Dynamic Evolution And Application Of Water Conducting Fractured Zone During Extraction Of Jurassic Coal Seams In Northern Shaanxi

The development emphasis of coal resources has rapidly shifted to western China with fragile ecological environment.Threat of water inrushing,leakage of water resources,and induced ecological environment damage problems are the main limiting factors for high-intensity mining exploitation of the dominant coal resources in northern Shannxi.The Quaternary sandy phreatic water is not only the unique precious groundwater resources with large-scale water supply and ecological significance in northern Shaanxi,but also the important resoures of water filling for the underground water disasters.Whether from the perspective of mine safety or water resources protection,the evolution law of water-conducting fractured zone（WCFZ）which is the main channel of communicating the overlying water and mine pit,is the top priority of mine water control research.This paper takes Yushenfu mining area as the background,Jinjitan coal mine as the target,the rock（soil）body overlying the coal seam and Q₃s sandy phreatic water as the main objects.Based on the analyses of spatial structure of aquifers and aquicludes,and engineering geological characteristics of overlying strata,the monitoring method and dynamic process of overburden deformation and failure caused by underground mining,the development law and characteristics and abnormal mechanism of mining-induced fractures,permeability evolution of mining damaged rock and soil masses are analyzed and studied by field investigation,laboratory experiments,in-situ measurement,numerical simulation,analogue simulation,theoretical analysis,etc.In addition,the responses of underground water disasters and surface ecological environment to the high-intensity exploitation have been studied and evaluated.The main results are as follows:（1）Comparative analysis of the characteristics and applicability of common distributed optical fiber sensing technology shows that the BOTDR technology with the single-end testing advantage is most suitable for overburden deformation monitoring.On the basis of the optimization of site embedding and drilling backfilling,the deformation consistency between cable and surrounding rock is guaranteed and verified by the laboratory test.Based on the strength and accuracy analysis,MKS strain sensing cable is superior to GFRS and 10m-IFS,which can fulfill the monitoring task.（2）In the mining process,the strain curve of overlying strata develops upward in the shape of"steps".The overall stress state can be divided into two parts.The process of upper overlying strata is compression-tensile,however,the lower is the compression-tensile-compression.The vertical compression of the rock is inversely proportional to the young’s modulus and the hardness.The rock are destroyed mainly in the tensioning form.The failure height of overlying rock can be divided into three stages of development-maximum-stability during the mining process.When the distance between the panel and the monitoring borehole is about 200m,it reaches the maximum height which is about 1.1 times of the stable height.（3）The spatial shape of the WCFZ is approximately"arch shaped"instead of"saddle shaped",and the height increases slightly along the mining direction.Based on the mining practice,in-situ exploration and theoretical analysis,the non penetrating fractured zone（NPFZ）is proposed as the transition zone between the WCFZ and the continuous zone.The definition of the NPFZ is that the part rocks that develop a large number of fractures but they are not connect with each other or rarely connect,maintaining the original layered structure on the whole and their deformation and movement have similar continuity.The relationship between the height of the NPFZ and the WCFZ is established,and it is pointed out that the height of the NPFZ is about 0.2 times that of the WCFZ.（4）The inhibition effect of soil layer on the development of mining-induced fractures is studied,and the inhibition factor is put forward and determined.Based on the analysis of measured data,mathematical models for predicting the development of the WCFZ which is fully developed in the bedrock and in the soil-rock composite rock stratum are respectively established,and the accuracy is verified by mathematical statistics and numerical simulation.It is pointed out that the abnormal increase of the Lc to the eastern mining area in the north of Shaanxi mainly comes down to two reasons for the overall overlying rock structure and the lower position of the key layer.（5）Under the influence of mining,the permeability coefficient of rock in the NPFZ is obviously changed,which maximum increased by 2 orders of magnitude,and the permeability grade is changed to"weak permeability"or even"medium permeability"from"micro permeable".The permeability increase is closely related to the rock location in the NPFZ.The permeability coefficient of the upper part,the middle part and the lower part of the NPFZ is 10 times,20 times and 40 times of the original value before the mining.The permeability of damaged rock is decreased by nonlinear power function during the stress recovery process.The higher the degree of damage,the lower the degree of the permeability recovery of the rock mass.And the rock permeability coefficient of the upper NPFZ is basically restored to the original level.The permeability recovery degree of the damaged rock mass of different lithology has the relation of"argillaceous sandstone>siltstone>fine sandstone".（6）The soil permeability decreases with the thickness of soil layer,and the thickness of soil layer has more influence on the permeability of laterite than that of loess.However,the soil permeability increases with water pressure,and the influence of water pressure on the permeability of loess is greater than that of the laterite.During the mining process,the impermeability of loess and laterite both exhibit a dynamic process of“reduction-recovery”.The reduction is done instantaneously,while the recovery should take some time.The impermeability reduction of laterite has a greater impact on the phreatic level than loess,and the recovery degree of laterite impermeability is higher than that of loess after mining.（7）Taking the 103 working face of Jinjitan coal mine as an example,the stable height and maximum height of the WCFZ and the height of the NPFZ in overlying strata are analyzed.According to the mining practice and the mining hydrogeological model,the water inflows from bedrock confined aquifer,weathered bedrock confined aquifer,and Quaternary sandy phreatic aquifer are estimated separately.As a result,the normal and maximum water inflow of the panel are predicted.Based on theoretical calculation and numerical simulation,the change of groundwater level under high-intensity mining is studied.The post-mining water depth is reduced from the ecologically optimal water level to the warning water level,and a small amount of trees may appear to decay.