The post peak behavior of rocks has a significant influence on its strength and deformational characteristics and is crucial in many applications like underground coal mining. A clear understanding of post peak behavior of coal is necessary for economical and safe underground coal extraction. In this study, laboratory experiments and corresponding numerical modeling is done to understand the post peak behavior of a coal and coal pillar. A series of laboratory tests are conducted using coal samples and also artificially prepared gypsum samples with varying material strength. A numerical model is developed in FLAC3D and simulations are run with strain softening model to capture the post peak responses of the material. This model allows to attain the peak strength following the Mohr-Coulomb behavior and once it attains the peak, the strength parameters are softened with respect to plastic strain that the material experienced using a piecewise linear functions. The softening parameters are selected in such a way that a realistic behavior could be achieved. With the validated model, several parametric studies such as influence of dilation, confinement, and friction angle are performed. Understanding the influence of the post peak parameters gave a full extent of the usage of material performance in the numerical model for better design of underground excavations. The numerical model is subsequently extended to design coal pillar for a underground mines is briefly discussed.
Design of supporting rock pillars in underground excavations specially applications like mining is based on the maximum pillar strength as well on the post-failure behavior. The complete stress-strain behavior of the pillars play an important role for those pillar stability. Around the underground structures there is possibility of formation of plastic regions. Hence, the design steps together with support systems expected to be accommodative in accordance with the existence of such regions. To estimate the parameters related to the post-failure of supporting pillars, large scale in-situ compression tests are needed to be conducted, which is quite difficult and expensive. In this study, post peak characteristics of coal samples and artificially prepared gypsum samples are obtained using MTS servo controlled testing machine available in the Department of Civil Engineering, IIT Madras. The same experiments are also numerically modeled in FLAC3D and attempt was made to capture the post peak response. Simulation were done by moderating various parameters namely cohesion, friction angle and dilation. Mohr-Coulomb Strain Softening model is used as the decay of strength parameters with respect to the plastic strain to obtain the softening behavior (Itasca FLAC3D manuals, 2008). With the validated model, parametric studies are done in order to understand the influence of various parameters on post peak responses.
The failing process of coal sample under loading conditions is investigated by using both laboratory experiment and 3D finite-discrete element method in the present paper. The cohesive zone model was used to characterize nucleation, growth and propagation of cracks, while the potential contact detection and interaction of fractured solids were examined by means of the penalty method in ABAQUS software, where the parallel computation was employed to accelerate the calculations. Uniaxial and Brazilian tests were performed in the laboratory to obtain the mechanical properties of the coal such as Young’s modulus, fracture energy, cohesive strength, friction angle, uniaxial compression and shear strength. Further, these properties were carefully calibrated prior to being taken as input arguments in the continuous-discontinuous modelling. All the simulating results were basically in agreement with that obtained from the tensile tests in laboratory. This study shows that such computational mechanics of discontinua can be employed to gain powerful insight into the failure mechanism of coal, which could also be a useful tool to clarify the collapse mechanism of coal block caving in mining engineering design and rock test scheme optimization.
As a special kind of rock, coal is generally at a complex stress state under mining conditions. Thus, understanding for the mechanical behavior of coal plays a very important role in designing rock structures such as coal mining, underground excavation. In the literature, numerical methods such as continuum and discontinuum are often used to describe the failure mechanism of rock (Bobet et al., 2009; Li et al., 2015; Lisjak and Grasselli, 2014). For example, plastic deformation and damage softening are perhaps the most studied problems in the continuum method while the internal length of geomaterial is usually not considered in its formulation, which is the most serious drawback because of its predictions significantly depended upon mesh size. To bypass the shortcomings mentioned above, an enriched or higher-order continuum formulation for the softening was developed (de Borst and Pamin, 1996; de Borst, 2002) and nonlocal continuum was also introduced (Bazant and Planas, 1998). However, interaction between fragments during the evolution of multi-cracks cannot still be taken into account in the enhanced continuum methods. On the other hand, discontinuous modeling techniques that are known as discrete element methods (DEM) treat the material directly as an assembly of separate blocks or particles, which was originally proposed by Cundell (1971) from the viewpoint of analogous molecular dynamics simulation to better account for and understand the interaction between the blocks. In discontinuous methods, the length scale can be automatically incorporated into the modelling, which naturally accommodates the real size of elements or particles to capture failure zone of the shear process.
