The goafs generated by underground mining activities have severely induced geological disasters and caused damage to the ecological environment. How to effectively control goafs is not only a requirement for the construction of green mines, but also a requirement for the construction of ecological civilization.

At the 2015 China International Mining Conference Green Mine Forum, Ding Jixin, General Manager of Beijing Zhongse Resources Environmental Engineering Co., Ltd., who has been working on the green technology research in goafs, published “The Development and Application Status of the Hidden Dangers and Green Control Technologies in China's Goaf Areas. The keynote speech was devoted to explaining how to adopt advanced green governance techniques, eliminate hidden dangers in goaf areas, and promote green mines and ecological civilization.

Reporter: How was the mine goaf formed? What are the hazards?

Ding Jixin: The goaf area is a professional term for mine mining. Metal mines are called “empty areas” and “empty yards” in mining, and coal mining is also called “old beggars” and “old ponds.” The concepts of goafs are broad and In the narrow sense, the generalized concept of the goaf, that is, all the space dug out of the underground by human action, the narrow concept of the goaf, that is, the space left by the artificial extraction of the ore body, because the goaf is a mine The technical terminology, therefore, in the mining industry, "produced due to mining activities" should be the first feature of the goaf.

China has built more than 10,000 state-owned mines and more than 100,000 individual mines. Each year, about 5 billion tons of ore are mined underground. During the mining process of mineral resources, due to severely out-of-control stages, indiscriminate mining was once over-inflicted and untreated after mining, leading to numerous clusters of mines in China. According to a survey conducted by the State Administration of Work Safety Supervision on the goafs of 457 metal non-metallic large and medium-sized mines in 25 provinces and cities in China, as of 2008, the area of ​​goafs in China reached 432 million cubic meters. The existence of a large number of goafs and their instability have caused geological disasters such as ground subsidence around the mining area, destruction of the ecological environment, and the occurrence of a series of safety accidents.

Reporter: What are the characteristics of hidden dangers in goaf areas in China?

Ding Jixin: First, the proportion of large-scale goafs is small, but the total volume ratio is significant. According to the survey results of the State Administration of Work Safety, the scale of the evacuated areas of the 457 mines surveyed in 25 provinces and cities in the country was less than 500,000 cubic meters, 500,000 cubic meters to 1 million cubic meters, and 1 million cubic meters to 5 million. Cubic meters, 5 million cubic meters to 10 million cubic meters, more than 10 million cubic meters for classification and statistical analysis. From the statistics, we can see that from a nationwide perspective, the number of goafs is less than 500,000 cubic meters, accounting for 66.3% of the total. The number of goafs from 1 million cubic meters to 5 million cubic meters is 21.9 percent, and the number of goafs larger than 5 million cubic meters is only 4.6%. However, in terms of volume scale, the total volume of the country's more than one million cubic meters of goaf accounts for 82.5%, and the total volume of the gob of more than 5 million cubic meters accounts for 40.9%. It can be seen that the large-scale mined areas produced by large-scale mines are relatively small in number, but their size accounts for most of the country's mined-out areas. Therefore, it is necessary to pay close attention to the stability and safety of these large-scale goafs to prevent the occurrence of large-scale cluster disasters.

Second, there are differences in the structure of the number and size of goafs in different industries. The survey results are classified and statistically analyzed according to different industries in gold, black and non-ferrous mines. It can be seen from the statistical results that the number of different mined-out areas in gold mines is less than 500,000 cubic meters, and the scale is generally small, with no more than 1000. Million cubic meters of goaf. Similarly, the number of different goafs in black mines is also less than 500,000 cubic meters, which is generally smaller. However, the mined area of ​​5 million cubic meters to 10 million cubic meters and more than 10 million cubic meters of mines in this industry also occupy a large proportion. In contrast, there are relatively small differences in the number of goafs of different scales in non-ferrous mines, ie non-ferrous mines have a variety of types of goafs. In the goafs of the three industries, except for the largest number of goafs less than 500,000 cubic meters, the number of goafs with a volume of 1 million cubic meters to 5 million cubic meters occupied the second (but its total volume was All kinds of goaf occupy the first place). Therefore, the characteristics of the goaf area with a volume of 1 million cubic meters to 5 million cubic meters, hazards, and prevention and control require more attention.

