Surrounding Rock Stability And Asymmetric Support Of Large Section Gob-Side Entry Driving In Fully Mechanized Caving

In China, thick coal seam distributes throughout most of the main mining areas of the country. Roadway section size, abutment pressure ranges, mining impact and pressure behavior will change in large-scale mechanized caving. As the construction and development of resource-saving mine, the coal pillar width tends to be small. Traditional roadway pressure theory and control technology is difficult to adapt to the new caving mining production and roof rock which is protection of the roadway safe and smooth. In gob-side entry driving under large fully mechanized caving, Roof vertical subsidence is asymmetry with respect to the roadway cross-section vertical center axis, and it is also asymmetric in the horizontal direction. A lot of old supporting structure appears damaged failure and even roof seriously falling, which hampered development of high yield and efficiency in large mechanized caving mining.In the dissertation, field research, theoretical analysis, numerical simulation and mechanical testing are used to study on surrounding rock stability and asymmetric support of large section gob-side entry driving in fully mechanized caving. Based on field research results, mine pressure behavior and character of roadway maintenance are analyzed. Problems of original support scheme are put forward and coal samples physical and mechanical experiments carried out. Overburden structural features and the stability of the arc triangular block are studied. Then overburden strata movement pattern, top coal movement law, roadway displacement field, asymmetric stress field factors and symmetric and asymmetric load mechanical effects are researched. Asymmetric regulation system of gob-side entry driving in top coal caving mining is put forwar, and mechanics of asymmetric truss roof control is revealed. The support parameters are obtained, and surrounding rock stability are evaluated. Finally, asymmetric support industrial test and pressure observation are completed. The main conclusions are whereby.(1) Roof control of haulage entry 120210 should focus on the pillar side top corner, taking into account the middle of the roof, and optimized design should be a eccentric one which is close to the middle roof at the pillar side. Function of steel mesh is not obvious, so it should be instead by the diamond metal mesh.(2) Roadside coal of haulage entry 120210 escepially area without supporting deformate largely. Both the strength and density of the bolt are insufficient. Extended length and the number of anchor bolts shoule be increased and the steel bolts should be used in the coal pillar side.(3) Anchor cables of haulage entry 120210 in 0-560m are symmetric in accordance with section roadway axis. According to the observations of the roadway, surrounding rock behavior characteristics are asymmetry. The original symmetry arrangement is not suitable to strata behaviors, and supporting effect is not well.(4) Roof strata horizontal movement more prominent under large section gob-side entry driving in fully mechanized caving. Original supporting concept is amied at roof strata vertical descending control mainly, in particular, the vertical arrangement of monomers anchor. Because there is no supporting structure can effectively provide a reliable level control, support systems can not provide effective support force in the horizontal direction. The whole support system can not meet the roof strata horizontal motion, which lead to complexity and difficulty of the surrounding rock control.(5) Coal seam No.2 belongs to surrounding rock genus V, the immediate roof belongs to the surrounding rock genus Ⅲ. Natural coal density is 1412.62kg/m3 Splitting tensile strength is 1.046MPa. Uniaxial compressive strength is 13.89MPa. Elastic modulus is 2062.39MPa. Poisson’s ratio is 0.362. Cohesion is 2.3131MPa, within friction angle of 44.34°. Natural rock sample density is 2659.10kg/m3. Splitting tensile strength 7.597MPa. Uniaxial compressive strength is 63.28MPa. Elastic modulus 11579.69MPa. Poisson’s ratio of 0.271. Cohesion is 8.89MPa, within friction angle of 47.39°(6) There are four basic forms of top basic roof fracture under large section gob-side entry driving in fully mechanized caving. Firstly, the basic fault line located above top coal pillar. Secondly, top basic fault line located just above the roadway. Thirdly, the basic fault line located entity top coal inside wall. Finally, the basic fault line located outside of the top pillar, which can not form an articulated structure. Haulage entry 120210 fault line is first one.