| The middle reaches of the Yellow River,Jin-Shaan-Mongolian region is the strategic core area of China’s coal resources development,has the reputation of energy"Golden Triangle",where annual coal production accounts for about 70%of the total national production.After more than 30 years of development,coal mining technology development and promotion,has formed a safe,efficient and green mining technology system.The arid climate in the western ecologically fragile mining area,coal mining caused surface subsidence and ground fractures,which led to the decline of groundwater level and soil moisture loss in the mining area,further intensifying the process of soil desertification and causing serious harm to water resources,surface environment in the mining area.Therefore,the dissertation takes the typical mine in the area,the Ningtiaota Mine,as the engineering background,and makes comprehensive use of the theoretical knowledge intersected by several disciplines,such as surveying,mining subsidence,mining,rock mechanics,geology,land reclamation and ecological reconstruction,to conduct an in-depth and systematic study on the overburden rock deformation law,ground fracture generation and self-healing mechanism when mining without coal pillars in the windy sandy area,and obtains the following conclusions.(1)Based on the calculation and analysis of the measured data,we have mastered the law of ground movement and static deformation and characteristic parameters of the high-intensity mining of coal-free working face in the wind-cum-sand covered area in northern Shaanxi① Based on the measured data,we analyzed the spatial and temporal evolution law of surface movement deformation of coal-free column mining in the wind-sand area,and obtained the characteristics of surface movement deformation angular volume and dynamic and static change law.Based on the deformation curve characteristics of S12013 working face of Lime Strip Tower Mine in northern Shaanxi mining area,the surface movement integrated boundary angle of 71.3° towards,58.7° up hill and 35.2° down hill,integrated movement angle of 58.7° towards,63.1° up hill and 59° down hill,and fracture angle of 77° towards,79° up hill and 58° down hill for coal-free pillar mining in windy sand mining area in northern Shaanxi were obtained.Comparison with the parameters of typical surrounding mines such as Shendong and Shaanbei reveals that the characteristic parameters of surface angular volume of the wind cumulus mine without coal pillar mining are smaller on the side without coal pillar,which indicates that its mining has a large influence,strong subsidence degree and wider fracture development.②The dynamic characteristic parameters of surface movement of coal-free pillar mining in Fengjisha were obtained,including the initiation distance of 46 m,the influence angle of 55°and its influence distance of 86 m,the maximum sinking velocity of 290.5 mm/d and its hysteresis angle of 64.7°,and the duration of active surface movement phase of 56 days.Compared with similar mining areas in the vicinity,the surface initiation distance is small,the duration of surface active phase is short,the surface movement deformation is strong,and the speed of surface subsidence changes is fast.③The parameters of surface subsidence estimation were determined based on the probability integral method,which provides a basic basis for mining subsidence estimation in similar areas.After comparing with the typical working faces in Shennan coalfield of North Shaanxi mining area and Shendong mining area,it was found that the subsidence coefficient of the working face without coal pillar mining was larger than that in Shendong mining area and smaller than that in Shennan coalfield of North Shaanxi mining area,and the tangent value of the main influence angle was larger than that in Shendong mining area and smaller or equal to that in Shennan coalfield of North Shaanxi mining area.④The comprehensive analysis of the measured data reveals that the surface movement of coal-free pillar mining in the wind-accumulated sand mining area has significant characteristics such as fast sinking speed,strong degree,wide influence area,large distribution area of cracks,and discontinuous surface movement curve.It proves that the method of coal-free pillar mining and the environment of the wind-cum-sand area have changed the law of surface and overburden movement damage,and have obvious influence on the surface and overburden movement.