Progress and trend of theoretical research and practice in open-pit to underground cover (2)

According to the theory of ore-delanging ellipsoid, the interaction between the ore during the mining is studied, and different kinds of scattered bodies are used to conduct the seepage experiment at the interface of the rock and rock to study the leakage. Finally, from the theory of ore-delivery ellipsoid and the prevention and drainage Comprehensive determination of the reasonable thickness of the cover layer.

1.3 Numerical analysis of the cover layer The numerical study of the mechanical properties of engineering rock and soil is divided into the study of continuum theoretical mechanics and the study of discontinuous medium theoretical mechanics [20]. The former research object is mainly a complete whole. The research methods mainly include finite element method and fast Lagrangian method. The calculation tools used are FLAC, GTS, ANSYS, etc. The model is composed of multiple modules and connected by The nodes connect the modules into a complete geometric model, and apply the stress to the nodes. After the refinement process, the external deformation and internal stress changes of the model are studied by computer. The latter research object is a collection consisting of a broken whole or a plurality of discrete mutually independent particles with mutual contact faces (points). The research methods are mainly block analysis and discrete element method, the main tools used. It is DDA, PFC, FLUENT, UDEC, etc.; the method is mainly to establish a large number of particles to make point or surface contact, set particle properties and contact conditions in advance, generate a whole bulk model, and then calculate the bulk by computer In the movement process during the movement, the changes of physical properties and the changes of physical and mechanical parameters of the bulk during the movement are recorded, and the nonlinear problem of the dispersion is studied.
The open-air to underground cover layer belongs to the discontinuous medium in the rock and soil, and the discrete element method is mainly used. The Discrete Element Method (DEM) is a numerical method developed by the American Academy of Engineering and the Royal Academy of Engineering, PerterCundall in the 1970s to study the properties of bulk, giving discrete units the properties required for experiments and making reasonable faces. Contact connection, the model is established by Newton's second law, the nonlinear static problem is transformed into a dynamic problem, and finally the integral solution of the nonlinear physical quantity is obtained, and then the physical parameters and changes of each unit at any time are obtained.
China's research on discrete elements is lagging behind. In the late 1980s, Professor Wang Yongjia first quoted the concept of discrete elements in the first national rock mechanics numerical calculation and model experiment discussion, and introduced the discrete element method into China [21]. Subsequently, the method has developed rapidly in the research of geotechnical mechanics in China, and has been widely applied in the fields of mining, metallurgy, agriculture, food, chemical, pharmaceutical and environmental. At present, the most famous discrete-method commercial program development company is the ITASCA international engineering software development company headquartered in Minnesota, USA. The founder is the Charlesfairhurst academician who is known as the father of rock mechanics. The company currently has 14 companies in 11 countries around the world, not only with a number of high-level research institutions in China, but also provides high-end analysis software, and has a consulting company in China responsible for the completion of multiple Engineering in the field.
At the end of the last century, with the participation of the leader of the academician PerterCundall, ITASCA developed a software for the study of bulk, the Particle Flow Analysis Program (PFC) [22]. With the widespread use of the software worldwide, ITASCA is constantly updating its programs and updating its version. The latest version is 5.0. The particle flow analysis program (PFC) can analyze the nonlinear problems such as physical quantity changes and mechanical properties of discontinuous bulk media in essence. Although there are certain limitations [23], it is the current study to open the ground. The most ideal and most common means of covering the law of the movement of loose bodies.
University of Science and Technology Beijing Liu Zhinuo use PFC numerical simulation software for East Daye iron ore mining plant design modeling experiment of the six groups of different structural parameters of stope to the ore dilution rate of release of economic indicators, the analysis of the ore process, The optimal structural parameters of the ore-mining were found out, and the reasons for the instability of the recovery rate and the depletion rate in the numerical experiments were analyzed in detail. The variation of the imbalance between the particles during the ore-boring process was also studied [24].
Based on the stochastic medium ore-mining theory, combined with the actual production of Xingshan Iron Mine, PFC simulates the ore-mining process under overburden. The ore discharge rate and depletion rate are technical indicators, and are divided into three groups according to the thickness. Observe the movement of the cover and the release of the ore to find a reasonable thickness of the cover [25]. On the basis of the above conclusions, the particle size of the overlying layer is divided into three groups to find out the reasonable particle size for the ore mining, which provides a theoretical basis for the actual production of the mine.
