First, the impact of coal development on water environment and its protection technology
1. Impact of coal development on water environment
Coal development will have a great impact on the water environment, mainly in the following aspects:
(1) The relationship between groundwater recharge and discharge has changed. This change has taken place in almost all mining areas, and it is very obvious. For example, before mining in Pingdingshan mining area, groundwater supplemented surface water, and there were more than 40 springs exposed in front of Pingdingshan Mountain. Zhanhe has flowing water all the year round. However, with the exploitation of coal, the groundwater level has been declining, surface water has replenished groundwater, spring water has disappeared, and rivers are often cut off. The recharge relationship between aquifers has also changed. For example, before mining, there was a layer of bauxite mudstone with a thickness of more than 10 m between Carboniferous limestone and Cambrian limestone, and there was no hydraulic connection between them. However, in the process of mining, bauxite and mudstone in some areas were destroyed, which made them interconnected and formed a recharge relationship from CAMBRIAN limestone aquifer to Carboniferous aquifer. For drainage, groundwater is discharged in the form of spring water and river water before mining, and in the form of mine drainage and artificial mining after mining.
(2) Dynamic change of groundwater. This change is manifested as the change of layer flow direction turbulence, and changes with the temporal and spatial changes of mine drainage. Generally speaking, the closer to the drainage roadway, the greater the change of water flow state, and vice versa. Similarly, the flow pattern changes greatly in the initial stage of drainage, but with the continuous drainage and the stability of flow rate, the flow pattern changes less and less. The criterion for judging the change of water flow state is the increase of Reynolds coefficient in the region. When the Reynolds coefficient is greater than 100, the groundwater flow becomes turbulent.
(3) Dynamic changes of groundwater. This change is manifested in two aspects: on the one hand, it is a micro-dynamic change, and its changing law is similar to that under undeveloped conditions. Affected by rainfall and micro-groundwater tide, but only the superposition of trend changes; On the other hand, it is the dynamic change of macro-trend, and its changing law is that the water level keeps falling, and the water volume gradually decreases and tends to be stable.
(4) Environmental changes of water-rock interaction. In coal mine roadway, fresh rock is exposed to ventilation environment, which changes the original reducing environment into oxidizing environment and increases the Eh value. If the sulfur content in coal and the neutralization ability of rock reach a certain proportion, acidic water will be formed and the pH value will be reduced. In this way, the change of Eh value and pH value will cause the change of water-rock interaction environment.
2. Environmental problems caused by mine development
(1) Water resources are exhausted. Due to the change of water environment caused by mining, a large number of water supply systems are scrapped and water sources cannot be used. This situation has reached a quite serious level. The comprehensive table of water supply survey results in China coal mining area shows that 86% of mining cities are short of water.
(2) Water pollution is serious. The change of water environment leads to the strengthening of rock leaching and the increase of toxic and harmful components in water. According to statistics, 18 of the 30 rivers with a distance of more than 500km are polluted by coal mining. Such as Jinsha River in Sichuan, Taizi River in Liaoning, Wenhe River in Shandong and Fuyang River in Hebei. Rivers in coal mines are more serious, and almost all rivers are polluted. Water contains more organic and inorganic suspended solids. The pH value is very low, and the contents of Ba2+, F-, phenolic compounds, iron, germanium, magnesium and radioactive substances seriously exceed the standard. The water supply system is polluted, aquatic organisms die, the land is hardened, vegetables and grain are polluted, which brings serious consequences to people's production and life.
3. Water environmental protection technology
Among the four major impacts of coal exploitation on water environment, the recharge-discharge relationship, flow pattern and dynamic changes are caused by the destruction of aquifer structure and the change of water cycle law, while the change of water-rock interaction environment is caused by the change of hydrochemical phase balance. Water environmental protection technology starts from these reasons to prevent the side effects of coal development on water environment.
(1) Prevent or mitigate the destruction of aquifer structure. Coal mining makes the surrounding rock deform and move. Caving zone, bending zone and fracture zone appear in the overlying strata of goaf until the surface. The aquifer structure of the stratum where the strata move is destroyed, the relative relationship between the aquifuge and the aquifer changes, the water storage structure is destroyed, the occurrence conditions of groundwater change, the water storage capacity of groundwater decreases, the groundwater level is greatly reduced, and the flow pattern and dynamics have also changed obviously.
