| The large-scale mining construction activities,on the one hand,they can promote the development of the economy;on the other hand,they have also causeed the problem of Mine Geological Environment,making the Geological Disaster in the mine area and its vicinity more and more serious.For example,the karst collapse in the Lengshuijiang Mining Area occurs from time to time with the continuous mining and construction of the mining area,and it presents an increasing trend year by year.The investigation and analysis of the mining area found that there is a constant discharge of groundwater in the mining area,groundwater fluctuation is pareicularly violent.Then,what is the impact of the dramatic fluctuation of water level dynamics on the formation of subsidence in this mining area? This is a key question worth studying.The study of this problem is of great significance for the prevention and control of ecological environment and the pretection of villagers' life and property in the mining area.Therfore,this paper takes the karst subsidence area in the Lengshuijiang Mining Area of Xinhua District in Central of Hunan Province as the research area.Using a variety of methods such as field investigation,experimental research and numerical simulation,to investigate and study the physical geographical conditions,hydrogeological conditions,etc.;to investigate the characteristics of karst development in the mining area,analyze the distribution and genesis of karst collapse;to study the change of groundwater level and its effect on karst collapse in mining area,and put forward the early waring process of karst collapse.The main results obtained in this paper are as follows:1.After the investigation,the characteristics of distribution and development of solvable rock were obtained:Dissoluble rock in the study area is distributed almost all over the region.The rate of encountering karst hole/cave of the borehole is up to 40%,and the height of cave is between 0.5?1.3m,in addition to karst caves,the honeycomb cavity and dissolution gaps are very well developed.The developed depth of karst fissures varies from a few tenths of a meter to several tens of meters.2.Taking into account the basic condition of karst development,this paper analyzes the influence of various factors in the process of karst development is analyzed emphatically,including the hydrodynamic condition,topography and geomorphology,soluble rock components etc.The results indicate that:(1)The content of Ca O in bedrock's composition affects karst development,and the higher the ratio of Ca O/Mg O is,the easier it is to develop karst.(2)The circulation between groundwater and surface water are controlled by landform,which indirectly restrict the development degree,scale,temporal and spatial distribution of karst development.The study area belongs to the landform of karst hills and depressions,which is conducive to the development of underground karst.(3)The karst development degree of the same lithologic combination stratum is different in different structural paits.The study area is located in the Hanpoao syncline,and there is a large fault running through the eastern limb of the syncline and is accompanied by several small faults,the area with wide tensile cracks is easy to form heavy runoff zone by rainfall infiltration,while the other areas are difficult to form it.(4)Groundwater movement is the main factor affecting karst development,and the hydrodynamic zoning determines the karst distribution to a certain extent.The closer to the surface,the faster thee velocity will be,which is more conducive to the dissolution of soluble rock.3.The temporal and spatial distribution rules of collapse in the study area are obtained by investigation and analysis:The number and density of karst collapse are the largest in the karst hills and depressions,and they are mainly distributed along the alternate areas with frequent groundwater runoff and drainage activities.The collapse is distributed in the 0?20m Soil layer,and the collapse in the 0.0?10.0m thick soil layer accounts for 60.72%.There is a concentrated outburst period for karst collapse development,with stages;for example,11 karst collapses developed in 2006 and 10 in 2016.4.The inducing factors and formation mechanism of karst collapse were analyzed:(1)Rainfall infiltration makes the soil saturated,reduces the soil strength and increases the load,causes the fluctuation of groundwater level,causes the positive or negative pressure of the underground karst cavity,all of these phenomenon leads to the occurrence of karst collapse.(2)Extreme dry weather leads to the decrease of groundwater level,which increases the vertical penetration and concentrated penetration point effect of groundwater.(3)Geomorphologic types affect groundwater recharge,runoff and drainage;and indirectly induce collapse.(4)Underground karst can not only provide space for collapse,but also store collapse materials.(5)The thinner the soil layer,the more vulnerable to groundwater erosion and induced collapse.(6)With the development of mine construction and mining activities,drainage of mine water,the underground wate level decreases greatly,and the hudraulic gradient increases,which is easy to produce the seepage pressure effect,and the soil layer is absorbed by the latent erosion.(7)The mechanism of collapse induced by osmotic pressure effect includes vertical osmotic effect,water absorption softening effect of soil,hydrostatic loading effect and the negative pressure suction and erosion in closed space caused by vertical permeability,that is,the surface water seeps into the soil layer and induces collapse.(8)The mechanism of collapse induced by negative pressure erosion,that is,in a relatively intact closed karst cavity,negative pressure was formed due to the decrease of groundwater level,resulting in negative pressure expansion and vacuum erosion,and induced collapse.(9)The mechanism of the loss of support and the increase of load induced collapse,and the groundwater level was originally located in the water-blocking cover layer,when the groundwater level fell,the dead weight of soil in cover layer increases the additional stress of surface water and loses the support of groundwater,which leads to collapse.5.The hydrogeological conditions in the study area are generalized,the hydrogeological conceptual model is established,and the variation of groundwater level is calculated,analyzed and predicted,the results show that the time of frequent fluctuation of groundwater level is consistent with the time of karst collapse,which is from April to August every year.Conbined with the monitoring data of groundwater level,the influence of groundwater level fluctuation on karst collapse is analyzed,the results indicate that the main inducing factors of karst collapse are atmospheric rainfall and mine drainage.6.According to the results of groundwater level monitoring and analysis,the process of early warning of karst collapse is put forward,which provides a basis for the prevention and control of karst collapse and protects the safety of people's life and property.The basic idea is: the water level is kept in the water-blocking cover layer,namely the highest water level;or the lowest water level is at a certain position below the rock-soil contact interface.If the water level is above the maximum water level or lower than the minmum water level all the time,it is possible to avoid collapse,otherwise it may induce collapse. |
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