Based on the theory of discrete element method, Northeastern University Wang Peitao uses PFC to numerically simulate the nugget with non-pillar sublevel caving method with cover layer using the ore recovery rate and depletion rate standard. Two kinds of ore-mining schemes were compared in the same case [26]. The results show that the recovery rate of plane ore mining is higher than that of ore surface. On the basis of the experimental conclusions, by changing the coefficient of friction between particles in the grading plan, on the other hand, the means to improve the ore recovery rate and the influence of the internal friction angle on the ore-mining results are found.
In addition to the above studies, many scholars use the discrete element principle to stabilize the mining ground pressure [27], surrounding rock stability [28], ore drop impact [29], slope surrounding rock stability [30-37], mining process The optimization [38-42], the rationality of low-depleted ore mining [43], surface subsidence [44], and the analysis of the movement and mechanical safety of the mill media [45-50] have also been studied and obtained. A rich result.
2 Application practice of many mines were from open pit to underground mining, the first century belongs to Jiangsu Phoenix Mountain Iron 1960s, other transitional Copper Mine copper mines such as Anhui official, white silver bow Copper Mountain, Gansu, Jiangxi Liangshan Iron Mine and so on. After a lot of theoretical research and long-term engineering practice in China, the research theory of the overburden has already played a guiding role in the actual production of many mines, and some even obtained good benefits, so that the mine can achieve a safe and smooth transition.
From the west to the east, the WISCO Daye Iron Ore Body is the six major ore bodies of Tiemenkan, Longdong, Jianlinshan, Xiangbishan, Shizishan and Jianshan. In 2001, the eastern open-pit mining ended and entered underground engineering. Beijing University of Science and Technology studies the cover layer and its porosity based on the surface air leakage problem, which provides a theoretical basis for the determination of the thickness of the mine cover [51-52]. Based on the stability of surrounding rock, Wuhan University of Science and Technology provides a plan for the formation of a cover layer (collapsed surrounding rock and slope cut) [53-55], in which the cover layer formed by Lion Rock is -48 to 0 m. After the formation of the cover layer, the ground pressure monitoring is carried out, and the earthquake prevention problem, the recovery sequence and the surrounding rock stability during the formation of the cover layer are analyzed to prepare for improving the quality of the ore discharge.
Hegang Miaogou Iron Mine was built in 1986 and put into operation in 1989. The maximum production scale of open pit mining is 2.6 million t/a. In 2010, Sinosteel Engineering Design and Research Institute evaluated its open-pit mining reserves and years, and it can continue for 10 years. The open-pit mining pit is set at 372m level. In 2011, the open-pit to underground mining design was carried out. Based on the overlay stratification theory [19], the North China University of Technology provides two types of overburden formation schemes for the Miaogou iron ore based on the mining engineering specifications and the conditions and conditions of the mine site: the chamber blasting slope scheme and the dumping site. Waste rock backfilling program. After comparing and analyzing a series of problems such as the construction period, investment quota and safety factor of the two schemes, it is finally determined to form a cover layer for waste rock backfilling. In addition, the scheme includes detailed parameters, engineering quantities and economic budget estimates for the formation of the cover layer. This program is expected to be implemented in 2017.
Shougang Xingshan Iron Mine was originally an open-pit mining iron ore. At the end of 2004, there were not many open-pit mining resources. The deep mineral resources below -33m were underground mining. China Metallurgical North Engineering Technology Co., Ltd. based on the waterproof, anti-freeze and anti-leakage of the cover. In theory, the thickness of the cover layer designed for the mine is not less than 40m [56]; the optimization theory of the overburden structure based on the stochastic medium ore-mining theory of North China University of Technology and the natural grading theory of the back-filling of the underground cover layer, considering the cover layer The basic function has established a detailed cover formation plan and technical support measures for Xingshan Iron Mine [3, 12, 57]. The method of forming the cover layer is a high-step one-time turning and unshaping forming, the main structure is divided into a downward moving layer and a flowing layer, and the corresponding layered thickness and the grain size ratio scheme are designed. The Xingshan Iron Mine began underground infrastructure construction in 2007, and the cover layer was completed in 2011. Real-time monitoring of the formed cover layer shows that the cover layer can meet the production requirements and the monitoring index is good. The successful implementation of this overlay is important for overlay research.
With the development of economy and technology, more open-pit mines will enter the transition period in the future. Through continuous in-depth coverage research, understand and master the internal mechanism of the cover layer to provide more scientific and rigorous theoretical support for the mine.
