| As the focus of coal resource development in China has been gradually shifting from the eastern good eco-environment areas to the western arid ecologically fragile areas.Based on the investigation of the ecological status,geological conditions and environmental situations of the arid and semi-arid coal mine district in northern Shaanxi Province,taking Yushen coal mine district as an example,after systematic collection and analysis of geological condition and ecological environment data in the coal mine district,and different eco-geological environment types under the natural conditions of the coal mine district had been defined.Vegetation development space distribution and temporal variation characteristics were also analyzed in different eco-geological environment types.At the same time,after the spatial distribution and interannual variation of groundwater level analyzation in the coal mine district,and the quantitative relationship between vegetation development and groundwater buried depth of each eco-geological environment type was established.In view of the eco-geological environment of phreatic water desert shallows oasis,which accounts for the largest proportion of the coal mine district,Jinjitan minefield was taken as the research object,the development of water-conducting fractured zone in coal seam roof overburden rock under different mining thickness conditions,the variation characteristics of soil relative aquifuge permeability,the dynamic variation rule and mechanism of groundwater level above stope during mining,and the zonal response of surface vegetation to groundwater level variation sensitivity after mining were analyzed by means of theoretical analysis,field monitoring,in-situ and indoor tests,numerical simulation and other research methods.The main conclusions were obtained as follows,(1)There were 3 types of eco-geological environment in the coal mine district,surface runoff(loess)gully,surface water river-valley oasis and phreatic water desert shallows oasis.Different eco-geological environment were classified into 3 types based on ArcGIS and MATLAB.Surface runoff(loess)gully accounted for 5.94% of the total area in the coal mine district,it had some characteristics of large amount of Lishi loess exposed accompanied by a small amount of Baode laterite,large terrain gradient,abundant longitudinal joints and easy erosion.A few areas were covered by aeolian sand,and vegetation species were poor mainly drought-tolerant herbaceous plants and a small number of shrubs.Surface water river-valley oasis accounted for 16.09% of the total area.Surface water and shallow groundwater resources were abundant.It has some features of many tall trees,very abundant herbs and shrubs along both sides of river valley,and there was a number of farmlands existing in the eco-geological environment.Phreatic water desert shallows oasis occupied 77.97% of the coal mine district.Surface was generally covered by aeolian sand and the Salawusu Formation sand seam with Lishi loess sporadically exposed.Shallow groundwater resource was relatively abundant,and surface vegetation was relatively sparse with small herbaceous shrubs.Therefore,phreatic water desert shallows oasis was the most important eco-geological environment in the coal mine district and the most affected by coal mining in the future.(2)Underground eco-phreatic water mainly contained 4 types of the Quaternary loess water,alluvial water,the Jurassic burnt rock water and the Salawusu Formation water.The Quaternary loess water was distributed in the surface runoff(loess)gully eco-geological environment in the eastern part of the study area.Alluvial water and Jurassic burnt rock water were mainly distributed in the surface water river-valley oasis eco-geological environment,which often forms the same aquifer with the Salawusu formation aquifer.The Salawusu Formation phreatic water was distributed in the phreatic water desert shallows oasis eco-geological environment with the largest proportion of the coal mine district,which was related to loess phreatic water,alluvial phreatic water and burnt rock phreatic water.Therefore,the Salawusu Formation sand seam phreatic water was very important for vegetation normal growth and development,as well as for the industrial production activities,agricultural production and irrigation,and local resident life in the coal mine district.(3)The distribution of groundwater buried depth showed obvious spatial variability in the coal mine district.The groundwater buried depth varied from 0 to 30 m in general,which can exceed 30 m in some areas.The groundwater buried depth was larger than 12 m only in northern,central and southern part of the study area.The groundwater buried depth in eastern part of the study area was relatively large.The groundwater depth was generally less than 6 m with some areas larger than 12 m.The groundwater buried depth level was between 0 and 29.8 m in surface runoff(loess)gully.The groundwater depth ranges from 0 to 32.5 m in surface water river-valley oasis,and the data distribution frequency was the highest between 0 and 1 m.The groundwater buried depth was between 0 and 32.3 m in phreatic water desert shallows oasis,of which 2 to 6 m was the most concentrated.