The overlying strata is often destroyed in large-scale during shallow coal seam mining, and the sliding instability of the caved roof seriously threatens the safety of the mining field. Based on the monitoring data of the roof weighting of a typical shallow coal mining, the load distribution characteristics of the roof along the strike and trend of the mining field were analyzed, and the mechanical model of the pressure-arch in the surrounding rock was established. Then the evolution characteristics of the pressure-arch and the elastic energy of the surrounding rock were revealed during shallow coal mining by theoretical analysis and numerical simulation. The results show that the continuous pressure-arch was formed when the horizontal stress exceeded the primary vertical stress of the mining field, and the elastic energy of the roof was released by the mining unloading effect. The caved zone of the overlying strata was formed below the inner boundary of the pressure-arch. The elastic energy was accumulated in the pressure-arch and the energy arrived the highest at the front arch foot. The accumulated energy at the arch foot was released by coal mining and the shear zone could be formed. So the sliding of the caved zone along the shear zone would induce the strong roof weighting. The concentrated stress and the released energy during each mining increased with the panel advancing, and the height of the shear zone also increased. The conclusions obtained in the study are of important theoretical value to direct the similar engineering practice.
The instability of overlying strata during shallow coal mining, such as the large-scale roof falling and step-like ground subsidence, is a key problem that can restrict the safety mining in the mines (Ju &; Xu 2015). The self-bearing structure of the pressure-arch can form in the overlying strata after the coal mining, and this structure can support the load of the upper strata and soil layer, so the weighting intensity of the panel is determined by the caved rock in the unloading zone under the inner boundary of the pressure-arch. A large amount of elastic energy is accumulated in the pressure-arch under the concentrated stress, and the released energy for mining is the internal cause of rock failure (Wang et al. 2017). It is an important problem to reveal the distribution characteristics of the stress and energy fields in the mining field, and to analyze the stability of the overlying strata during shallow coal mining based on the evolution characteristics of the pressure-arch.
Hamanaka, Akihiro (Kyushu University) | Itakura, Ken-ichi (Muroran Institute of Technology) | Su, Fa-qiang (Henan Polytechnic University) | Deguchi, Gota (Underground Resources Innovation Network) | Kodama, Jun-ichi (Hokkaido University)
Underground coal gasification (UCG) is a process of producing combustible gases by the in-situ conversion of coal into gaseous products. Coal resources abandoned under the ground for either technical or economic reasons can be recovered with economically and less environmental impacts by UCG; therefore, this technology is regarded as a clean coal technology. UCG has several advantages of low investments, high efficiency, and high benefits compared to conventional coal gasification. However, some environmental risks such as gas leakage, surface subsidence, and underground water pollution are difficult to control because the process is invisible. The reactor in UCG is unstable and expands continuously due to fracturing activity caused by coal combustion. It is, therefore, considered that acoustic emission (AE) is an effective tool to monitor the fracturing activities and visualize the inner part of coal. For this study, UCG model experiments were conducted using coal blocks of 0.55 × 0.60 × 2.74 m to discuss the applicability of AE monitoring for the estimation of the crack generations during UCG process and the extent of the gasification area. Temperatures were also monitored because the crack generations were strongly related to thermal stress occurred by coal combustion and heat transfer. The monitoring results of AE agreed with the measured data of temperatures and the gasification area; the source location of AE was detected around the region temperature increased and the gasification area. Additionally, the gasified coal amount can be predicted by using the data of product gas. Therefore, AE monitoring combined with the prediction of reacted coal amount are expected to be a useful tool as monitoring system of the gasifier in the underground.
Underground coal gasification (UCG) is a technique to extract energy from coal in the form of heat energy and combustible gases through the chemical reactions in the underground gasifier. This technique enables to utilize coal resources that remain unrecoverable in underground due to either technological or economic reasons. Most coal mining in Japan was closed by 2001 because of complicated geological conditions for mining development and high prices of domestic coal. However, abundant unused coal resources remain underground, but they are not recoverable because of technical and economic reasons. Such coal resources are estimated to be 30 billion tons. For that reason, UCG has a great potential to recover vast amounts of energy from these coal resources.