Entry driven along goaf-side (EDG), which is to develop the entry of the next longwall panel along the goaf-side and isolate the entry from goaf with a small-width yield pillar, has been widely employed in the past few decades in China. The width of such a yield pillar has a crucial effect in EDG layout in terms of ground control, isolation effect and recourse recovery rate. On the basis of a case study, this paper presents a methodology of evaluation, design and optimization of EDG and the yield pillar by considering results from numerical simulation and field practice. In order to analyze the ground stability rigorously, the numerical study begins with the simulation of goaf-side stress and the ground condition. Four global models with identical conditions except the width of the yield pillar are built and the effect of the pillar width on ground stability has been investigated with comparison in aspects of stress distribution, failure propagation, and displacement evolution in the entire service life of entry. On the basis of simulation results, the isolation effect of the pillar acquired from field practice is also taken into consideration. The suggested optimal yield pillar design is validated from a field test in the same mine. Thus the presented methodology provides references and can be utilized for evaluation, design and optimization of EDG and yield pillars under similar geological and geotechnical circumstances.
The stability of roadways is a long-standing issue in underground coal mines, especially for entries that serve and ensure the safe production of longwall panels. The ground stability and failure mechanisms of entries vary depending on stress, geological and geotechnical conditions.
Entry driven along goaf-side (EDG), which is the development of an entry of the next longwall panel along the goaf-side and the isolation of the entry from the goaf with a small-width yield pillar, has been widely employed in China over the past several decades (Li et al. 2015; Wang et al. 2015, Zhang et al 2017). A yield pillar, which is designed to deform progressively during its service life, can transfer its load to adjacent abutments and control the mining-induced stress distribution around the entries (Peng 2008). Hence, it contributes to preventing coal bumps and excessive ground deformation by employing yield pillars, and it has been successfully applied in many coal mines of China and USA (Peng 2008; Li 2015; Chen et al. 2014). For instance, Carr et al. (1985) employed a yield-abutment-yield pillar layout to a four-entry longwall system in the Blue Creek seam in Alabama to control its severe floor deformation. The application of yield pillars in Utah coal fields has achieved a notable effect on preventing coal bumps (Peperakis, 1958; Agapito et al., 1988).
Utilization of coal reserves in the protection area of the coal mine gives the possibility of unusual mining methods. Just use methods room pillar in lignite coal mining is quite unusual opportunity. Application of the room pillar method can become very beneficial to the place where it can be somehow limited extraction using longwall face. This method is respectful of the overburden and excavated areas stay stable. The paper deals with the issue of stability and stabilization of coal pillars by using rockbolt support. The problem with this method is currently designing pillars so as to sufficiently handle pass the load.
Coal mining under the end slopes of surface lignite mines is becoming quite an important topic especially in areas where coal mining is slowly finishing up. The first experience with the method of pillar mining was acquired in the North Bohemian ČSA Mine, where this method was tested for mining in protected zones. This method is based on the room-and pillar mining method, where the principle is driving parallel galleries, while pillars that ensure stability are left between galleries. Therefore, it is not the case of caving mining, but the pillars remain stable even after the excavation. This brings a number of problems that must be solved. These include the size of galleries, which is partly influenced by mining mechanisms (milling machines), as well as the stability of the pillars, which depends on the strength of coal, thickness and the properties of the overlying rocks, but also the dimensions of the pillars. The advantage of this method is continuous extraction while the stability of the extracted space is ensured.
To ensure the long-term stability of the mine excavations, they must be secured (stabilized) by means of supports. Due to the right-angled profile of the working and the applied technology of driving using road headers, separate roof bolting in combination with welded meshes is used to ensure the stability.
The applied methods of dimensioning separate roof bolting as well as the methods of calculating the stable pillars between the mining galleries are mostly based on empirical and empirical-analytic procedures based on knowledge and in-situ measurements. The method of pillar mining and separate roof bolting has not been applied yet in the conditions of this lignite deposit, therefore, the results of in situ measurements are not available, either. For that reason, we adjusted the results of general analytical and empirical methods for determining the minimum parameters of roof bolting to the results of mathematical modelling [1, 2].
Rib-failure related accidents in underground coal mines continue to cause injuries and fatalities. Investigation of the coal-mass failure process through numerical simulation is one approach to mitigate rib failures.
The paper presents a framework to describe the strength and deformation of a coal-mass. The peak strength is evaluated by the Hoek-Brown failure criterion. The residual stiffness and strength are evaluated by the Fang and Harrison degradation model. The dilation is defined by the Alejano and Alonso peak-dilation model.