(4)The relationship between groundwater buried depth and vegetation index reflecting vegetation growth and development was analyzed in different eco-geological environment types.It showed that groundwater buried depth was more effective in controlling the vegetation development of surface water river-valley oasis and phreatic water desert shallows oasis,and its influence on the vegetation development and distribution of surface runoff(loess)gully type was relatively limited.The sensitivity of vegetation to groundwater buried depth in the coal mine district was classified into three types.Type-? sensitive area,vegetation growth increased with the decrease of groundwater buried depth,in surface runoff(loess)gully,the critical buried depth was about 8 m,type-? sensitive area accounted for about 3.6% of the total area.3m was the critical groundwater buried depth in surface water river-valley oasis,type-I sensitive area accounted for 9.7% of the coal mine district.The groundwater critical depth in phreatic water desert shallows oasis was about 2 m,and type-I sensitive area accounted for 16.8%.Type-? sensitive area,vegetation growth decreased with the decrease of groundwater buried depth,in surface runoff(loess)gully,the critical buried depth was about 16 m,type-? sensitive area accounted for about 1.6% of the total area.19 m was the critical groundwater buried depth in surface water river-valley oasis,type-? sensitive area accounted for 10.5% of the coal mine district.The groundwater critical depth in phreatic water desert shallows oasis was about 10 m,and type-? sensitive area accounted for 40%.Non-sensitive area,the relationship between vegetation growth and groundwater buried depth was not obvious,in surface runoff(loess)gully,the critical buried depth was larger than 16 m,in surface water river-valley oasis,the critical buried depth was larger than 19 m,in phreatic water desert shallows oasis,the critical buried depth was larger than 10 m,vegetation growth and development remains basically unchanged.In general,groundwater buried depth was larger than 2 m in phreatic water desert shallows oasis and 3 m in surface water river-valley oasis,vegetation development started to decrease.Vegetation growth keep stable in surface runoff(loess)gully.Vegetation had the strongest sensitivity to groundwater level variation in phreatic water desert shallows oasis.(5)Taking Jinjitan minefield as an example,the maximum development height of roof water-conducting fractured zone under three different mining thickness conditions was studied by field measurement,in-situ monitoring and numerical simulation.The maximum development height of water-conducting fractured zone was 111.32 m under a condition of slice mining with mining thickness of 5.5 m.The maximum development height of water-conducting fractured zone was 194.88 m under a condition of full seam mining with a mining thickness of 8 m.The maximum development height of water-conducting fractured zone was 203.46 m under a condition of comprehensive mechanized cover caving mining with a mining thickness of 11 m.Comparing the thickness distribution of the roof overburden rock and soil seam,water-conducting fractured zone usually did not penetrate directly key soil aquiuge into sand aquifer under three mining thickness conditions,resulting in short-term rapid decline of groundwater level,which threatened the normal growth and development of surface vegetation.In order to be safe,however,restricted mining thickness should be adopted in areas with small thickness of key soil aquiuge to prevent water-conducting fractured zone from penetrating directly into sand aquifer.(6)By arranging groundwater level monitoring holes directly above and around the mining stope,the dynamic variation of groundwater level before and after mining were monitored,and combining with numerical simulation method,it was determined that groundwater buried depth would decrease by 2.07 m in the mining process with 5.5 m mining thickness in 103 mining stope,and groundwater level would rise to the highest point in the later period,with a decrease of about 0.34 m,which takes about 94 days.After one year's recharge,groundwater buried depth would rise by about 0.15 m.Groundwater buried depth would decrease by 1.31 m in the mining process with 8 m mining thickness in 108 mining stope,and groundwater level would rise to the highest point in the later period,with a decrease of about 0.44 m,which took about 85 days.After one year's recharge,groundwater buried depth would rise by about 0.58 m.Generally,although the mining of coal seam causes the rapid decline of groundwater depth in a short period of time,the groundwater depth could basically be recovered during about 90 days.After one year of recharge,the groundwater depth had increased,which was conducive to the absorption and utilization of groundwater by vegetation.(7)Combining the dynamic monitoring data of groundwater level before and after mining with groundwater simulation software,the permeability variation rule of key soil aquifuge was inverted.The results showed that the permeability coefficient of Baode laterite key soil aquifuge increased slowly from 0.0012 m/d to 0.0187 m/d,and then rapidly increased to 0.8485 m/d,gradually decreased and stabilized at 0.01884 m/d because of the soil seam self-repairing characteristics under the condition of 5.5m mining thickness.It took about 91 days for permeability coefficient to recover from 0.0187m/d to 0.01884 m/d.The permeability coefficient of Lishi loess key soil aquifuge increased slowly from 0.011 m/d to 0.0823 m/d,and then rapidly increased to 4.9526 m/d,gradually decreased and stabilized at 0.0842 m/d because of the soil seam self-repairing characteristics under the condition of 8 m mining thickness.It took about 82 days for permeability coefficient to recover from 0.0823m/d to 0.0842 m/d.(8)Through field investigation and model forecasting,the influence of mining on the surface eco-geological environment was mainly reflected in the formation of surface fissure and waterlogged area caused by surface subsidence after mining.Groundwater level distributions in the minefield was simulated separately after four stopes and the No.1 panel mining,and different vegetation to groundwater level variation sensitive areas was zoned.The comparison results showed that surface waterlogged area increases greatly,phreatic water desert shallows oasis eco-geological environment transforms to desert wetland.Although groundwater level in the unmined area had decreased,it had little effect.It was expected that vegetation development in the No.1 panel would be improved.The thesis contains 157 figures,48 tables,and 336 references. |
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