Reporter: What are the characteristics of subsidence disasters in China's goaf areas and what are the damages to the ecological environment and safety production?

Ding Jixin: The subsidence disasters in gob areas are characterized by concealment, abruptness, and large damage. The insufficiency of the mined-out area will directly lead to the occurrence of ground subsidence, ground fissures, collapses, landslides and other geological disasters, damage to the infrastructure of local roads, bridges, buildings, etc.; destruction of farmland, crops, forests, etc.; changes in groundwater quality. Hydrological systems such as water volume and flow status. In recent years, due to the subtle and sudden characteristics of subsidence in goafs, many geological hazards have occurred. This can be seen from some of the disasters that have been reported. For example, on May 22, 2002, five people were missing from lead-zinc mines in Nanchang, Lanping County, Yunnan Province due to a sudden collapse of the goaf. On December 26, 2005, an iron ore mine at the Duoli Industry and Trade Co., Ltd. of Anyang, Henan Province, collapsed due to a sudden collapse of a goaf, forming a collapse pit measuring approximately 100 meters long, 50 meters wide, and 60 meters deep. On August 19, 2006, the Tiande gypsum mine in Shimen County caused a large area of ​​collapse due to a large area of ​​goaf. The area of ​​the collapse area was approximately 18,000 square meters, and the center of the collapse area sank approximately 15 meters, resulting in 9 deaths. disaster. On February 13, 2011, the Datongkeng Mining Area in Dingshu Village, Tingzu Town, Ezhou City suddenly collapsed and formed a large deep pit of about 5,200 square meters. According to incomplete statistics, there have been more than 200 collapses of large-scale goafs in the country, with a total area of ​​over 700,000 hectares and economic losses exceeding 50 billion yuan.

Reporter: Governance has not been moved, and the foundation is first. What basic work should be done before the collapse of goafs?

Ding Jixin: First, investigate and identify the goaf. In order to eliminate the hidden dangers of the goaf, it is necessary to carry out the investigation of the goaf area to ascertain the distribution, scope and scale of the goaf. Only with a full understanding of the spatial distribution of the goafs can governance be targeted.

The identification of goafs is mainly based on surveys of engineering geology and mining conditions, engineering drilling, and geophysical exploration, supplemented by deformation observations and hydrological tests. During the identification process, a detailed survey of engineering geology and mining conditions should be conducted first to determine the location, extent, and surrounding engineering geological characteristics of the goaf. Then, based on the terrain and geological conditions of the survey area, appropriate geophysical exploration methods should be selected. Detection. With the development of science and technology and the application of computer technology, many new methods and technologies have been continuously introduced into the field of geophysical exploration, opening up a vast space for the further development of geophysical methods. For example, commonly used methods for geophysical exploration include: gravity exploration method, which uses the quality loss or surplus of underground goafs, and observes the gravity anomalies caused by them on the surface to determine the distribution, size, and boundary of underground goafs. Electromagnetic method, that is, according to the different conductivity of the geological body and magnetic permeability, the use of electromagnetic induction principle to detect the gob area, including high-density resistivity tomography, transient electromagnetic method, very low frequency electromagnetic method and geological radar Law and other methods. Seismic exploration, that is, the use of the difference in elasticity and density of subsurface media to detect goafs. Finally, according to the investigation and geophysical exploration results, the drilling was conducted for verification to comprehensively analyze the geological, geophysical and drilling data, and the distribution, extent and scale of the goaf were delineated.