The sudden destruction of aquifer structure generally occurs when inappropriate coal mining methods are adopted. In order to prevent or reduce the damage of aquifer structure, the following measures should be taken: ① try to adopt filling mining and strip mining, and at the same time appropriately reduce the mining thickness of the first and second floors, and require that the exposed area of the roof should not be too large at one time; (2) When the roof strata are hard and not easy to fall, artificial caving should be adopted; (3) Investigate the geological mining data such as goaf, abandoned roadway and karst in the old kiln, so as to prevent the aquifer structure from being suddenly destroyed due to draining the accumulated water in the old kiln and the water level in the karst aquifer.
(2) Using the change of water cycle law. In order to meet the needs of coal mining, external water supply and mine adjustment have been increased. At the same time, the aquifer structure is destroyed in the process of coal mining, which leads to great changes in the water cycle law and water balance conditions, thus changing the water environment conditions in the mining area. Taking Chi Jiujian and others' comparison of water circulation law before and after coal mining in Shanxi as an example, we can draw the following conclusions: under the condition of coal mining, the confluence conditions of surface water in the basin are constantly changing, and the leakage recharge is increasing; The recharge runoff and discharge conditions of groundwater are different, and surface runoff, basic flow and undercurrent are partially or completely converted into pit water; The movement of groundwater changed from lateral movement to vertical movement before coal mining, and the groundwater was mainly discharged from the previous basic flow and undercurrent, which changed into mine drainage.
From the point of view of water environment protection, it is necessary to design the combined holes of "water purification and drainage roadway" and "drainage and drainage" under the mine according to these water circulation characteristics, and design the water supply system to realize "combination of drainage and supply" or "combination of drainage and supply" to solve the problem of water resource depletion.
(3) Protect the hydrochemical phase balance. The change of water-rock interaction environment is caused by the change of hydrochemical phase equilibrium. Taking the Pe-pH hydrochemical phase equilibrium of Fe as an example, the equilibrium relationship of water-rock interaction under different Pe and pH conditions is reflected. Other metal ions have similar hydrochemical equilibria. Therefore, the change of hydrochemical phase equilibrium will inevitably lead to the change of water-rock interaction environment and direction, and also lead to the dynamic change of metal ions in water.
The decisive factors of hydrochemical phase equilibrium are Pe and pH. Therefore, it is necessary to deeply analyze the dynamic change conditions of hydrochemical phase equilibrium, design parameters to protect hydrochemical phase stability, realize the control of mine water pollution "source area", and adopt different control methods according to different conditions to realize the comprehensive utilization of mine water.
Mature technologies to realize comprehensive utilization of mine water are: ① Coagulation and sedimentation; ② electrodialysis; ③ neutralization method. The technologies to be further studied and developed are: ① two-stage contact biological oxidation purification technology; ② Geo-ecological engineering technology; ③ Closed circulation technology of washing water; ④ Sand purification process.
Second, the impact of coal development on the land environment and its protection
1. Impact of underground mining on land
(1) Surface subsidence. More than 95% of China's coal production is mined by underground workers, and surface subsidence has become the main form of land environmental impact. According to the different topography, landforms, natural environment, geology and mining conditions, the impact of ground subsidence on the land environment can be divided into three categories: ① hilly and mountainous subsidence areas. After mining, there is no obvious change in topography and geomorphology, basically no water accumulation, which has little impact on the land environment and causes environmental engineering geological problems such as landslides in some areas; (2) Plain subsidence land with medium and low water level. Due to the deep groundwater level, there is a small part of perennial water accumulation on the ground after mining subsidence, and seasonal water accumulation occurs in the gentle slope area, resulting in soil erosion and salinization. Most of these mining areas are located in most plain mining areas north of the Yellow River. (3) High-water level plain subsidence land. Due to the high groundwater level, most of the surface water is accumulated all the year round, which leads to the failure of cultivated land and seasonal water accumulation in most slopes, causing serious damage to surface farmland water conservancy facilities. Most of these mining areas are located in the central and eastern parts of the Huang-Huai-Hai Plain.
