Study On The Strata Movement Rule And Regional Control Of Deep Mining With The Super-Thick And Weak Cementation Overburden

As the national economic strategy is westward shifted,coal resources in the west China,especially deep coal resources in the Dongsheng coal field of the Ordos Basin,have become a crucial energy reserve for national development.Compared with the deep mining area of the Yanzhou coal field and the Jining coal field in the east,under the same mining degree,the surface subsidence of coal mining under the ultra-thick and weak cementation overburdens in the deep mining area of the western Dongsheng coalfield is significantly smaller.The existing strata movement theories cannot rationally elucidate its particularity.This phenomenon produces pronounced safety threats to the mining of high-yield and high-efficiency working faces,thereby severely confining the large-scale mining of coal resources under ultra-thick and weak cementation overburdens.Yingpanhao coal mine in Dongsheng Coalfield was studied in this paper:Firstly,the physical and mechanical features and microstructural characteristics of weakly cemented sandstones have been investigated experimentally.Then,the differences in rock mechanical properties,overburden structure characteristics and surface movement and deformation rule were compared and analyzed in eastern-western deep mining areas.This work also identified the influencing factors that caused the surface subsidence of ultra-thick and weak cementation overburdens in deep mining being relatively small.Lastly,mechanical analysis,numerical simulation and physical simulation were applied to learn the surface movement and deformation rule,strata movement and failure characteristics of ultra-thick and weak cementation overburdens in deep mining,and the regional strata movement control method of ultra-thick and weak cementation overburdens in deep mining was proposed.The specific findings can be summarized as follows:(1)Although the ultra-thick and weak cementation overburdens were weak in lithology,the overall rigidity of the rock strata was relatively large.(1)The uniaxial compressive strength of the Cretaceous Zhidan group sandstone was between 10~20MPa,but its lithology was weak;The inner fissure of the rock strata was great,but the cracks and joints were barely developed;The cementation was calcareous and insoluble in water,and the mechanical features of the cement were similar to those of the rock particles;The single strata was thick and no folds and faults were grown.Therefore,the above factors cause the overall rigidity of the sandstone of the Zhidan group larger.(2)The uniaxial compressive strength of the sandstone of the Zhiluo formation was between 20~40MPa,and the lithology was medium-hard;The rock strata were dense,and there were fewer cracks and voids;The cementation was a clay mineral,but it was not easy to dissolve in water;The single strata was thick,and the folds and faults were not developed.Therefore,the above factors make the overall stiffness of the sandstone of the Zhiluo formation huger.(2)There were obvious differences in the surface movement and deformation rule of coal mining in the east-west deep mining areas.Under the same mining conditions,when the mining area was small,the movement rule of ultra-thick and overly cemented overburdens was similar to that of hard overburdens in eastern China,and the surface subsidence coefficient of ultra-thick and weak cementation overburden in deep mining was considerably smaller than that of soft overburden in deep mining.When the mining range was great,and the mining degree was close to or even reached to fully mining,the surface subsidence coefficient of the deep mining areas in the eastern region was usually larger than that of the deep mining areas in the western region,and the surface movement rule of ultra-thick and weak cementation overburdens was still shown the characteristics of the inadequate mining.(3)Impact of mining space on the surface movement and deformation rule of ultra-thick and weak cementation overburdens in deep mining:when D1/H0>=3 and D3/H0>=3,the surface reached to fully mining;With the increasing mining degree in advancing direction,the advancing boundary angle decreases rapidly in Boltzmann function,the subsidence rate increases in Boltzmann function when a single working face of deep mining was mined under super-thick and weak cementation overburdens;With the increasing mining degree in dip direction,the subsidence rate increases in Boltzmann function,the horizontal motion coefficient decreases in a sine function,the tangent of the main influencing angle was increased in Boltzmann function when multi working faces of deep mining was mined under super-thick and weak cementation overburdens;The relationship between surface subsidence coefficient,horizontal movement coefficient and main influence angle tangent of coal seam repeated mining and that of coal seam initial mining were attained via numerical simulation studies.