According to the typical structure of the hanging roof blocks after shallow horizontal mining, taking the fully mechanized mining face with large mining height in Shendong Mining Area of China as engineering background, theoretical analysis and numerical simulation were used to analyze the arching law of the principle stress and boundary distribution of the horizontal stress in the roof block structure. Three typical mechanical models of the roof structure were proposed, such as the symmetrical stress arch of two key blocks, the semi arch of multiple key blocks and the squeezed arch of multiple key blocks. The results indicated that the horizontal stress displayed a nonlinear distribution at the abutments of the symmetrical stress arch and there was a linear stress distribution with higher peak value at mid-span of the arch; the high horizontal stress at the arch abutment was necessary to form the squeezed arch of multiple key blocks; the horizontal stress was relatively less at the arch abutment of the semi arch structure; the main key block was easier to slide in this structure for the boundary horizontal stress displaying the nonlinear distribution. The causes of step subsidence and strong roof weighting of shallow large height mining face were revealed by using the semi arch model. Moreover, the instability of the whole compressive arch of the multilayer bedrock would lead to long periodic roof weighting. The results are of instructive significance for roof weighting forecast and strata control during shallow horizontal mining for thick coal seam.
The Shendong Mining Area was characterized by large thickness, shallow buried and flat dipping coal seam, which was a typical high efficiency mining district of shallow thick coal mining in China. The large-scale shallow mining for full thickness of coal increased the failure range of the thin bedrock roof, resulting in strong strata behavior, and the bedrock and thick alluvial sand layer being suffered seriously damaged. So it was an important problem of the roof weighting and ground subsidence induced by shallow mining to restrict the development of similar mining in the west of China (L.Q. Ma et al., 2009; D.S. Zhang, et al., 2011).
Lian, C. J. (Shandong University of Science and Technology) | Hou, J. Z. (Shandong University of Science and Technology) | Gao, G. L. (Taian Taishuo Strata Control Science and Technology Co. Ltd.) | Wang, G. (Taian Taishuo Strata Control Science and Technology Co. Ltd.) | Song, W. T. (Henan Polytechnic University)
ABSTRACT: It has wide distributions and large recoverable reserves of Jurassic period coal seam in China. It is difficult to maintain stability of the development roadways with long service term for Jurassic strata because there are abundant argillaceous rocks and some minerals in the high argillaceous rocks will be expanding while meeting with water. The fractures of the roadways develop well under high stress and are suitable to filling with grouting. But the poor cementing performance of cement with argillaceous rock as well as a heavy water filtration rate of cement slurry had resulted in failures of many engineering cases adopting cement grouting to reinforce this kind of roadways. In this paper, according to characters of the high argillaceous rock in Jurassic strata, a marlaceous inorganic grouting material which possesses the well cementing performance with argillaceous rocks and little filtration rate was introduced; and the grouting reinforcement mechanism, construction technique and engineering application effect about it was clarified. It will be of great significance for reinforcement and maintenance of the development roadways with high argillaceous rocks.
According to statistics, 60% of the proved coal reserves in China distributes in Early-Middle Jurassic period of northern North China, southern Northeast China and Northwest China, along with late Jurassic to early Cretaceous period of Northeast China and east Inner Mongolia. The period of coal forming is short and argillaceous rocks are abundant in Jurassic strata. Moreover, there are quite a few expanded minerals in some strata. So the roadways are easily to be deformed and damaged when affected by mining-induced stress. In addition, this kind of soft rock roadways has an obvious time effect. For the development roadways with long service term, serious deformations are frequently observed and part of the roadways has suffered deformation and maintenance time after time. The stability support of the development roadways really need much cost.
Qian, D. (Kyushu University) | Sasaoka, T. (China University of Mining and Technology) | Shimada, H. (Kyushu University) | Wahyudi, S. (Kyushu University) | Tsedendorj, A. (Kyushu University) | Wang, C. (Kyushu University) | Matsui, K. (Henan Polytechnic University)
Rock masses are often heavily jointed and instability involving falling or sliding of rock block may arise upon excavation. As such, supports or reinforcements are required to stabilize the unstable rock blocks. Accurate prediction of mode of rock block failure and volume of unstable rock block are essential for the design of underground excavations in jointed rocks. Traditionally stability analysis involves a tetrahedral block formed by an excavation free face and the mean orientation of three rock joints and this is termed as the deterministic analysis. However, the orientation of rock joint may be widely scattered and Fisher distribution is commonly employed to evaluate the scatter of joint orientation. In the present study, joint orientation surveys obtained from the field are evaluated. It is found that Fisher distribution may not represent the true dispersion of the rock joint orientation for many cases. Goodness to fit test is employed to evaluate the fitting of the field data. The results show that Kent distribution is more appropriate to represent the dispersion of rock joint orientations. A probabilistic simulation is then employed to generate the distribution of volume of unstable tetrahedral rock block based on the mean orientation of the rock joint and its orientation dispersion obtained from Kent distribution. The simulation results are then compared with that determined from deterministic approach which only employs the mean rock joint orientations.
