(1 School of Water Resources and Environmental Engineering, China Geo University, Beijing,100083; 2 Shanxi Geological Survey Institute, Taiyuan, 03000 1)

This paper briefly introduces the present situation of environmental geological problems in six coalfields in Shanxi, and puts forward some thoughts on the problems existing in the current investigation and the work to be carried out in the future. For the comprehensive evaluation of mine environmental geology, the author thinks that the analysis of various coal mine environmental geological problems should be extended from the professional perspective to the social and economic aspects, and the latter should be used as the combination point of unified comprehensive comparison. At the same time, it is suggested that research work should be carried out in the following aspects: the change of ground stress caused by karst water caused by coal mining, the land desertification caused by coal mining, the calculation of various security coal pillars and the determination of movement angles, the environmental geological model of coal mines and the establishment methods of various environmental geological models of coal mines.

Keywords coal mine environmental geological investigation thinking

1 Overview of Shanxi Coalfield

The coal-bearing strata in Shanxi are mainly Taiyuan Formation of Upper Carboniferous and Shanxi Formation of Lower Permian in Late Paleozoic, which are distributed in coal fields of the whole province, and the coal reserves account for 98% of the total reserves in the province, with great mining potential. Followed by Mesozoic Jurassic Datong Formation, distributed in Datong coalfield and northern Ningwu coalfield, its reserves account for about 1.6% of the whole province, and there are few recoverable coal seams left; Third, Cenozoic Tertiary coal-bearing formations, distributed in Fan Shi, Yuanqu and other places, have few reserves. According to the comprehensive factors such as sedimentary environment, geological structure characteristics and coal quality deterioration law, there are mainly six coalfields in Datong, Ningwu, Xishan, Huoxi, Qinshui and Hedong in Shanxi from north to south. The environmental geological problems of coal mines are also common here, and the problems of small coalfields in other areas are becoming less and less serious. See table 1 for the basic information of the six coalfields.

Table 1 Statistical Table of Basic Situation of Six Coalfield in Shanxi Province

2 Shanxi coal mine environmental geological problems present situation

According to statistics, the current coal mining area in Shanxi Province is about 5500km2. According to the Report on Geological Investigation and Evaluation of Mine Eco-environment in Key Areas of Shanxi Province submitted by Shanxi Geological Survey Institute on June 5438+ 10, 2002, the main geological disasters are mining subsidence, ground fissures, collapses and landslides in Datong, Ningwu, Xishan, Huoxi and Qinshui coalfields (including Dongshan, Yangquan, Lu 'an and Jincheng in Taiyuan). See Table 2 for various environmental geological problems.

Table 2 Statistics of various environmental geological problems

3 Shanxi coal mine environmental geological survey thinking

Shanxi is a province with frequent geological disasters in northern China and the most serious environmental geological problems in coal mines. However, due to various reasons, the investigation of environmental geological problems in Shanxi coal mines has gradually attracted attention since 1990, and most of the work carried out before 1999 was carried out after contradictions and disputes caused by environmental geological problems, providing a basis for Party A and Party B to coordinate or resort to legal cases. The representativeness is limited and the overall legality is not strong. It is difficult to macroscopically determine the overall scale, degree and loss of environmental geological problems in Shanxi coal mines, and it is difficult to analyze and determine different environmental geological problems modes under different geological environments, different mining minerals, different mining horizons and different mining methods. The geological disaster investigation report of Datong mining area submitted by Shanxi Environmental Geology Station 1999, the environmental geological investigation report of Shanxi Province in February 200010: 500,000, and the comprehensive investigation and evaluation of mine eco-environmental geology in key areas of Shanxi Province completed by Shanxi Geological Survey Institute in June 2002 (10: 250,000) reveal the environmental geology of Shanxi coal mines.

According to the environmental geological survey of1∶ 500,000 in Shanxi province, the environmental geological problems in Shanxi province are divided into 4 regions and 34 sub-regions according to the disaster degree, and the qualitative analysis method is used to evaluate and predict them. Comprehensive investigation and evaluation of mine eco-environmental geology in key areas of Shanxi Province (1∶ 250,000) According to the actual number of eight types of mine eco-environmental geological problems such as ground subsidence, ground fissures, house cracks, solid waste, collapse, landslide, debris flow and spontaneous combustion of coal seam, the disaster distribution statistics are carried out in a unit of 2km×2km, and the digital superposition analysis is carried out, and the comprehensive evaluation of mine eco-environmental geology is carried out by using the disturbance coefficient method. It is determined that the eco-environmental geological damage area of key areas is 1925km2, of which the severely damaged area is 562km2, the moderately damaged area is 696km2, and the lightly damaged area is 667km2. According to the production plan of each mine 20 10 years ago and the national tenth five-year plan, the development trend of eco-environmental geology in each mining area is divided into deterioration area, recovery area and natural state area. The map of 1∶65438+ million in the 2km×2km unit is evaluated and predicted respectively.