(2) Coal gangue occupies land and affects the ecological environment. Coal gangue is the largest solid waste discharged from coal mines, and the impact of coal gangue waste accumulation on the environment is mainly: occupying a lot of land, affecting ecology and destroying landscape; The spontaneous combustion of coal gangue pollutes the atmosphere, and the coal gangue mountain will also explode and collapse, blocking traffic; Leaching water from gangue hill makes the water quantity strongly acidic or contains harmful and toxic elements, which pollutes the surrounding soil and water body and harms the growth of crops and fish.
2. The impact of open-pit mining on land
(1) Excavate and destroy the stripped land. The most direct influence of open pit mine on land environment is the stripping of topsoil and rock above coal seam in mining area, so it has the greatest damage to topography and the whole ecological environment in mining area. For example, more than 20 well-known open-pit mines in northern China, such as Fushun Open-pit Mine, Yuanbaoshan Open-pit Mine and Fuxin Open-pit Mine, have to excavate an average of 0. 10 ~ 0. 16km2 of land. In 1949 ~ 1989, the total area of excavated land in China's open-pit mines reached 9600 km2, and in 1990 ~ 1998, the annual growth rate of open-pit mines in China was 1200km2, that is, the total area of excavated land by the end of last year. After 2000, it will increase by 8-9% every year, and by 2020, 3600km2 of land will be excavated and destroyed every year.
(2) The garbage dump covers an area. According to the investigation and estimation of more than 20 well-known open-pit coal mines in the north, the land occupied by the dump pressure per 10,000 tons of coal is 0.10 ~ 0.22km2. From 1949 to 1989, the national open-pit coal mine dump covers an area of about 8000km2 1990 ~ 1998, with an annual increase of 1000km2, which was occupied by the end of last year. After 2000, it will still grow at an annual rate of 8-9%, and it will reach 3000km2 by 2020.
3. Land environmental protection of coal development
(1) mining technical measures. Mining technical measures refer to the prevention of environmental damage of mine surface land by reasonably designing mining methods from the perspective of land environmental protection and in line with the principle of paying equal attention to coal mining and environmental protection. The technology includes: ① setting protective coal pillar; ② Roof management by filling method; ③ Strip mining; ④ Eliminating the influence of mining boundary; ⑤ Coordinated mining, etc. These technologies are discussed and required in detail in the study of "Three Downs" coal mining, so I won't repeat them in this paper because of the space limitation.
(2) Land reclamation technology. Land reclamation in mining areas does not insist on restoring to the original state before the collapse, but on the basis of the specific conditions of each mining area, based on the principle of combining local conditions with economic, environmental and social benefits, it can be developed into entertainment places. According to the purpose of reclaimed land, it can be divided into seven categories: building reclamation, agricultural reclamation, forestry reclamation, fishery reclamation, grassland reclamation, amusement park reclamation and ecological agriculture reclamation. The specific technology can be found in the research status and trend of mine land reclamation.
Third, the mine geothermal disaster.
1. Present situation of mine heat damage
Mine geothermal disaster is mainly manifested as mine thermal disaster. Before liberation, due to the limited mining depth in China, there were few mine heat hazards in various mines; After liberation, China's mining industry developed rapidly, and a large number of new mines were built. The old mines gradually extended to the deep, and some mines were gradually threatened by underground hot water and geothermal water. With the increase of mining depth, the underground temperature and humidity gradually increase, which seriously affects the health and work efficiency of workers, thus forming a mine heat hazard. The control and research of mine heat damage in China began in 1950s. With the increase of coal mines with heat damage in coal system, the Ministry of Coal Industry held five technical symposiums on mine ground temperature research and cooling work from 1975 to 1978 to exchange temperature measurement and cooling technology. The problem of mine heat damage has appeared abroad. Peru, South Africa, Japan, the United States, Zambia, Mexico, Nicaragua, the former Czechoslovakia, the former Soviet Union, and the former Federal Republic of Germany all have mine heat damage problems, which have been studied and dealt with to varying degrees.
