| Traditional longwall top coal caving(LTCC)operation,the cutting height of which is 3.5~5.0 m,caused some problems,such as long hanging length and poor cavability of top coal during the extraction of extra-thick and hard coal seam in Yushen mining area.Based on the geological conditions of the eastern region of No.1 panel in the Jinjitan Coal Mine,the LTCC method with super-large cutting height(≥6.0 m)was studied,which combines the longwall with super-large cutting height(SLCH)and longwall top coal caving(LTCC).The relationship of structural coupling between hydraulic roof support and surrounding rock could be optimized by means of increasing the mining height,etc.The caving lumpiness of top coal was reduced,the caving structure of top coal was improved,and the cavability of top coal was improved.Structural coupling relationship between hydraulic roof support and surrounding rock was studied using methods of theoretical analysis,numerical simulation,physical test and on-site industrial trail.The field industrial production trial shows that the shallow-covered,extra-thick and hard coal seam was extracted with safety,efficiency and high recovery rate.The following results were achieved with a series of study:(1)Principle of structural coupling between hydraulic roof support and sur rounding rockOn the basis of “Three Coupling” support principle of hydraulic roof support-surrounding rock,the concept and principle of “structural coupling” were introduced,forming the “4S Coupling” theory,which includes structure,strength,stiffness and stability coupling.Structure is the carrier and manifestation of strength,stiffness and stability.To understand the structural characteristics of hydraulic roof support and surrounding rock as well as their mechanical and kinematic characteristics is a necessary condition for analyzing the multi-field coupling states,such as stress field,displacement field and energy field,of the coupling support systems of the hydraulic roof support and surrounding rock.The coupling support systems are classified as primary coupling of near field “small structure” and secondary coupling of far field of “large structure”.The primary coupling of “small structure” is the internal loading of hydraulic roof supports(providing setting load),aiming at the reinforcement of surrounding rock by active support.The secondary coupling of “large structure” is the collapse load of overburden structure,and the supporting system is in the passive bearing state.This state should not only have enough rigidity to bear load,but also have sensitive flexibility to yield,so that the “small structure” has an improved adaptability to “large structure”.The coupling relationship between the structure of hydraulic support(especially the structure of caving mechanism)and caving structure of top coal should be considered in a LTCC face.The rational relationship of structure coupling is beneficial to the caving and drawing of top coal,as well as the improvement of recovery rate of top coal.(2)The relationship of structural coupling between the hydraulic roof support and overlying strataFor the central part of the LTCC face with super-large cutting height in Jinjitan Coal Mine,the structure of key strata of lower part in the near-field is an alternation between the structure of composite cantilever and the structure of a unstable voussoir beam,and the structure of key strata of upper part in the far-field is the structure of a stable voussoir beam,and the roof periodic pressure is characteristic of large and small periodicity with periodic weighting span being 10~20 m.In the two ends of the longwall face,the roof structure is a lateral hanging structure with curved triangular shape,the maximum hanging length of which is more than 20 m,causing strata behaviors appear not intense.The advantages of bearing capacity for 2-legged shield-type hydraulic roof support were explained by using the coupled load-bearing capacity curve of roof support and surrounding rock.The on-site industrial trail shows: ZFY21000/35.5/70 D type,2-legged and shield-type,hydraulic roof support for LTCC face can not only guarantee the reliability and adaptability of support system at the working face,but also crush the hard top coal effectively;The actual cutting height is 6.3~6.5 m,with daily advance speed of 10.0 m/d.Slabbed spalling happens at part of the coal face during the roof weighting,and the maximum depth of spalling is less than 0.8 m.(3)The relationship of structural coupling between the hydraulic roof support and the structure of top coal cavingThe increasing of cutting height can not only facilitate the coordination of coal cutting and coal drawing through “more cutting and less drawing”,but also improve the caving structure and cavability of the top coal by increasing the intensity of roof weighting,reducing the hanging length of hard top coal,and enlarging the size the drawing opening.The transient and dynamic loose arch structure,small and unsteady arch structure as well as large and stable arch structure may be formed during the drawing process of caved top coal with various fragmentation.The arch formed on the rear canopy can be broken by the more efficient strategy of “small arch-small swing,large arch-large swing” of the rear canopy.The fragmentation and cavability of top coal are characteristic of double periodicity.The strike periodicity is corresponded with periodic caving span,and the vertical periodicity is related to the position of top coal.The on-site industrial trail shows the recovery rate is 87%,and the cavability in the middle of the longwall face is satisfactory while the cavability is unsatisfactory within 20 m of the two ends of longwall face.(4)The relationship of structural coupling between the hydraulic roof supp ort and the coal faceThe site investigation shows that the main failure mode of the coal face is slabbed buckling spalling,and there are many characteristics of the slabbed spalling,such as slab-like spalling,onionskin-like spalling,ejection-type spalling and spalling caused by disturbance of face sprag of hydraulic roof support.The slab structure is formed by the propagation and coalescence of discontinuous vertical cleats under the influence of mining stress,and various types of failure of the slab structure occurs under the corresponding stress environment.The slabbed spalling of coal face is related to the strength of coal mass,the structure characteristics of coal mass and the stress environment.Many factors contribute to the longitudinal tensile failure of coal face,such as the Poisson effect caused by vertical stress,the unloading rebound of the coal face after cutting and the reflection of the stress wave on the free surface of the coal face.Bearing capacity curves of both the integral-type sprag and the split-type sprag were proposed and calculated,which could also be used as an evaluation index of bearing capacity of a face sprag.It is concluded that the integral-type sprag has the advantages of strong bearing capacity,excellent flexibility and high adaptability of structural coupling,which was analyzed from the perspective of mechanical and kinematic properties.It is suggested that the integral-type sprag with two plates should be selected in priority under the premise of the required protective height of coal face is satisfied.(5)The relationship of structural coupling between the face-end supports,transition supports,gate road supports and the surrounding rockThe lateral structure of hanging roof leads to the low intensity of the roof weighting at the end of the longwall face.The structure of face-end supports,transition supports,gate road supports are designed reasonably,which guaranteed the safe and efficient operation of support system and various equipment at the two ends.(6)Weakening technology for top coal at longwall face endsIn order to solve the problems of unsatisfactory cavability of the top coal at both ends of the first longwall face and the long hanging length of roof in the gob,highenergy gas splitting and hydraulic fracturing measures were taken to weaken the hard top coal.Pre-splitting trail of high-energy gas device shows:(1)The caving fragmentation of the top coal at the face end was obviously reduced and the cavability of the top coal was improved;(2)The weighting intensity at the face end was strengthened,and the coal wall had a certain degree of controllable spalling(the maximum depth of spalling was less than 0.5 m);(3)The effect of roof cutting of was remarkable,and the maximum hanging length of the roof was reduced from about 20 m to 2~6 m.Presplitting trail of hydraulic fracturing shows:(1)Hydraulic fracturing had a certain effect on improving the cavability of top coal at the face end,but there were still many large-sized coal fragments,and the breaking effect of the top coal was weaker than that of high-energy gas pre-splitting device;(2)The roof at the face end caved in time,and the maximum hanging distance was 5~6 m. |
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