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.