(2)Analysis of the "ripple-like" crack distribution pattern and asymmetric "M" type dynamic development characteristics of surface cracks in coal-free pillar mining in the wind-sand area of northern Shaanxi① GNSS was used to collect data on the dynamic development of surface edge fractures for more than 50 days and 10 times to obtain the dynamic and static evolution of surface fractures in coal-free pillar mining in the north Shaanxi mining area.Based on the analysis of the geometry of the dynamic fracture distribution of the coal-free pillar mining in the wind-accumulated sand mining area in northern Shaanxi Province,a"ripple-like" fracture development pattern was proposed.The characteristics of the distribution of fractures in the coal pillar-free mining in Fengjisha mine were found.The fracture zone on the side of the coal pillar is located in the mining void area,the fractures generated by the secondary disturbance of the coal pillar-free mining are located above the old mining void area,and the coal mining collapse fractures in the wind-sand area have a strong "self-healing" effect,and the characteristics of the four development stages of the marginal fractures and the parameters of the marginal fracture zone were clarified.The width of the fractures in the fracture zone is smaller than that in the wind-sand area of Shendong.② The whole life cycle of the dynamic development of surface fractures in the windy sand mining area without coal pillars in northern Shaanxi province was mastered.Using GNSS and in-situ monitoring methods,two types of dynamic fractures were monitored,single-peak type developing 7-15 days,fast developing(4-6 days),slow decaying(3-7 days)to closure.The asymmetric "M" bimodal fractures develop in 12-18 days,with development(8-13 days),initial closure,stagnation phase(1-2 days),and secondary development(1-2 days)to closure.It is concluded that the dynamic fracture self-healing effect of coal-free pillar mining in the wind-cum-sand area of northern Shaanxi mine is significant and exists for a short period of time.The dynamic surface deformation parameters of dynamic fracture cracking in coal-free sand mining in northern Shaanxi province were analyzed to obtain the threshold value of horizontal deformation of fracture generation from 2.7 mm/m to 3.7 mm/m.It is proposed that the main area of fracture generation at the edge of coal-free sand mining should pay attention to the "self-healing"effect of fracture,supplemented by artificially induced repair.It is proposed to combine the surface deformation parameters with the rootstock strength of sandy soil plants,and study the damage threshold of plant rootstock to improve the theory of ecological restoration of mining in the western wind and sand area.③ Constructed a full life-cycle dynamic fracture mathematical model equation T=H0(1/tanδ+1/tanφ)/V,and a dynamic fracture generation timing model Y=0.637x+2.715+d in the north Shaanxi mining area.showed that the full life-cycle fracture development of surface movement was influenced by mining depth,maximum sinking velocity The timing of fracture generation is mainly controlled by the daily recovery schedule and the location of the working face.The scientific and accurate prediction of surface fractures and geometric characteristics is of practical value for identifying and delineating ecological restoration areas and improving damage restoration of coal pillar-free mining in windy and sandy areas with the power of self-healing.(3)The dynamic evolution mechanism of overburden and surface fracture was studied by using similar material experiment and fractal theory to reveal the dynamic selfrepair process of overburden damage and fracture in shallow buried thick coal seam in windy and sandy area① Based on the similarity principle and the geological mining conditions of S12013 working face in Lizhota mine in northern Shaanxi province,we made a similar material model and obtained the rock and surface movement damage law.The results show that the overlying rock and surface movement basin has a bias feature.Above the mining hollow area,the lag of rock beam breaking under the mining stress causes the paramorphism of the top rock mass,the essence of which is the different mechanical mechanism of rock beam breaking of the overburden rock on the cut-eye side and the advancing side of the working face.The rock fracture on the cuthole side(coal pillar side)is mainly caused by the fixed support beam fracture,while the cantilever beam fracture above the working face is mainly caused by the upper fixed support beam fracture,and there is a certain difference in their rock fracture angles.The rock rupture angle of rapid mining of shallow thick coal seam under high intensity conditions is large at the working face side.Statistical analysis shows that the working face advance distance and the ultimate rupture angle of each rock layer are correlated.The self-repair was guided by simulating the overburden breaking deformation law and characteristics through similar materials.② Based on the overburden fractal theory,the division of the fractal dimension of fracture into three stages,i.e.rising(1.2839～1.3347)-falling(1.3347～1.1369)-stable trend(1.1369～1.23),is proposed for coal pillar-free mining in windy sand mine.Among them,the ascending section corresponds to the direct top stage,where the overburden breaking damage and fracture development correspond to the fractal dimension of fracture;the descending section corresponds to the near-field non-critical hard layer and critical layer stage,where the overburden loses bearing capacity,large bending settlement is destroyed,and the fracture area appears to be certain;the stable stage is the surface collapse to complete collapse stage.Most of the transverse and longitudinal fractures are compacted,and the phenomenon of "self-repair" of overburden fracture is obvious.Adopting the fractal dimension of overburden fracture can better characterize the overburden transport and pressure change characteristics of the quarry,and provide scientific theoretical support for repairing the overburden fracture.