Evaluation Of Ordovician Ancient Weathering Crust And Floor Water Inrush Vulnerability In Typical Part Of Fengfeng Mining Area

As one of the most important energy sources in China,coal is of great strategic significance to the development of China’s national economy.With the continuous development and utilization of shallow coal resources,how to mine deep coal resources safely and effectively has become an important problem waiting to be solved in many areas of our country.Due to the influence of the Ordovician limestone aquifer and complex geological conditions,the mining of lower group coal in North China coalfield is facing huge technical resistance and difficulties.The problem mainly focuses on the high confined water in the thin water-resisting section of the floor,which has seriously affected and restricted the efficient and sustainable development of the mine.Therefore,to study the water inrush problem from the Ordovician limestone of the lower group coal in North China coalfield has becoming a significant research direction.The key to solving this problem is that clear the hydrogeological conditions of Ordovician limestone firstly.On this basis,we can make the research results of the water inrush problem of the floor more accurate and reliable.In this research,the Fengfeng mining area and its typical part（Niuerzhuang mine and Xintun mine）as the research object,this thesis first evaluates the regional Ordovician limestone hydrogeological conditions through the comprehensive study of the regional tectonic field,groundwater chemical field and groundwater seepage field.Secondly,based on systematically summarizing the characteristics of geological and hydrogeological conditions of the typical part in the Fengfeng mining area,the drainage and precipitation of the typical part is predicted using the Ordovician limestone groundwater numerical simulation model,and the hydrophobicity is analyzed and evaluated.Meanwhile,the countermeasures are provided for the prevention and control of Ordovician limestone water inrush in the mine floor;Then,the variable weight aquifuge index method based on fuzzy c-means clustering is used to evaluate the water impermeability of Ordovician limestone ancient weathering crust.Finally,the vulnerability index method coupled with AHP（analytic hierarchy process）and CV（coefficient of variation method）is used to evaluate the floor water inrush vulnerability of multi coal seam with single aquifer.A three-level evaluation system is formed based on"evaluation of hydrogeological conditions,evaluation of water impermeability of ancient weathering crust,and evaluation of water inrush from the floor".The research results are more in line with the production practice of the Fengfeng mining area and the typical part.The main research contents and achievements of this thesis are as follows:（1）By analyzing the evolution process,distribution characteristics,structural scale and development degree of the regional structure,this thesis studies the control of geological structure on the regional boundary,burial distribution of the Ordovician limestone aquifer and the law of karst water movement.The research results show that the large-scale structure controlling the boundary of the Heilongdong spring area,which makes the area closed to a certain extent as a whole.The distribution characteristics of groundwater in the unit are mainly controlled by different levels of the fault structures,and the burial distribution characteristics of the aquifer are also affected by them.The movement law of regional groundwater is strictly controlled by structure and magmatic rock mass,so that the collection and migration of groundwater show obvious strip and direction.Through the study of the structural field in the study area,it is revealed that the geological structure plays a major role in controlling the Ordovician karst groundwater.And the structures of different levels control the boundary conditions of hydrogeological units,the buried distribution,and the movement state of groundwater respectively.（2）By collecting the test results of hydrochemical components(p H,K⁺,Na⁺,Ca²⁺,Mg²⁺,SO₄^2-,Cl^-,HCO₃^-,NO₃^-,TH and TDS)of 14 water samples in the Fengfeng mining area,SPSS25.0 software is used to carry out Q-type cluster analysis.Taking the shortest distance as the identification principle,the hydrochemical data in the mining area are divided into three groups.Based on the statistical characteristics analysis of hydrochemical parameters,the general law of hydrochemical field distribution characteristics is obtained:the groundwater in group A is mainly distributed in the plain area on the east and west sides of Gushan,which is the runoff area of the groundwater flow system.The groundwater in Group B is mainly close to the new urban area of the Fengfeng mining area and close to the Heilongdong spring.The hydrochemical characteristics of water samples are close to the excretory area of the groundwater flow system.The groundwater in group C is mainly distributed in the area below 600m underground of the Xintun mine and the whole area of the Wutongzhuang mine.Combined with the hydrogeological conditions and water blocking structure analysis in the study area,it is a local detention area.（3）The numerical simulation model of regional Ordovician limestone karst groundwater flow is established by using Visual Modflow software.The model is calibrated and verified by using historical observation data.Then the characteristics of the groundwater seepage field of Ordovician limestone seepage field under natural and man-made conditions are simulated.Through the numerical simulation of Ordovician limestone groundwater,it can be found that the seepage direction of Ordovician limestone seepage field is consistent with the movement of Ordovician limestone water in the natural state.However,due to the massive exploitation of Ordovician limestone karst groundwater and a local groundwater funnel is formed.The direction of local groundwater flow caused by the groundwater funnel is inconsistent with that in a natural state.（4）By analyzing the geological and hydrogeological conditions of the typical part of the Fengfeng mining area,three different drainage schemes of Ordovician limestone karst water in the typical part of the Fengfeng mining area are designed by using Ordovician limestone groundwater numerical simulation model.From the simulation results,it can be seen that the effect of drainage and pressure reduction in typical locations is not ideal,and it is theoretically difficult to reduce the karst water level to a safe head by arranging pumping holes on the ground.However,other measures such as floor grouting reinforcement and transformation should be taken to reduce the water inrush threat of Ordovician limestone aquifer faced by coal mining.（5）Given the widespread phenomenon of Ordovician limestone weathering crust with certain water resistance in the Fengfeng mining area,this thesis evaluates the impermeability of Ordovician limestone weathering crust in the typical part using the"variable weight aquifuge index method"based on the variable weight theory.Firstly,the main controlling factors affecting the water impermeability of Ordovician limestone ancient weathering crust are determined based on the analysis of geological and hydrogeological conditions.And the thematic map of each main controlling factor is established.The key point is that the fuzzy c-means clustering method is introduced into the state variable weight classification of the variable weight model to divide the weight adjustment interval.Finally,the water impermeability index is divided into five levels using the natural discontinuity classification method.The evaluation results fully reveal the distribution law of the water barrier performance of the Ordovician ancient weathering crust in the typical part of the Fengfeng mining area:the water barrier performance of the Ordovician ancient weathering crust in the study area has an overall trend of gradually strengthening from southwest to northeast with the increase of the buried depth of Ordovician.Compared to the results of variable weight evaluation and constant weight evaluation,the former has better accuracy and applicability.（6）The mining of lower group coal in North China coalfield is generally faced with the problem of thin floor aquifuge and high pressure of the Ordovician limestone water.The water inrush vulnerability of the typical part is evaluated,which uses the floor water inrush vulnerability index method coupled with AHP and CV.Firstly,the main control factors affecting floor water inrush are optimized and determined.Then,the corresponding thematic map is established through the information extraction of relevant data.Importantly,the subjective weight and objective weight of each main control factor are determined by AHP and CV methods.Meanwhile,the comprehensive weight is determined by the principle of minimum discriminant information.Finally,the zoning threshold of floor water inrush vulnerability of multi coal seam with single aquifer is determined by the fuzzy c-means clustering method.In this research,the five-level zoning of floor water inrush vulnerability evaluation of each coal seam in the lower group of coal is obtained.The results show that the evaluation accuracy of the evaluation method coupled with AHP and CV is higher than that of the traditional vulnerability index method.The evaluation results fully reflect the important role of the waterproof performance of the deep Ordovician limestone ancient weathering crust in the prediction and evaluation of floor vulnerability.