(4)Effect of overburden structure changing on the surface movement and deformation rule of ultra-thick and weak cementation overburdens in deep mining:With the increase of mining degree,the control effect of super-thick sandstone on the overburden movement is strengthened at first and then weakened.Due to the control effect of super-thick Zhidan group sandstone,the surface subsidence coefficient decreases by 0.18 at most,28.2%of the surface subsidence coefficient without control effect.There are obvious differences in the effect on surface movement and deformation rule caused by the alterations in the relative spatial distances and relative order of the main key layer and the sub-key layer.The alterations in the relative spatial distances of the main key layer and the sub-key layer owned a small effect on the surface subsidence rate,the horizontal surface movement coefficient and the main influence angle tangent.The changes in the relative order of the main key layer and the sub-key layer had a huger influence on the surface subsidence rate,and led to a smaller effect on the horizontal surface movement coefficient and the main effect angle tangent.In this paper,when the relative spatial position of the main sub-key layer changes,the maximum value of the surface subsidence coefficient changes is0.09,15.8%of the corresponding subsidence coefficient in the geological prototype.The maximum value of the surface subsidence coefficient changes is 0.31,67.4%of the corresponding subsidence coefficient in the geological prototype when the relative order of the main sub-key layer changes.The maximum value of the horizontal surface movement coefficient and the main effect angle tangent is small,less than 20%of the corresponding parameters in the geological prototype when the relative spatial position and the relative order of the main sub-key layer changes.(5)Influence of horizontal tectonic stress on the surface movement and deformation rule of ultra-thick and weak cementation overburdens in deep mining:Horizontal tectonic stress was one of the chief reasons that the impact range of deep mining was far larger than the current cognition,and it was also one of the key reasons for the small surface subsidence of ultra-thick and weak cementation overburdens in deep mining.The decrease value of surface subsidence coefficient reaches 0.49 at most,61.3%of subsidence coefficicent of geological prototype.S_w value increases 1110m at most,205.6%of S_w value of geological prototype.S_u value increases 1710m at most,247.8%of S_u value of geological prototype.(6)Monocular digital photography based on the displacement parallax method with the geometric sequence correction was used to monitor the similar material model with the accuracy of 0.47mm.The new simulation notions for overlapping similar material model in deep mining and an innovative monitoring approach for single-binocular close-range photogrammetry were proposed.It overcomes the limitation of studying strata movement in deep mining by conventional similar material simulation.These two techniques offered new ideas and technical support for the investigation of the dynamic development process of regional deformation caused by mining multiple working faces and multiple mining areas.(7)The failure mode of the ultra-thick and weakly cemented overburden was the"beam-arch shell"failure,and the damage boundary was arch-shaped cracks.With the continuous expansion of the mining area,the complete"arch shell"failure was gradually transformed into"semi-arch shell"failure.As the thickness of a single strata layer increased,the failure mode of ultra-thick and weak cementation sandstones changed from"arch shell"to"arch shell-beam-arch shell"failure.Before the"arch shell-beam"failure of the ultra-thick and weak cementation sandstone showing,its movement process demonstrated a clear time correlation,and after that,its movement process disappeared in the time correlation,indicating the characteristics of mining and sinking.Due to the"arch shell-beam-arch shell"failure mode of super-thick Zhidan group sandstone,the surface of deep multi-working face mining of the super-thick and weak cementation overburden shows a leaping subsidence.(8)Combining key layer theory,rock beam theory,and granular media theory model of random media,this paper reveals the affecting mechanism of thick and weak cementation sandstone and horizontal tectonic stress on the movement rule of thick and weak cementation overburden in deep mining.It provides a theoretical basis for reasonably explaining the small surface subsidence of the ultra-thick and weak cementation overburden in deep mining and the wider influence range of deep mining.Based on the rock beam theory and the pressure arch theory,this paper analyzed the failure mode of super-thick and weak cementation overburden,derived the equations of tensile stress and shear stress at any point on any section of rock beam,established the equilibrium equations of bending moment,shear force and axial force in any section of arch trace.(9)This paper proposed an approach,partial backfill mining based on the main control layer,for the surface subsidence control and energy-polling reduction of ultra-thick and weak cementation overburdens in deep mining.We investigated the influencing factors and response rule that affected the surface subsidence control and energy-polling reduction effects of partial backfill mining based on the main key layer,and found that the influence degree on the control effect of strata movement from large to small is working face width,filling rate,section pillar width and filling face width,the influence degree of energy-polling from large to small is section pillar width,filling rate,working face width and filling face width.Then,with analyzing the stress distribution characteristics of composite backfill and overburden strata,this paper also indicated that the control mechanism of partial backfill mining based on main control layer was the synergy between the composite filling body and the main control layer structure.The dissertation has 185 figures,184 tables and 163references.