Using laboratory and in-situ coal numerical models, regression equations were developed to scale-down the strength and degradation parameters of coal material. Through a try-and-error process, the appropriate parameters of coal-mass could be defined by matching the behavior of modeled rib to field observations. If such observations are not available, preliminary parameters of coal-mass are calculated for a specimen size of 1.5 m. Young’s modulus and Poisson’s ratio are 3.0 GPa and 0.25, respectively. The Hoek-Brown peak strength parameters “m”, “s,” and “a” of the coal-mass are 1.659, 0.015, and 0.5, respectively. The unconfined peak and residual strengths of the coal-mass are 4.3 and 0.35 MPa, respectively. The degradation parameter and the critical plastic shear strain of coal-mass are 0.485 and 0.2, respectively.
Potential rib instabilities, such as brow formation, toppling, bumps, etc. are threats in underground coal mines. Efforts to improve the stability of underground mine ribs have continued for decades. Despite the ongoing rib-support-related research, there has been a continual occurrence of rib-related fatalities in underground coal mines. While these efforts have contributed to the overall improvement in rib safety, the average fatality rate is still 1.3 fatalities per year over the last 18 years . Because of the wide variation in requirements and design criteria nor a universal methodology for rib support, the National Institute for Occupational Safety and Health (NIOSH) is conducting research towards developing guidelines for determining the appropriate level of support and design methods for support systems to minimize the risk and severity of injuries to miners from rib falls in underground coal mines.
ABSTRACT: In order to understand the top-coal movement law influenced by caving sequence in steeply dipping seam, taking NO. 1201 working face of Shanxi Dayuan coal Ltd. as engineering background, established the PFC2D numerical model and simulated caving process under three different caving sequences: caving from top to bottom, caving from bottom to top and mixed caving sequence. The characteristic of recovery ratio, top-coal draw body and the boundary of coal and rock under the condition of steeply dipping seam are obtained from the simulation. The results show that the recovery ratio is the lowest when caving from top to bottom, while it is the highest when caving from bottom to top; the top-coal draw body has the tendency of developing towards the top of working face; the boundary of coal and rock shows significant asymmetry during the developing process; under the condition of steep coal seam, the top-coal at the top-of working face left blank seriously, which has a significant impact on the stability of the air-return way, so special measures are suggested to reinforce the top-coal above the air-return way.
With the exhaustion of the eastern mining area's coal resources, the focus of coal mining in our country gradually shifted to the west part of China, and the occurrence conditions of coal seams in west part of China are complex, the steeply dipping and steep seam are widely distributed in western China (Wu 2001). Therefore, the research which focuses on the top-coal movement law of steeply dipping seam caving mining technique has important significance to ensure the security and efficiency in the caving mining and to improve the resource recovery ratio. At present, many scholars have conducted researches on the law of top-coal movement under the steeply dipping seam condition and the failure mechanism of the top-coal (Wang et al. 2006, Wang et al. 2013). However, the former researches on the top-coal movement law under steeply dipping seam conditions always place extra emphasis on the analysis of top-coal recovery ratio, while rare researches have been conducted on the aspect of combination with top-coal caving pattern and evolution about boundary of coal and rock (Wang et al. 2005, Yang et al. 2010).
Cui, F. (Xi'an University of Science and Technology and Key Laboratory of Western Mines and hazard Prevention) | Lai, X.P. (Xi'an University of Science and Technology and Key Laboratory of Western Mines and hazard Prevention) | Cao, J.T. (Xi'an University of Science and Technology and Key Laboratory of Western Mines and hazard Prevention)
Abstract: Majority of the coal seams in Urumqi coal field, located in Xinjiang Province, China, are dipping about or above 45 degrees, called steeply dipping coal seams. The maximum width of the coal seams group is about 50m, and the minimum width is only 20m. and the vertical height is more than 400m. At present, the mining method used for these seams is the horizontal section top-coal caving mining. In this paper, the caving effect of steeply dipping and thick coal seams after weakening are studied with numerical simulation. The migration path and the falling quantity of discrete coal particles are analyzed and the distribution characteristics of hinged force between flowing blocks are intuitively presented. The load change of supports in the process of topcoal caving is monitored in real time, and the expression equation for caving space in flow model is established. The mining practice shows that the coal mass can be separated by using the advance pre-blasting method, which improves the top-coal recovery rate and reduces the loss of resource.
Ogura, Kazumi (The Kansai Electric Power Co., Inc.) | Kawamura, Hiroshi (The Kansai Electric Power Co., Inc.) | Yoshida, Mitsuhiro (The Kansai Electric Power Co., Inc.) | Harada, Tomoya (The Kansai Electric Power Co., Inc.) | Tsuboi, Hideo (NEWJEC Inc.) | Sone, Akito (NEWJEC Inc.) | Endo, Nobuyuki (NEWJEC Inc.) | Sato, Hiroaki (NEWJEC Inc.) | Matsui, Tamotsu (Ritsumeikan University)