Rock burst is one of the major disasters in deep mining and it appears that the frequency and intensity of rock burst will increase with the buried depth of rock engineering. Burst tendency is the inherent factor that determines whether rock burst occurs or not. We put forward a new rock burst tendency index—Yield Degree using the stress-strain curves of rock specimens under the conditions of uniaxial and triaxial compression. All in all, rock burst tendency is bigger when YD is smaller. Then we respectively analyzed rock burst tendency under different conditions which are submergence, confining pressure, and ratio of height to diameter. The results show that YD becomes bigger when the specimens have been submerged in water, increases with the increase of confining pressure, and decreases with the increase of the ratio of height to diameter of the specimens. Above conclusions could provide a certain thought to control rock burst for rock engineering.
Rock burst is one of the major disasters which occur in underground engineering frequently. According to the theory of rock burst mechanism, the occurrence of rock burst is concerned with its own properties, the magnitude of stress that rock bears and the energy storage and release of rock. The breed and occurrence of rock burst include the physical process of loading on rock mass, energy storage, energy dissipation and sudden deformation failure with instantaneous release of large amounts of energy, and its performance is transformation of energy’s basic form. Rock burst tendency of rock mass is the necessary condition when stain-type rock burst occurs, and it is the important issue of studying on rock burst and is the foundation of rock burst forecast and prevention control (Chen, S. J. et al. 2007, Zhang, X.Y. et al. 2007).At present, scholars have put forward various rock burst indexes, such as elastic energy index, impact energy index, the maximum plastic deformation velocity and brittleness index etc (Pan, Y. S. et al. 2010). Elastic energy index reflects the energy storage ability in prepeak phase of the uniaxial compression stress-strain curve of rock specimen. Impact energy index considers the relationship between energy storage in pre-peak phase and energy dissipation in post-peak phase (Tang, L. Z. &Wang,W. X. 2002). However, these rock burst indexes have some shortcomings. For example, elastic energy index cannot research post-peak energy dissipation. And impact energy index cannot consider the actual energy accumulation value in pre-peak loading phase (Zhu, F. C. et al. 2002). What’s more, it is difficult to determine the stress level to confirm elastic energy index, and it is difficult to determine the origin of residual stress phase to confirm impact energy index. The brittleness index emendation value (BIM) which can escape these shortcomings was put forward (Aubertin, M. et al. 1994). Otherwise, Plastic deformation energy is dissipated energy with plastic deformation of rock specimen. As we know that plastic deformation energy can’t release and only pre-peak elastic energy can release in post-peak phase because plastic deformation can’t be recovered, so that prepeak yield has close contact with plastic deformation. Therefore, a new rock burst tendency index and rock burst tendency evaluation method were put forward after studied the characteristic of pre-peak yield phase and its relationship with BIM (Chen,Y. & Guo, B. H. 2013).
The plastic characteristics of the coal seam floor has been explored in this article on FEM stength reduction with the software of ANSYS, and the plastic area perforation process has been given. The slip scope given on FEM strength reduction are much close to the theoretical solution, the plastic failure fields depth of the coal seam floor is similar to Prandtl type,comparing some parameters with the theoretical solution,The solution error between FEM strength reduction and theoretical solution is less than 10%, which showed that the finite element intensity reduction method used to solve floor damage process is feasible. The scope of its plastic area can be used to reserve the safe thickness in water outburst from coal seam floor and the maximum effective spacing of protected seam.
Meng, L. (China University of Mining &Technology) | Liu, M.J. (Henan Polytechnic University) | Jiang, Y.D. (China University of Mining &Technology) | Zhao, Y.X. (China University of Mining &Technology) | Wang, Y.G. (Henan Polytechnic University)