③The results of the above studies are combined to analyze the dynamic realization process of overburden and surface self-healing in the wind-sand mining area.In the windsand area under high-intensity conditions of rapid mining,the early mining stage of the mining area overlying rock and soil sand layer crushing and bubbling down relatively fast,strong and intense,accompanied by continued mining,small deformation damage in the working face mining area overlying rock and soil sand layer conditions,less deformation time,continuous rock cracks in the top plate and overlying strata,the structural integrity of the rock and soil sand layer increases,which increases the overall uniform settlement area of This increases the overall uniform settlement area of the rock and soil sand layer,reduces the movement and deformation time of the overlying rock,reduces the degree of damage to the rock and soil sand layer,and realizes the "self-repair" of rock damage under the rapid advancement of shallow and thick coal seams.(4)On the basis of field survey,similar simulation,numerical simulation and mechanical theory analysis,the formation and dynamic development mechanism of surface fractures along the side of the hollow retention lane are revealed.The synergistic evolution of "rock-soil-sand" self-healing mechanism of coal pillarless mining in windsand area is proposed.① The results of field survey,similar material simulation experiments and numerical simulations show that there are obvious zoning features in the final form of fracture evolution in the overlying rock layer of coal-free pillar mining in the wind-sand area of northern Shaanxi,with the formation of permanent fracture zone of articulated rock beam structure on the cuthole side,fracture zone of rock block in the riser zone and fracture zone of rock body broken above the working surface,and typical "self-healing" features in the part of rock layer above the riser zone."Self-healing" feature,i.e.self-healing function area of rock fracture.② Based on the "O-X" fracture theory and the theory of roof masonry beam,the formula of fracture location mechanics model of overlying key rock layers along the air stay lane is derived.The model considers that the dynamic ground fracture multi-cycle phenomenon of coal mining subsidence is related to the breakage and sinking speed of key rock layer.On the basis of the analysis of the mechanism of rock formation breakage and ground fracture development along the side of the air-holding lane under the mining conditions of coal-free working face,the characteristics of rock formation breakage development and the mechanical model perspective are combined to elucidate the selfhealing characteristics of ground fractures along the side of the air-holding lane under the influence of rock block articulation structure and secondary rotation and the synergistic transmission driving mechanism under the influence of rock,soil and sand coupling.(5)According to the mining plan,a zoning management strategy based on fracture self-healing characteristics was proposed to optimize the surface restoration management model for the next five and ten years of mining along the hollow retention lane in the Ningtiaota Mine.① Based on Boltzman’s function,a model was developed to predict the subsidence of coal column-free mining in the windy sand area of the Shennan coalfield,and to calculate the degree and extent of surface subsidence along the hollow lane mining in the near term(5 years)and medium and long term(38.9 years)since the mine was built.The calculation results show that the maximum surface subsidence is 7953 mm,and the comparison with the measured results of S12013 working face shows that the error in Boltzmann function model Ⅰ is ± 159.01 mm,and the error in Boltzmann function modelⅡ is ± 187.19 mm,both within 10%.The calculation results of the boltzmann function are better than the probability integral method under the mining conditions without coal pillars.②Based on the self-healing characteristics of surface damage,subsidence basin and land ecological damage of coal-free column mining in the wind-sand area,a zoning management model was established for the dynamic change characteristics of ground cracks.Considering the self-healing characteristics of surface fractures,the mining impact area is divided into self-(natural)repair area(NRR),and artificially guided to promote repair area(ARR).And combined with the difficulty of treatment and the degree of influence of mine geological environment problems,the zoning treatment and terrain reconstruction planning of mine subsidence after terrain reconstruction treatment,the whole area is divided into a total of 3 levels and 7 different treatment zones. |
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