Opportunities and challenges of deep mining



1. Introduction

The mining of the earth's resources has a long history, the shallow coal and mineral resources are gradually depleted, and the mining of coal and mineral resources is constantly pushed deeper into the earth. At present, 1000m deep mining is a common phenomenon, the mining depth of coal has reached 1500m, the development of geothermal has exceeded 5000m, the depth of non-ferrous metal mines has reached about 4500m, and the depth of oil and gas mining has reached about 7500m. In the future, deep mining will become common. As early as the 1980s, Poland, Germany, the United Kingdom, Japan and France had coal mining depths of more than 1,000 m, and China now has 47 coal mines mining depths of more than 1,000 m[1,2]. In the case of metal mines, according to incomplete statistics, there were at least 80 mines more than 1,000 m deep before 1996, mainly located in South Africa, Canada, the United States, India, Australia, Russia and Poland. The average depth of metal mines in South Africa reaches 2000m, of which the WesternDeep Well gold mine has reached 4800m[3].

The deep rock mass is characterized by high primitive rock stress, high temperature and high water pressure. Compared with shallow resource mining, deep mining may involve rock burst, large-scale collapse and large-scale outburst of coal, gas and water mixture. These events are often complex in nature and difficult to predict and control. The characteristics and boundary conditions of deep mining rock mass are the initial causes of deep mining disasters [2]. For example, when the mining depth reaches about 1000m, the primary rock stress caused by the overlying rock layer, the structural characteristics and the stress concentration caused by the mining operation can lead to the fracture and damage of the surrounding rock [4]. Under high stress, accidents may occur more frequently because the accumulated deformation energy is more obvious.

Under the conditions of high stress, high temperature and high water pressure, the disturbance generated by mining operations can lead to sudden and unpredicted damage of rock mass, which is manifested as large-scale instability and collapse [5]. In addition, at very deep depths, the deformation and fracture characteristics of rock mass often show strong time-related characteristics [6]. The disturbance stress and the time-dependent characteristics of rock mass deformation caused by deep mining engineering may lead to the occurrence of disasters which are very difficult to predict.

Various new problems in rock mechanics and mining engineering arising from deep mining have been studied. At present, most of the research work focuses on regional fracture of deep surrounding rock [7-10], large extrusion failure [11], brittle to plastic transformation of rock mass [12], energy characteristics of dynamic failure in deep mining [13], visualization of stress field [14,15], and rock mass deformation and displacement caused by deep mining [1,16]. Although the results of these studies have revealed some mechanical characteristics of deep mining, some theories, processes and methods related to deep mining are still in the initial stage. Xie[2] believes that this is due to the limitations of current rock mechanics theories, which are based on material mechanics and have little relationship with deep mining problems and engineering geological activities. Therefore, for deep mining, it is necessary to consider the characteristics of primary rock and the mechanical properties of rock mass caused by mining.

2. Rock mass support of deep mine

In mining and other underground engineering, the primary rock stress is the main factor affecting the deformation and failure of underground rock mass. With the increase of mining depth, the influence of primary rock stress on the fracture and stability of surrounding rock becomes more obvious, so it is very important to choose rock support technology.

He et al. [4] developed the asymmetric coupling support technology of soft rock roadway, including floor heave control technology, dual anchoring control technology of large-section roadway intersections, and strengthening design technology of pump station cavity. These techniques have been successfully applied in field support work [17]. According to the field test results, Niu et al. [18] suggested that in order to resist creep deformation, the dynamic reinforcement process of rigid-flexible coupling should be adopted to provide the initial flexible support for the stable broken surrounding rock in the early stage, the method of reserving deformation should be used to cope with the unloading of high stress in the middle stage, and the support with high strength and high stiffness should be adopted for the whole section in the later stage. He et al. [17] further developed a test system called rock burst in deep mining. In order to solve the damage problem of common supporting materials of large deformation surrounding rock, an energy-absorbing bolt with large extension and constant resistance was developed, as shown in FIG. 1 (a) and (b) [17]. Through its own large deformation, this kind of bolt can resist the large extrusion of rock mass caused by sudden deformation energy. The output range of the bolt is usually 120~200kN, and the deformation is 0.5~1m. Li et al. [19] developed an energy-absorbing rock mass support device for rock burst prone surrounding rock and extruded surrounding rock, that is, D-bolt [Figure 1 (c)]. For a 200mm D-bolt, the average impact load is 200~300kN, and the accumulated kinetic energy absorbed is 47kJ· m-1.