The above two projects, as well as the large-scale environmental geological survey in some mining areas and the investigation and zoning of geological disasters in1∶ 50,000 counties (cities) in recent years, have accumulated data and laid a foundation for summarizing the macro-laws of environmental geological problems in coal mines in Shanxi Province, establishing formation models and putting forward prevention and control countermeasures. The author puts forward the following thoughts on the problems existing in the current investigation and the work to be carried out in the future.

3. 1 Comprehensive evaluation of mine environmental geology

The comprehensive evaluation of mine environmental geology is marked by the intensity scale, activity times, distribution density and loss of various environmental geological problems. These factors determine the damage intensity, frequency and scope of environmental geological problems, and are the key factors for comprehensive evaluation and determination of loss degree. However, different types of coal mine environmental geological problems have different signs of risk factors. When there are a variety of environmental geological problems in mines, the problem of determining the weights of various problems will emerge, which is closely related to mining types, geological environment, mining scale, mining horizon and mining methods, with diversity, complexity and uncertainty, making different departments, different regions, different periods and even different departments. Although many scholars and research institutions have put forward some suggestions in recent years, so far, there is no unified comprehensive evaluation method of mine environmental geological elements in points, areas or regions, which is the primary problem to be solved in this kind of work in the future, and corresponding norms or requirements should be established as soon as possible.

At present, the methods to determine the weight of various environmental geological problems mainly include expert scoring method, investigation and statistics method, sequence synthesis method, formula method, mathematical statistics method, analytic hierarchy process, complexity analysis method and so on. The comprehensive evaluation methods of mine environmental geology mainly include expert judgment method, engineering analogy method, factor score method, attack coefficient method, probability statistics method and cluster analysis method. These are all for evaluating the risk of environmental geological problems, which are professional and difficult to compare comprehensively with different evaluation methods and results. The author thinks that the analysis of various mine environmental geological problems should be extended from the professional point of view to the social and economic aspects, and the latter should be the combination point of unified comprehensive comparison. It is necessary to demonstrate the destructive effects of these problems on human beings, towns, buildings, transportation, land, water sources, crops and vegetation. And then determine the specific affected body, then determine the damage level and value loss rate of the affected body according to the relevant national standards, and finally evaluate the damage loss of mine environmental geological problems from the economic point of view. For some regional evaluations where it is difficult to determine the number and damage scope of disaster points only through regional investigation, the method of hazard intensity index (hazard index × vulnerability index) of environmental geological problems can be used for evaluation. Taking its size or strength as the division standard of the disaster degree of mine environmental geological problems can achieve unified standards and comprehensive comparison. In a word, the risk analysis and vulnerability analysis of mine environmental geological problems should be organically combined, and the work should be carried out with the evaluation of its damage loss (economic value) or hazard intensity as the goal.

3.2 the relationship between coal mining, geostress and karst water

For local strata, coal mining is equivalent to manual unloading. After coal mining, the geostress in a certain range around the goaf will inevitably change, and the geostress change will inevitably affect the migration of karst groundwater in carbonate fractures and dissolution spaces (caves, caves). Firstly, due to the change of geostress, the movement speed and direction of karst groundwater in the hydrodynamic field will inevitably change; Secondly, coal mining and in-situ stress changes lead to the development of cracks, which strengthens the connection between coal measures strata and underlying Ordovician karst strata. For the mining area under pressure, the increase of coal mining displacement accelerates the decrease of karst water resources; For non-pressure mining areas, groundwater leakage in coal measures strata pollutes karst water resources. In our past work, the measured data of in-situ stress are few, and the research in this field is still blank, which should be strengthened in the future. It is necessary to study the influence mode and mechanism of geostress change on karst water migration, and the influence degree of geostress change on karst water caused by coal mining in Shanxi Province.