3 Technical problems to be solved Although a large number of researches have been carried out on the combined mining technology and technology of the open-air to underground transition period at home and abroad, certain research results have been obtained and the production practice has been given a certain guiding role, but at this stage it is only covering A certain function of the layer is studied separately and has a single purpose. The requirements for the mine only satisfy the safety structure, ensure the integrity of the external structure and maintain the thickness, neglect the subtle changes between the internal particles, and the internal mechanism of the coating is unclear. There is no real understanding of the movement of the cover layer, which leads to the limited application of theoretical research to the promotion of production practice guidance. Based on the above analysis, the technical problems that need to be solved are summarized:
(1) The change law of the cover layer structure under mining disturbance. During the ore mining process under the overburden, the space left by the ore discharge needs to be replenished immediately after the overburden, so the structure of the overburden changes with the collapse of the ore. The lack of understanding of the changes in the structure of the ore-mining process, the movement trend is not clear, may lead to the destruction of the integrity of the overburden structure during the mining process, resulting in potential crises, such as air leakage, water leakage and other issues, Mine production caused huge losses.
(2) The change of the internal stress field of the cover layer under mining disturbance. At the same time as the ore mining begins, the internal equilibrium stress of the cover layer is instantaneously broken, the stone begins to move in a direction of equilibrium, and will soon reach equilibrium, and each subsequent mining process will be carried out. Breaking the balance of the stress field, and then reaching a new balance, the ore-mining process is a process of constant imbalance of the overburden stress field and the evolution of the equilibrium cycle until the ore is released to the cut-off grade. Because the understanding of this stress field change is unclear, the stress between the stones cannot be effectively controlled during the ore-extraction process, which may cause excessive local stress and affect the quality of the ore.
(3) Multiphase coupled field action. The cover layer itself is a kind of bulk flow which is coupled by multi-phase bulk. During the movement of the cover layer, the fluids of each phase interact and interact with each other, and the effect is not clear. The fluids of each phase are flowing. The process is either promoting or hindering, and the results are not known. Research on the current cover layer is rarely involved.
(4) Seepage process and driving force source, seepage velocity and trend of water in the cover layer. During the movement of the cover layer, the water moves in the way of seepage, and the flow rate will have certain influence on the cover layer, and the benefits or disadvantages of this effect and its transition nodes are unknown. Understanding the water seepage trend and flow rate is important to ensure the integrity of the overburden structure.
(5) The trigger mechanism and nodes of the debris flow. Disasters such as mudslides will bring great harm to underground work. Mine mudslides occur frequently. If you understand the trigger mechanism and nodes, take effective preventive measures to avoid unnecessary losses, thus greatly improving mine safety.
The technical problems of the above-mentioned overburden layers are internal coupling effects. The research of engineering experiments and physical experiments is to derive the mechanism in the process from the experimental results. Especially the industrial experiments not only need to invest a lot of manpower and material resources, but also have a large workload. Many errors and blind spots cannot directly and accurately understand the mechanism from the process of change.
In addition, with the development of modern science and technology, the popularization of digital applications, the use of numerical simulation technology will become the most extensive and authoritative technical means of coverage research. The application of numerical simulation technology not only saves the macro research investment, but also can fully understand the changing laws of each field during the movement. Although the current technical conditions can not fully simulate the complex environment of the site, with the rapid development of information technology, the above technical problems will eventually be solved.
4 Conclusions The research on the cover layer has been carried out for more than half a century. In this long exploration process, the cognitive transformation of the cover layer from the shape to the essence is realized. The formation method is from empirical reference to targeted design. The application examples in production practice prove that the previous exploration concept of the cover layer is successful and can guide the on-site safety production. However, the current cover layer research theory system is not rich enough, and the existing theoretical knowledge for the entire cover layer. The theoretical system is still only a preliminary understanding. More technical problems have yet to be resolved. In terms of the law of the movement of the open-cover to the underground cover and the formation methods, it is necessary to test and improve the existing technology through production practice. In the future, the formation method of the mine cover layer is to combine the formation mechanism of the cover layer with the geological conditions and technical level of the mine itself to form a protective layer that is completely suitable for its own mine. It is no longer necessary to learn from other successful examples, and the formed cover layer can Optimize the economy while ensuring mine safety.
Author: Chen Chao, well Boxiang; Institute of Mining Engineering, North China University of Technology, Mining Development and Security Technology Laboratory in Hebei Province;
Article source: "Modern Mines": 2016.2;
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