(4)The field measurement shows that the space of the inclined upper and lower goaf of Lvshuidong 3222 working face is in the shape of arch shell,but a triangular free interface is formed in the upper goaf.There are a large number of broken and collapsed roof accumulated and filled in the middle of the slope.which is consistent with the experimental research conclusion.The field measurement verified the existence of large-scale gangue accumulation in the inclined central goaf,resulting in the asymmetric filling phenomenon that the accumulation degree of the collapse roof in the middle and upper part is greater than that in the inclined lower part.The influencing factors of the filling in the inclined middle of the pitching oblique stope are coal thickness of working face,roof lithology and pseudo inclined arrangement of working face.It has important theoretical significance for the roof stability control and prediction of incoming pressure for this kind of mine.