2. The classification of mine heat damage
(1) Classification of geothermal types in mining areas by geothermal chamber of Institute of Geology, Chinese Academy of Sciences. 198 1 year, the geothermal room of Institute of Geology, Chinese Academy of Sciences edited and published the book Introduction to Mine Geothermal, and classified the geothermal types in mining areas. Based on the actual data of several mining areas in eastern China, this book divides the known mining areas into six types for geothermal research and heat damage prevention in mining areas. That is, basement uplift type, basement depression type, deep fracture type, intense groundwater activity type, deep circulating hot water type and sulfide oxidation type.
(2) Classification of geothermal types of Yanrusui deposit. 198 1 year, Yan Rusui classified the geothermal of mineral deposits in the article Some Problems of Geothermal Work of Mineral Deposits published in the third issue of Hydrogeological Engineering Geology (198 1). His division principle is: it can not only fully reflect the clear concept of the occurrence state and genesis of geothermal energy, but also facilitate the exploration of the main characteristics of different geothermal types in exploration methods and cooling measures, regardless of mining methods. According to this principle, combined with some practical data, the geothermal types of the deposit are divided into three types: hot water type, rock temperature type and mixed type.
(3) Classification of geothermal types of Wangrui deposit. From 65438 to 0999, in the book "Exploration, Prediction and Treatment of Mine Thermal Hazards" edited by Wang Rui, according to the principle of dividing geothermal occurrence, the carrier nature of deep heat source conducting heat to the upper crust and mine thermal hazards caused by human factors, mine thermal hazards were divided into normal geothermal warming type, geothermal anomaly type (including abnormal subtypes of rock temperature and geothermal water) and carbon and sulfide oxidation heat type.
3. Prediction and evaluation of mine thermal damage in general survey and exploration of mineral deposits.
In recent years, due to the increasing number of mines with thermal hazards, it is as important as geology, hydrogeology and engineering geology to investigate and study the geothermal conditions of solid deposits in general survey and exploration. The national standard GB12719-91issued by the State Bureau of Technical Supervision and compiled by the State Bureau of Mineral Reserves requires the investigation and study of ground temperature, but it is not specific and comprehensive enough. In the book "Prediction and Treatment of Mine Thermal Hazard Exploration", Wang Rui put forward the arrangement and requirements of geothermal work in each exploration stage according to various examples of mineral geothermal work investigation and research, and studied some technical methods that should be paid attention to in mineral geothermal investigation and research.
4. Mine heat damage control measures
At present, countries are trying to study the prevention and control measures of mine heat damage. The main contents of the study are: ventilation cooling, heat source isolation to prevent heat diffusion, mechanical refrigeration and air conditioning cooling, dehumidification cooling and humidification cooling.
(1) Ventilation for cooling. The focus of ventilation cooling research is to develop and optimize ventilation system, increase ventilation volume and strengthen ventilation management.
(2) isolate the heat source. The methods to isolate the heat source include: draining the aquifer (zone) in front of the middle section, strengthening the management of hot water in the cave, isolating the heat source by fully filling the goaf, and using heat insulation materials for heat insulation.
(3) Mechanical refrigeration underground air conditioning. Brazil, South Africa, the former Federal Republic of Germany, Belgium, India, the United States and Japan have already used this method. The use of mechanical refrigeration and air conditioning in mines in China began in the early 1960s.
(4) Dehumidification refrigeration, humidification refrigeration. According to Japanese data, in high temperature mines, the humidity decreases by 1.7%, which is equivalent to a temperature decrease of 0.7℃. When the underground humidity is high, the humidity reduction measures are effective. Humidification and cooling means that when the relative humidity is less than 20% ~ 30%, water is sprayed into the intake air to increase the water content of the air, and the air temperature is reduced by using the vaporization heat absorption of water.
5. Comprehensive utilization of mine geothermal energy
It includes the comprehensive utilization of underground hot water and geothermal water.
The industrial utilization of underground hot water in mines has not been carried out at present. The main reason is that the newly-built mining area is far from the industrial zone and is not easy to use; The management system and drainage project in the old mining area have been formed, which is inconvenient to transform. In terms of domestic water use, the hot water supply system was built in the pit of Chenzhou uranium mine in Hunan Province in July 1979, which achieved good economic and management effects.