3. Smart mining

As an inevitable product of the information age and knowledge economy, digital mining originates from the geological information system of mine or mining [20]. The purpose of digital mining is to improve the exchange of mine information, support automated mining and intelligent mining, ensure the safe, efficient, green and sustainable development of mining, and realize scientific mining. Digital mine construction is a gradual process and a complex system project [20].

Research and development of automated mining technology began in the mid-1980s. In Canada, NorandaInc has developed various automation equipment, including loading machine (LHD), light guide system, LHD remote control system, etc., to meet the needs of automation in underground hard rock mining [21]. In 1994, Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) launched the Mining Robotics Research Project. CSIRO researchers have developed an open-pit bucketcruise system, an accurate unloading model and an underground metal mine LHD automation system. Then, DynoIndus-trierASA of Norway, INCO Ltd. of Canada, and Tamrock of Finland implemented a $22.7 million mining automation project to increase labor productivity and reduce operating costs. Later, Sweden implemented the "Grountecknik2000" strategic plan for mine automation. At present, unmanned working face and unmanned mine based on fully automated mining/unmanned mining process have become an important research field [20].

According to Wu et al. [20], in order to construct a multidimensional and dynamic virtual reality system for coal mines, the new task of the digital coal mine is to establish a coal mine that uses the digital mine integrated platform in real time. In the new situation of deep mining, digital mining has four main directions: (1) digital mine integrated platform; ② Mining simulation system; (3) Underground positioning and navigation technology; ④ Intelligent perception of mining environment.

4. Strengthen continuous mining and roadway cutting machine mining

Gu and Li[22] have suggested that intensive mining and high plains rock stress induced rock cracking techniques should be adopted in deep metal mines. However, there are four key problems in deep hard rock mining: (1) The characteristics of high stress field and geological structure of deep mining and their mastering methods; (2) Knowledge of hard rock blockfracturing (full-blockfracturing) under the action of highland rock stress; (3) Support measures to control rock burst under high temperature conditions; ④ Knowledge of the coupling and flow of all solid-gas-liquid media in leaching mining of low grade deposits.

Due to the complex anisotropy of the target rock mass, it is difficult to use the roadway cutter (TBM) for mining. In mines, more than 70% of TBM damage is due to geology-related problems [23]. The use of TBM in hard rock mines, as well as the average length of tunnel drilling, has increased in recent years, but several limitations still limit the use of TBM in mines. In hard rock mines, when TBM is used for cutting, rock burst and sheet slope caused by stress redistribution of high stress rock mass is a major disadvantage, which will affect the operation safety and installation of roadway support.  The highly fractured and block-like rock mass is another factor in the application of overhead TBM cutters in mining. Loose chunks of rock have been known to clog and damage conversion funnels and cutter loading buckets. Therefore, in order to expand the application of TBM in deep mining, TBM needs to be improved, such as impact rods, to avoid damage to cutting machines, rock loading buckets and belt conveyors.

In addition to these problems encountered in hard rock mines, other concurrent problems involving water inrush and gas explosions have affected the use of TBM cutters in coal mines. A novel process of combined drilling and trenching has been implemented in China's Pingdingshan coalfield for CBM pre-extraction, which enhances both coal and gas recovery and reduces the possibility of gas explosions. Underground water in unfavourable geological bodies (such as faults and karst caves) can cause coal mine collapses.

5. Fluidized mining

Xie et al. [24,25] have pointed out that there is a theoretical limit of mining depth for traditional methods. Theoretically, it is estimated that once the buried depth of underground solid mineral resources exceeds 6000m, various existing mining methods will become unusable. Therefore, it must be recognized that the development and utilization of greater depths of mineral resources requires disruptive innovations in theory and technology. For this purpose, Xie et al. [25] proposed a theoretical and technical concept of fluidization mining of deep underground solid mineral resources (FIG. 2). Based on a mining model similar to TBM, the idea is to realize in-situ, real-time and integrated utilization of deep underground solid mineral resources through mining, selection, smelting, filling, power generation and gasification of solid resources, that is, to convert solid resources into gas, liquid or a mixture of gas, liquid and solid materials. As a result, the coal mines of the future will have no workers going down the mine, no coal being extracted, no coal piling up, no dust polluting the air, and instead will have electricity and energy delivered in a clean, safe, smart, environmentally sound and eco-friendly way.

