Dynamic Response Of Overburden-Soil Composite Structure And Early Warning Of Water Disaster Caused By Fully Mechanized Caving Mining With Super Large Mining Height

The ecological environment of Yushenfu mining area in Northern Shaanxi is vulnerable.The high-intensity exploitation of coal resources has caused serious problems,such as mine water inrush disaster and the ecological environment deterioration,which severely restricts the sustainable development of this area.Mining disturbance causes the deformation and failure of overlying rock and soil,leading to the hydraulic conductivity mutation and increased permeability of overburden and the water inrush and leakage of overlying aquifer.This is the direct cause of mine water inrush disaster and the surface ecological environment deterioration.This paper focused on the super-high-mining and fully-mechanized working face of Jinjitan coal mine in the Yushenfu mining area.The hydrogeological and engineering geological conditions of the study area were analyzed systematically.Field investigation,laboratory test,insitu test,theoretical analysis,and numerical simulation were used to conduct in-depth researches on the engineering geological property of overlying rock and soil in Jurassic strata,the damage and permeability evolution law of mining overburden,the dynamic evolution law of mining overburden-soil composite structure,and the prediction and warning of roof water hazard.The obtained results provide basic theory and technical support for promoting the coordinated development of coal resource secure exploitation and regional ecological environment.The results obtained are as follows:(1)The mineral composition and microstructure characteristic of overlying rock and soil of Jurassic coal seam were determined and analyzed.The mineral composition of roof Jurassic sandstones is dominated by quartz and feldspar,and contains a small amount of organic matter.Clastic particles in sandstones are mainly fine grains with poor roundness,scattered arrangement,and argillaceous cementation.The mineral composition of roof mudstones is mainly quartz,followed by clay minerals and feldspar.The structure of mudstones is relatively dense,and the cracks are not developed.The contents of montmorillonite and illite/montmorillonite mixed layer in the Neogene Baode laterite are relatively high.Moreover,mineral grains in the laterite are closely arranged and pores are relatively developed.The particle component of the Lishi loess is mainly silt grain with less clay grain.(2)The physical-mechanical properties of overlying rock and soil of Jurassic coal seam were tested and analyzed.The average bulk density of roof Jurassic rocks gradually increases with decreasing grain size(from coarse sandstone to mudstone).The average value of P-wave velocity increases from coarse sandstone to sandy mudstone,but decreases from sandy mudstone to mudstone.In the roof overburden,the proportion of weak rocks and semi-hard rocks is large,while the proportion of hard rocks is relatively small.In addition,the average uniaxial compressive strength of sandstones and mudstones increases with decreasing grain size,respectively.In the natural condition,the vertical permeability and horizontal permeability of the Lishi loess belong to slightly permeable and medium permeable,and the permeability of the Baode laterite belongs to slightly permeable and weak permeable.(3)The quantitative relationships between petrographic characteristics and physical-mechanical properties of roof sandstones of Jurassic coal seam were established.Mean grain size(φ-value)and grain-to-grain contacts show significant positive correlations with mechanical strength of sandstones,while non-uniform coefficient,fractal dimension,sphericity,floating contacts,feldspar content,and organic matter content show significant negative correlations with sandstone mechanical strength.Poisson’s ratio has a significant positive correlation with grain-tocement contacts,but significant negative correlations with grain-to-matrix contacts and organic matter content.In addition,mean grain size is positively correlated with bulk density and P-wave velocity of sandstones,while non-uniform coefficient,fractal dimension,sphericity,feldspar content,and organic matter content are negatively correlated with bulk density and P-wave velocity of sandstones.(4)The damage and permeability evolution law and their internal relations of overburden under different mining stress paths were revealed.Under the conditions of monotonous triaxial compression and axial stress cyclic loading and unloading,the mechanical strength of sandstone samples increases significantly with increasing confining pressure or decreasing permeation pressure.The peak strength of sandstone samples under the cyclic loading and unloading condition is stronger than that under the monotonous triaxial compression condition.As confining pressure decreases or permeation pressure increases,the peak volumetric strain of sandstone samples decreases,and the volume expansion advances.In addition,the failure modes of sandstone samples are closely related to confining pressure and stress loading paths.Under the two stress loading paths,the overall variation of permeability coefficient is closely related to stress state and volumetric strain of sandstone samples.Increased confining pressure has an inhibitory effect on the permeability of sandstone samples,which is opposite to increased permeation pressure.There exists a corresponding relationship between acoustic emission characteristic and permeability variation of sandstone samples.Increased confining pressure slows down the damage evolution process,while increased permeation pressure accelerates the damage evolution process of sandstones.In addition,the stress loading path has a significant effect on the damage evolution characteristic of sandstones.(5)The dynamic response characteristic of overburden-soil composite structure caused by fully mechanized caving mining with super large mining height was revealed.The overlying strata mainly occur tensile deformation caused by coal mining,and compressive deformation only occurs at local positions of the strata.The strain characteristics of optical fiber indicate that the geotechnical layer effect in mining process is apparent,i.e.,large strains occur mostly at lithologic interfaces,thinner layers,interfaces between bedrock and soil,and weak soil interior.When the working face reaches 94.36 m beyond the cable borehole,the water flowing fractured zone develops to its maximum height of 225.43 m and enters loess layer 4.35 m.The ratio of the maximum height of the water flowing fractured zone to the mining height is 23.68.The fracture angle of roof overburden is 67.29°.Both bedrock and lower loess layer reach their maximum displacements of 76.65 mm and 59.61 mm when the working face reaches 83.16 m beyond the cable borehole.When the working face reaches 94.36 m beyond the cable borehole,the water pressures of weathered bedrock fissure confined water drop abruptly,and the water flowing fractured zone has developed to the top boundary of weathered bedrock,resulting in the leakage of fissure confined water in weathered bedrock.(6)The zoning evaluation method for inrush risk of phreatic water in sand layer overlying the first super-high-mining and fully-mechanized working face in Northern Shaanxi mining area was proposed,and the monitoring and early warning technology of dangerous area was established.The inrush risk zoning of phreatic water in sand layer of 117 working face is divided into safe area,relatively safe area,transition area,relatively dangerous area,and dangerous area.The dangerous areas are mainly distributed in the northeast and central-southwest of the working face,and the safe area is mainly distributed in the southwest of the working face close to the stopping line.In addition,the risk gradually decreases from the open-off cut to the middle of the working face,whereas the risk gradually increases and then decreases from the middle of the working face to the southwest.The field monitoring and early warning results show that the water contents of loess layer with different buried depths suddenly drop after the working face reaches 97.76 m beyond the monitoring hole,and the water flowing fractured zone has developed to the buried depth of-50.1 m.The maximum ground settlement above the 117 working face is 3.84 m,and combined with the exposure situation of phreatic water,indicating that sand phreatic aquifer does not occur leakage.The water column height in the monitoring boreholes increases significantly during the working face passes through the boreholes from 50 m to 150 m,and its final stable value is greater than the initial value,indicating that sand phreatic aquifer does not occur leakage.There are 85 figures,20 tables,and 223 references in this dissertation.