Study On Protection Scope Of Up-protective Layer Mining In Steep Coal Seam By Physical Similarity Simulation

The protective layer mining is the most effective regional measures to control coal and gas outburst in coal seam group at present,and according to the mechanism of protective layer mining,it is known that the key to protective layer mining in coal seam group is the delineation of protection scope.As to the problem of protective scope delimitation to steep coal seam group protective layer mining,the Yanshitai coal mine deep and steep coal seams' up-protective layer mining was settled as the engineering background,the protection scope of upper protective layer mining was studied by using physical similarity simulation,the pressure relief and expansion deformation curves of the protected layer were gained during up-protective layer mining,protection scope of protected coal seam was delimited,and the result was compared and verified with the results of numerical simulation and field test.The main results and conclusions are as follows:(1)Based on the physical similarity simulation of the upper protective layer mining,the height distribution characteristics of the "three zones" above the goaf of steep up-protective layer as well as the breaking and collapsing characteristics of the overburden on steep and thin coal seam goaf were obtained;it was shown that the height of the collapsed zone in the "three zones" was about 3.4cm,2.3 times of the thickness of the protective coal seam,the height of the fracture zone was about 10 cm,which was 6.7 times of the thickness of the protective coal seam;at the same time,combined with the monitor result based on the digital speckle method,the overburden would break into several relatively complete giant rock blocks when coal seam was thin and be connected into a number of triangular support structure,and the formation of roof rock would be held by these rock support structures on the some points.(2)In the physical similarity simulation of the upper protective layer mining,the displacement of the surrounding rock of the working face of the protective layer was monitored by the digital speckle method,and the displacement contour line of the surrounding rock in the upper protective layer mining was obtained;the result showed that when the displacement of the surrounding rock was stable,2 obvious displacement anomalies appeared in the bottom of the goaf,these displacement's contours showed a downward semi-ring,and the displacement was downward as well,which showed that overburden was supported by these two points obviously.(3)Based on the physical similarity simulation of the upper protective layer mining,the pressure relief curve of protected seam and its shape characteristics were gained in upper protective layer mining.It was found that the shape of pressure relief curve was "U" as a narrow bottom and wide opening,and the opening position appeared stress concentration,the stress concentration factor could reach 1.04,the pressure relief at bottom was waving around a certain value,and the maximum could reach 0.046 MPa.(4)Based on the physical similarity simulation of the upper protective layer mining and the expansion and deformation criterion of the coal-rock mass to the protection scope,the expansion and deformation curve of the protected layer during up-protective layer mining and the pressure relief angles of effective protection scope after up-protective layer mining were obtained,which included the upper boundary relief angle of 68 °,the lower boundary of 88 ° in inclination;and the physical similarity simulation result was compared with the field test result and the numerical simulation result,which shows that it is reliable to delimit the protection scope of deep and steep coal seam protective layer mining by physical similarity simulation method.