3.3 Land desertification caused by coal mining

Open-pit coal mining, stripping the overlying rock and soil layer and vegetation, will inevitably lead to land desertification; The ground subsidence and ground fissures formed in the goaf of underground coal mines often bring the fine particles in the soil layer into the deep underground, which makes the upper land gradually desertification, accompanied by the phenomenon of fertilizer loss from cultivated land, and it is difficult to recover by general reclamation methods. What are the formation mode, harm degree, evaluation method and prevention measures of these environmental geological problems in coal mines? It was neglected or neglected in our previous investigation work, and it is also a topic that we should pay attention to in the future.

3.4 Safety Pillar Design Issues

The vast majority of coal mines in Shanxi are located in hilly and mountainous areas, and the ravines are vertical and horizontal, which makes the load distribution in the upper part of coal seam goaf very uneven. However, the measured values for calculating the key parameters (uphill, downhill and strike) of safe coal pillars in the Rules for Coal Pillars in Buildings, Water Bodies, Railways and Alleys used in the past and now have not been implemented in most major mining areas in Shanxi. Only No.1, No.2, No.3 and No.4 coal mines in Yangquan City and Xiqu, Guandi, Zhenchengdi and Mingxi coal mines in Taiyuan City have been measured, and their representativeness is limited. Therefore, the calculation of many security coal pillars in Shanxi is inherently insufficient, because most of them cite the movement angle, and the calculation formula does not consider the uneven distribution of the upper load. In the upper part of goaf with uneven load distribution, it is easy to form shear action, which makes the expression form and disaster degree of environmental geological problems in coal mine different from that in the upper part of goaf with uniform or uneven load distribution. This is also one of the main reasons why many coal mining enterprises in Shanxi have reserved safety pillar according to the relevant requirements of the state, but in the process of coal mining, mining subsidence, ground cracks and house cracks still occur in the protected area. The research in this field should be strengthened in the future.

3.5 Determination of environmental geological problems mode

The geological and structural conditions in Shanxi are complex. Because the geological environment, coal seam, mining scale and mining mode of each coal mine are different, the lithology and thickness of overlying strata are also different, and the degree and mode of environmental geological problems in coal mines are also quite different. For example, in Datong mining area, the stability time of goaf is difficult to predict because of the hard, brittle and high compressive strength of the coal seam roof of Jurassic Datong Formation. When the pillar support conditions are insufficient, a one-time collapse earthquake often occurs, which is extremely harmful. Because the roof hardness of Shanxi Formation and Taiyuan Formation is relatively soft and the surface is flat, the surface subsidence gradually exceeds the goaf in Lu 'an mining area, and the stability time is relatively easy to predict. Coal mines distributed in mountainous areas often cause environmental geological problems of coal mines in security pillars because of uneven upper load distribution. On the basis of 65,438+0 ∶ 250,000 mine environmental geological survey, it is very necessary to carry out 65,438+0 ∶ 65,438+0,000 ~ 65,438+0 ∶ 50,000 or even larger scale survey, conduct comprehensive research, and determine the mode of environmental geological problems in various mining areas.

3.6 Establishment of analysis and prediction model

The establishment of an analysis and prediction model of environmental geological problems in a single coal mine is of great significance for grasping the development trend of disasters, making early warning and prediction of disasters, and making disaster reduction and prevention plans and macro-decisions for government departments. At present, Shanxi's work in this field is still in its infancy, and few models have been established, which is difficult to meet the needs of the work. The project of "Comprehensive Investigation and Evaluation of Mine Eco-environmental Geology in Key Areas of Shanxi Province" uses the three-dimensional visual seepage software Visual Modflow to establish a visual three-dimensional seepage model which conforms to the mine water in Anjialing Coal Mine in Pingshuo, and makes a simulation and prediction. Other projects have also established corresponding models in Cuimeng mining area of Changzhi and Xishan mining area of Taiyuan, which have attracted the attention of the central leadership. The establishment of the model should be based on the determination of the geological disaster model, using the spatial information system to study the disaster body from a three-dimensional perspective, and then combining with the long-term observation data for simulation, and finally for early warning and prediction. At present, there are many softwares used to simulate the influence of coal mining on groundwater, such as GMS, Peflow, Modflow, etc. However, there are few softwares for mining subsidence, ground fissures, collapses, landslides and debris flows, and research should be strengthened in the future.

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