(7) The curved triangular block swing downward motion is the reason of roof asymmetric deformation, horizontal shear deformation, and coal pillar failure. It rotate about the roof fracture, which lead to roof compressional deformation, pillar asymmetric deformation and inclined coal level extrusion deformation.(8) There are crackings in roadway top coal where is 1/8 from center line of roof and close to pillar. Due to breakage and leakage of the damage coal around the cracks, there are spaces for displacement and deformation of top-coal, and meanwhile destroy the balance of overlying structure, which lead to basic roof’s rotation that aggravates extrusion of top coal cracks. Roadway roof experiences leakage-extrusion-leakage until the formation of long-term stable balance.(9) Roof subcidence in the same heigt increase from integrated coal to the coal pillar, so do the roof horizital displacement, the left side coal will restrict the right coal move to the left, which lead to strong extrusion deformation. In the shallow coalside, horizital displacement decrease from the middle to roof and floor. Horizital displacement in the deep is smaller than the shallow.(10)Vertical stress concentrated is mainly above integrated coal and contiguous gob.In the shallow roof, vertical stress decreased from the center to two sides in the same elevation.In the deep roof, it increase from integrated coal to the coal pillar.In the roof, it increase as the elevation in the vertical direction,which rapid changes in the shallow roof, but changes slowly in the deep roof.(11)asymmetric stress field factors under large section gob-side entry driving in fully mechanized caving include coal pillar width,roadway cross section,surrounding rock properties and mining depth.When the pillar carry large and small,the main roof forms separately uniformly distributed and monocline vertical load,and the strata below the main roof forms separately diclinic and half groove vertical load.(12)At an equivalent average load,the mechanical effect of diclinic load is the strongest,which most likely to lead to material tensile shear failure,and the half groove vertical load is stonger than the other two.Uniformly distributed load is the weakest.Symmetrical load displacement is about two times larger than that of asymmetric load.(13)"Multi cables-channel-steel ring combination beam"is a truss structure that can reinforce support to weak roof on coal pillar side.When the load of combination beam is lower than static friction force,trus swork is stable and can restrain roof horizontal compression-expansion deformation.When the load is higher than static friction force,the length of the combined beam can be lengthened or moderately reduced,which can suit roof horizontal compression-loose expansion deformation.(14)When"multi Cables-channel-steel ring beam combination"is used,roof moment reduction amount is calculated as follows:0≤x≤s, (?) (?) (?) (?)(15) According to numerical simulation analysis, the optimum support of asymmetrical truss cable length is 8m,eccentricity of 400mm,span of 1.4m.Different anchor lengths lead to substantially the same amount of floor heave.It nearly doesn’t impact the floor heave,but impact roof and two sides.(16) Roof and two-side stability evaluation of haulage entry 120210 shows that the supporting design could satisfy roadway stability requirements. Safety factor when uses asymmetric anchor supporting design is much higher than the actual needful safety factor in haulage entry 120210. Compression failure will not occur when roof bolt support using only in the design, but will shear failure. So an additional anchor support is needful.(17) In the course of face advance, surrounding rock deformation of haulage entry 120210 is not large. Two coal sides are sensitive to the effects of mining, but both deformation speed and amout in security. Roof and floor deformation increase slowly affected by mining, largely unaffected by mining excavation. There is nearly no bed separation in haulage entry 120210, and roadway maintenance status is relatively stable.(18) Abutment pressure affects range of haulage entry 120210 is about 40m, and peak pressure is about 23MPa. The stress concentration factor is about 2.48. Under large section gob-side entry driving in fully mechanized caving, mining influence abutment pressure distribution range is large, and stress concentration factor also increase. Coal pillar stress is sensitive to mining influence.(19) Roof deformation characteristic is obvious asymmetry about the center line. This asymmetry is mainly embodied in different roof failure depth and deformation in the same depth. Roof deformation increases from integrated coal to coal pillar.