The utilization of geothermal energy in mine is mainly to use abandoned roadway and mined-out area (with a certain length) as the air inlet duct of mine in winter to improve the air inlet temperature and prevent the shaft and roadway from freezing.
Four, the mine collapse disaster
1. Basic characteristics of mine collapse
In the process of mining, a large number of ores and brazing stones are mined from underground, and the underground space formed will be filled with rocks above and around it sooner or later, which often leads to extensive and serious mine collapse disasters on the ground. According to its shape, it can be divided into two categories: the first category is funnel-shaped collapse pit and stepped fracture, which are serious and can occur suddenly, causing unexpected damage to the ground, but the damage range is small; The second type is a flat subsidence basin formed on the ground, which is mainly caused by deep steep coal seam or gently inclined coal seam with the ratio of mining depth to coal seam mining thickness greater than 20. The formation process is slow, and it takes 1 ~ 3 years from the beginning of settlement to the final stability.
Since the beginning of this century, countries all over the world have invested a lot of money and technical strength to carry out the research on mine collapse. Russia, Poland, Germany, Australia, Britain, Canada, the United States and so on. In this paper, the theory of mine collapse and "three-underground mining" (that is, mining under buildings, underwater mining and railway mining) has been deeply studied and some achievements have been made. Since 1950s, China has established surface observation stations in major mining areas such as Huainan, Kailuan, Fushun, Fuxin, Fengfeng, Datong, Hegang, Yangquan and Benxi, and conducted research on mining subsidence. Over the past 40 years, China has not only accumulated the measured data of thousands of observation lines, but also further understood the basic law of mine collapse, and put forward the observation method with China characteristics.
The study of mine collapse involves a wide range of scientific research fields, which requires knowledge of surveying and mapping, mining, mechanics, computer, architecture, geology and other disciplines. In the past two or three decades, the achievements of these disciplines, especially the new achievements of computer science, have been continuously introduced into the study of mine collapse, which has made the study of mine collapse develop rapidly. At present, mine subsidence has become an independent frontier discipline, but compared with the needs of national economic development and modernization, the research on mine subsidence is still not enough, and more work needs to be done in theoretical research, scientific experiment, application technology development and simulation, testing technology research and so on.
2. The formation mechanism of mine collapse
So far, there is no recognized theory about the mechanism of mine collapse. Here are some popular assumptions.
(1) arch caving theory and arch pressing hypothesis. Arch caving theory was put forward by Russian M.M. proto Djakov in 1907; Germans Ha Yuan and Gillette Zell put forward the pressure arch hypothesis in 1928, which supplemented the arch-hole hypothesis.
This hypothesis borrows from the arch effect of roadway roof, and holds that space is formed in underground strata during mining, so that the overlying strata fall down until an approximately arch roof is formed. However, the pressure arch formed between the roof and the floor will increase with the continuous advancement of the working face until the arch reaches the ground. This assumption is basically consistent with the hard and simple overlying strata, but it is actually only suitable for explaining the collapse of small-span driving roadway and goaf, but it is difficult to explain the actual situation of the destruction and caving of overlying strata in large-scale goaf.
(2) Cantilever beam (slab) caving theory and caving filling theory. The theory of cantilever beam (plate) caving was put forward by Schulz and storck. It regards the roof above the working face and goaf as a beam or plate, which falls behind for the first time and one end is fixed on the front rock, only bending without breaking. When the cantilever beam (plate) is long, it will break periodically and produce periodic pressure. The theory of cantilever beam (slab) caving accords with the roof caving caused by large-scale mining.
According to the theory of falling rocks, falling rocks can naturally collapse and fill the space of mined-out areas, which limits the development of roof caving and tends to be stable.
In fact, these two theories can cooperate with each other to explain long-arm mining and large-scale overburden failure.
(3) The conclusion of caving rock hinge. Russia's Kuznetsov proposed that when the overlying strata on the working face collapsed, the rocks in the regular moving zone above it kept certain contact, twisted into multi-section hinges, and sank regularly above the goaf.