The second is the analysis and evaluation of the stability of the goaf. After identifying the goaf, it is necessary to perform calculation and analysis as to its stability, whether and how to take effective treatment methods. At present, most of the evaluation of the stability of the goaf area is based on qualitative assessment. Quantitative assessment methods are gradually developed and applied. For example, theoretical experience formula method, prediction method, block theory method, numerical simulation method and safety evaluation method can be used. The theoretical experience formula method calculates the height of the maximum falling zone and the height of the maximum water-flow fractured zone in the goaf, and uses the height of the maximum water-induced fractured zone as an indicator for evaluating whether the goaf forms ground collapse and cracks on the surface. When the buried depth of the gob area roof is less than the height of the maximum water-flow fractured zone, the horizontal projection range of the corresponding gob area is a destructive influence zone, which is an unstable goaf area. The prediction method is mainly used to calculate the capacity of the roof, the deformation of the remaining ground surface, the stability of the residual space, and the scope of surface destruction. The development of computer technology has promoted the application of numerical simulation methods in the stability calculation of the goaf. For example, finite element method (such as ANSYS), finite difference method (such as FLAC) and discrete element method (such as UDEC) can be used, which provides effective tools for geological and mining engineering analysis.

Reporter: What is the development and application of green control technology in goaf areas in China?

Ding Jixin: For the prevention of goafs, a prevention and control system for "pre-disaster prevention and post-disaster management" should be constructed. In pre-disaster prevention, the principle of “avoidance” should be fully followed: on the one hand, preparations for control of goafs should be made, and important project facilities such as villages and townships should be reasonably avoided; on the other hand, when building sites are selected, they should be avoided as much as possible. It is advisable to mine open areas, especially in mining areas with steep slopes.

In the case of underground mining of existing buildings, or when buildings are going through mining areas that are being mined and where they cannot avoid empty areas, various treatment technologies can be used to deal with them. At present, there are mainly caving method, filling method, support method and closed isolation method, and a combination of these four basic methods. Each of these methods has its own advantages and disadvantages. When it is used in a specific way, it should select the appropriate method according to the specific circumstances of the goaf and its surrounding areas, give full play to its superiority, and overcome its limitations. The main and commonly used methods are described below:

The roof caving method in the goaf area is to collapse the surrounding rock and fill the goaf, so as to fill the gob area or form a cushion layer, which can prevent the air waves caused by the sudden collapse of the goaf area. This method is simple in operation, low in the cost of control, and can effectively prevent a sudden collapse of a large area of ​​the goaf. It has been widely used in mine filling.

The goaf filling method is the most effective and environmentally friendly method of empty space treatment. It fills the goaf with filling material, supports the surrounding rock in the goaf, prevents the displacement and instability of the roof, and achieves the purpose of controlling the collapse. Conventional filling materials include pastes, vermiculite, aeolian sand, foamed cement, and high-water materials, as well as a variety of man-made special materials such as Remi filling materials in recent years. According to the filling position, filling volume, filling power and filling materials, the filling method can be divided into various types. In the filling process, how to eliminate the filling of the remaining goafs and make the filling and the top plate knots is the key in the construction process, and it is also a technical problem that the mines at home and abroad have been widely concerned and are urgently required to solve.

Gob grouting treatment technology is part of the filling method. According to the construction process, it can be divided into goaf strip filling technology, grouting filling technology in the falling area and partitioned grouting and filling technology. At present, when there are surface highways, railways and other important buildings passing through goafs, many projects have adopted methods of grouting filling and reinforcement in goafs.

In short, the management of goafs should pay full attention to pre-disaster prevention and fully follow the principle of “avoidance” for planning and design. At present, the prevention and control of goafs in our country is moving towards a quantitative analysis. The treatment of empty areas and the exploitation of resources are carried out in a coordinated manner. With the development of science and technology, the measures for preventing and controlling goafs will become more and more abundant, scientific, and economic.