| In order to solve the problems of regional cable anchorage failure and roof separation and movement of the large-section open-off cut in extra-thick coal seam,the push and pull anchoring device which would effectively improve the installation efficiency and anchorage safety was developed,and the research on anchorage safety and support reliability has been conducted on the basis of anchor cable truss bi-directional support mechanism and symmetric support layout with interval distance.The following conclusions are drawn.(1)The site investigation was conducted to make sure the situation of anchorage failure and roof separation and movement of the large-section open-off cut in Tongxin Coal Mine,and the mechanical test and analysis of its roof coal and rock was done.It is concluded that the basic quality grade of 8209 working face coal seam of Tongxin Coal Mine is V,which means that the coal seam is broken,and the basic quality grade of the roof rock stratum of the coal seam is a type II rock mass,which means that the rock stratum is firm and stable.The broken coal seam is the mean cause of anchor agent installation difficulties and cable anchorage failure.(2)The push chassis and U-shaped clip were developed as the core components of pull and push equipment,and its action mechanism of "push" and "pull" double force in same direction was illuminated.It was made clear that the push and pull chassis had the function of preventing anchor agent being broken and guided the anchor agent with the help of U-shaped clip.Mechanical test on the push chassis was conducted and the push and pull chassis was selected with a reasonable thickness range from 0.4 mm to 0.5mm.(3)The theory of anchoring agent force in the borehole was analyzed and its mechanical calculation was carried out.It can be obtained that the pushing force of anchoring agent with push and pull equipment is approximatelyWhen there is no push and pull equipment,the anchor cable pushing forceAdopting the push and pull anchor installation process,the anchor cable pushing force is the anchor agent’s own weight,which is obviously smaller than the pushing force under the traditional unconstrained anchor installation condition.(4)A similar simulation test was designed to determine the ability of the push and pull equipment threading the collapsed and detached zone.Through quantitative analysis,it is concluded that the push and pull equipment can improve rigidity and reduce frictional resistance of anchor agent cartridge,and can smoothly pass through the 300mm simulated separation spacing,and have a certain ability to dredge hole collapse.(5)According to the characteristics of separation and movement(separate in vertical direction and move in horizontal direction)of large-section open-off cut in extra-thick coal seam,the anchor cable and truss providing vertical and horizontal preload was selected to make the neutral axis of the roof rock layer move downward so that more anchorage body was in compressive stress range,which would improve the carrying capacity and anti-deforming ability of the anchorage body.(6)The regulation of roof reverse deflection provided by anchor cable and truss was analyzed on the basis of increased prestress theory.The maximum deflection W is equal to roof deflection under overlying loadq(x)subract deflection provided by anchor cable and truss.The calculation formula of the reduction is’On one hand,the moment of roof stratum in anchorage area is reduced because of the effect of preload anchor cable and truss.On the other hand,anchoring point is in stable position,which allows the support and roof stratum in anchorage area to constitute locking structure.Thus the moment caused by gravity is further offset and the deflection of the anchorage beam is reduced.(7)The mechanical model of the groove shape anchored cable truss was established,and its self-lock support theory was expounded,and the tensile stress formulas of the anchor cable and truss were obtained by calculation.Initial preload force formula of truss:(8)The traditional symmetrical support form has insufficient support space at the junction of the two excavation sections so that it can’t form an overall successive support system.According to these questions,symmetric anchor cable and truss support with interval distance was designed.This arrangement layout can significantly reduce the independence of the two digging sections.,it is advantageous to form a continuous load-bearing structure of the roof at the boundary line of the digging sections,and form an overall support mode for the large-section open-off cut in the extra-thick coal seam.(9)FLAC3D software was used to simulate the surrounding rock deformation,plastic zone expansion and stress distribution of large section open-off cut in extra-thick coal seam under different support parameters such as anchor cable truss span,cable length,cable angle,distance between bolt orifice and open-off cut side.The key parameters was determined,the truss anchor cable span was 2.1m,the anchor cable length was 9m,the anchor cable angle was 10°.(10)The common cause failure calculation model of anchorage with Push and Pull equipment and Bi-directional support of lager section open-off cut in extra-thick coal seam was established and the failure probability formula of anchor support was obtained according to the discretization process.Because of the discretization process,the common cause failure owns connotation of statistics.It can be calculated by "pull-out test" on site.(11)By the "pull-out test"(pull force limited by 200KN)investigation of 50m test section and 50m non test section in Tongxin mine,the result of common cause failure calculation isThe reliability of the anchor support in the test section(98%)was 19 percentage points higher than that of the non-test section(79%),which indicated that the support reliability of large section open off cut in extram-thick coal seam was effectively improve by anchorage with push and pull equipment and bi-directional support. |
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