For coal mining, the concept of fluidized mining includes the following five main processes: (1) unmanned mining; ② Automatic beneficiation; (3) fluidization conversion of solid mineral resources; ④ Controlled filling; Power transmission, intelligent power control and power storage. For metal mining, the concept of fluid mining includes the following three steps: (1) unmanned mining; ② Fluid conversion of solid mineral resources; ③ Controlled filling [24].

There are four technologies to realize fluidized mining of deep underground solid mineral resources [24] : (1) Conversion of solid mineral resources into gas, such as underground gasification of coal; ② Conversion of solid mineral resources into fluid fuels, such as underground liquefaction of coal and high-temperature biological and chemical conversion of coal; ③ Conversion of solid mineral resources into mixtures, such as explosive coal dust and coal water slurry; ④ Solid mineral resources are converted into electricity in situ, such as underground in-situ power generation of coal. Fluid mining is indeed a disruptive mining technology innovation, especially for deep mining in the future.

6. Enhanced simulation facilities for deep mining

Over the past 20 years, a large number of laboratory test equipment and numerical simulation software have been developed around the world to simulate the properties of real rock masses under raw rock stress conditions. In Australia, for example, the CSIRO Rock Mechanics Laboratory is equipped to simulate the real rock mass properties of deep mining using the latest bespoke triaxial devices, core displacement laboratory equipment and indoor digital modeling tools. In the United Kingdom, the University of Portsmouth has developed a mesoscale rock deformation machine to determine the failure mechanics of rock mass in seismic environments. The Department of Earth Science and Engineering at Imperial College London has a reservoir-condition core displacement laboratory facility with X-ray imaging capabilities, gas-liquid mass spectrometry, chemical testing equipment and advanced laboratory modeling tools ranging from rock pore simulation to very large scale model simulation. The Department of Earth Sciences at the University of Cambridge has facilities that simulate the microstructure and geochemical characteristics of rock masses using optical, electronic, infrared, nuclear magnetic resonance and X-ray diffraction (XRD) analyses. In the United States, the University of Minnesota has a variety of closed-loop, hydraulically servo loaders for uniaxial, biaxial (plane-strain) and traditional triaxial compression tests, as well as related digital imaging and acoustic emission (AE) tests. In Canada, the Rock Fracture Dynamics Laboratory at the University of Toronto has a variety of advanced equipment, including a multi-axis servo-controlled rock deformation system and a true three-axis system with AE and three-dimensional (3D) speeds.

Among the excellent geotechnical research centers around the world, the DeepEarthEnergy Laboratory of Monash University in Australia has a variety of advanced research equipment, which can carry out intensive research on rock mass characterization and fragmentation (3GDeep; http://www.3gdeep.com). The mesoscale equipment includes customized high pressure and high temperature enhanced triaxial test machines. The large equipment is represented by enhanced core displacement experiments and shear devices, including a high-pressure triaxial testing machine, a high-pressure hydraulic-mechanical test chamber, and a three-dimensional compression and monitoring Hopkinson bar for testing rock mass properties during rock mass failure. Microscale equipment includes X-ray microscopy for 3D contrast imaging, CT scanning, scanning electron microscopy (SEM), and XRD.

These test sets are complemented by basic simulation tools such as finite element method (FEM)/Finite Difference Method (FDM)/particle flow model (PFC). In order to understand the microscopic results of the rock mass environment (including true fracture, leaching, fluid flow characteristics, visible pore areas, and bending of liquids as they flow through the pore structure), the response characteristics of the ore body/very large rock mass, and the time-dependent impact on the surrounding environment, 3GDeep uses a comprehensive 4-stage test and numerical simulation rule (Figure 3) to study the entire mining process from micro to very large scale. The four scale ranges are as follows: (1) Micro scale: 0.03μm~20mm; ② Mesoscale: 20~100mm; ③ Macroscopic scale: 100~1000mm; ④ Ultra-large scale: 1000m. Studying the relationship between these four scales will help to obtain the underlying geological characteristics that are needed to synthesize very large scale models under real field conditions.

The macroscopic triaxial testing machine [FIG. 4 (a)] and the true triaxial testing machine [FIG. 4 (b)] are particularly important for conducting reliable rock mass fracture tests in these devices, that is, simulating more realistic rock mass fracture tests with non-traditional large samples under typical field conditions of high pressure and high temperature. The macroscopic triaxial test machine performs mechanical tests on rock samples with a diameter of 500mm, which is almost two orders of magnitude larger than the rock samples of traditional advanced triaxial equipment. The device can also simulate the flow of multiphase fluids (liquid and gas) through rock samples, using fluid pressures up to 25MPa.