④ Masonry Liang Pingheng said. Chinese scholars have developed the conclusion of caving rock hinge and put forward the theory of masonry beam balance, which holds that the working face area is greatly affected by fault zone. After the fracture zone is broken, the rock blocks are masonry-like, and the balance structure formed is called "masonry beam", which is actually an arch structure. After mining, the strata in the overlying fault zone are divided into three areas: the wall support area, the separation area and the re-support area. The structure of each layer is balanced by the friction generated by the horizontal extrusion force between the fractured rocks.
3. Mine collapse disaster assessment
(1) The form of surface collapse caused by mining. There are three forms of surface movement and subsidence caused by mining: subsidence basin, cracks and steps, and subsidence pit.
In the area affected by mining, the surface elevation drops, and a subsidence area with an area much larger than the goaf is formed above the goaf, which is called a surface subsidence basin or a surface moving basin.
The cracks caused by underground mining mainly appear at the outer edge of the collapsed basin, with a width of several millimeters to several tens of centimeters and a depth of several tens of meters, and some even communicate with the goaf. Sometimes there are gaps on both sides of the cracks, forming stepped or even graben-like open cracks and annular destructive gullies.
When mining steeply inclined coal seam, shallow inclined coal seam or mining depth is very small or mining thickness is very large, collapse pits and funnel pits will also occur on the surface due to uneven mining thickness and inconsistent failure height of overlying strata when using room and pillar coal mining or chamber hydraulic coal mining.
(2) Prediction of surface subsidence range.
① Full mining subsidence range. The full mining subsidence range is determined by the full mining angle φ. The full mining angle is the angle between the flat bottom edge of the mobile surface subsidence basin, the projection point of the surface horizontal line and the side of the coal seam in the goaf on the main section. For inclined coal seam, the full mining angle is φ 1 when going downhill, φ 2 when going uphill and φ 3 when going.
② Determination of surface subsidence boundary. Generally, the boundaries of subsidence basins are divided into three categories: the outermost boundaries of subsidence basins, the dangerous activity boundaries of subsidence basins and the fault boundaries of subsidence basins.
4. Prevention and control of mine collapse disaster
In the book "Introduction to Subsidence" edited by Ji Wanbin, the mine subsidence areas are comprehensively summarized and classified based on the form of subsidence amplitude, the topographic unit of surface subsidence as a sign and the degree of geological disasters as a measure. * * * is divided into two categories and six types, one of which includes subsidence area, subsidence area and serious subsidence area; The second category includes hanging rock, soil collapse area and collapse danger area, open-pit slope collapse area and collapse danger area, Qianshishan, waste rock pile collapse area and collapse danger area.
The prevention and control of mine collapse disaster is an extremely complex system engineering, involving many factors and aspects. Therefore, combining with the mineral reserves and geological conditions in the mining area, it is very important to formulate a reasonable mining plan and prevention plan. Only by adopting the policy of giving priority to prevention, comprehensive management and ensuring key points, rather than rushing to deal with them, can the limited manpower and material resources play a greater role. It is necessary to solve the relocation and reconstruction of severely affected towns and the living difficulties of residents, reuse the land and return the government to the people.
The development of mine collapse disaster is expanding, and the later settlement and deformation should also be considered in the treatment work; Only by coordinating ground management with underground mining can we get twice the result with half the effort.
In 1994, Ji Wanbin summarized the comprehensive treatment plan for subsidence area and serious subsidence area, including the following measures:
(1) The technical measures to reduce the collapse of underground workers include filling mining method, strip mining method, room-and-pillar mining method and overlying rock fracture zone.
(2) Grouting method in fractured zone.
(3) Reuse the land and return the government to the people. The methods include: changing dry land into paddy field, underground and aboveground drainage-lowering the height of underground water level, digging deep shallow or surrounding basins for farmland, developing "three-dimensional" ecological agriculture in subsidence areas, backfilling gangue and reclamation.
(4) Village relocation. Waste rock can be used for backfilling, dynamic compaction and manual relocation of foundation to design anti-deformation buildings.