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.
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).
Zhu, G. L. (Masdar Institute of Science and Technology) | Sousa, R. L. (Masdar Institute of Science and Technology) | Zhou, P. (China University of Mining and Technology) | Yang, J. (China University of Mining and Technology)
ABSTRACT: An innovative approach to the gob-side entry retaining non-pillar mining is being used to increase the coal seam re cycling rate and productivity in China’s coal mining. The retained entry with a sidewall formed by gob caved-in filling rocks is unique in this method and the stability of caved-in material is critical to ensure efficient and safe mining activities. In this paper a numerical investigation on the stability of the gob-side entry is conducted using a discrete fracture network (DFN) model developed by the Massachusetts Institute of Technology (MIT), GEOFRAC, in combination with discrete element modelling (UDEC). The proposed method is applied to a case study of a gob-side retained entry in an underground coal mine in China. Fracture traces are measured along the gob-side wall of the entry, and statistical methods are used to estimate the fracture intensity and the mean fracture areas, which are the key inputs to GEOFRAC. Fracture networks generated by GEOFRAC estimate the rock blocks in the filling body, and simulations with UDEC are done to evaluate the stability of the gob-side entry. Two models are developed, one considering the generated fractures and the other considering no fractures within the gob-side filling. The results show the effects of considering the fractures in the filling body on the distribution of displacement and field stress in the gob-side entry zone. Also, the stability under the mining impact loading, due to periodic roof caving, is simulated, providing the basis for the optimization of the design of the entry support.
Coal is one of the most significant energy resources, covering about 30% of energy consumption worldwide. China is the largest producer and consumer of coal in the world. Despite, China’s reduction of coal consumption in the past few years, and its target to reduce coal to 58% of total energy consumption by 2020, coal remains an important source of energy, and China is still one of the largest coal producer in the world. Fig.1 shows China’s annual coal production in recent years.
ABSTRACT: By using the characteristic of carbon dioxide phase transition of soft rock blasting, high pressure gas can be used. The mechanism of high pressure gas dynamic crushed rock is different of blasting and static rock breaking. By establishing the physical model of distributed optical fiber grating, the internal effect of carbon dioxide transformation induced by carbon dioxide in coal is tested. Analyzed the relationship between the gas pressures, the time of action, the strength of the coal and the occurrence of the crack. Using PFC software to establish different rock mechanics properties of soft rock numerical model, the hole and the layout of different parameters in the model, observed under different pressure, time conditions, and the expansion of the rock mass surrounding the fracture blasting holes. Through the slit number, length, distribution parameters to analysis the mechanism of dynamic pressure gas rock breaking, obtain the relationship between the crack rock, rock strength, rock burst and gas pressure and other parameters.
China more than 95% high gas and gas outburst coal mine belongs to low permeability coal, and 2/3 coal resources in 1 km, accounting for 53% of the total coal resources, mining depth 20m at an average annual rate of increase in the expected in the next 20 years, many coal mine depth will reach 1000 m to 1500 m, the increase of mining depth after and the gas content increases, and the permeability of coal seam is more reduced, therefore, the search for an efficient, economic and applicable to a wide range of antireflective method is an important aspect of governance at home and abroad and the development of coal bed methane in coal mine gas at present.
The technology of carbon dioxide phase transition cracking was taken seriously and developed in 1950s [Zhang et al., 2013]. It is specially developed for coal mining face in high gas mine [Sun, 2015]. Carbon dioxide presplit Air gas drainage technology by liquid carbon dioxide in a very short time and limited space in the rapidly transformed into gaseous carbon dioxide[Zhao, 2013], the initial release of 100 ~ 200 MPa pressure impact on coal, increase coal seam fracture[Nie. 2007], improve permeability; at the same time, the adsorption of coal and carbon dioxide than methane adsorption capacity the strong, through high pressure injection, carbon dioxide can occupy the methane adsorption, which will replace coal methane, improve mine gas drainage effect[Zhou et al., 2016] Therefore, it is very important to study the influence of carbon dioxide phase change cracking on coal body, which is of great significance for the identification of coal seam fracturing engineering parameters[Wen et al, 2016][Li, 2015][Wang, 2015][Zhang, 2016].
ABSTRACT: In underground coal mining practice, a majority of rocks are composed of clay minerals. When roadways are placed in them, clay minerals are exposed to water and humidity and will absorb water rapidly and generate pressures that can break apart the weakly bonded rock, leading to a progressive strength degradation and consequently a severe closure (i.e., squeezing) of the roadway. In this study, numerical simulation was carried out to investigate the mechanisms of roadway squeezing using UDEC Trigon approach. The strength degradation is simulated by gradually reducing the cohesion and tensile strength of the contacts between blocks in the UDEC Trigon model. The strength is not reduced everywhere throughout the model but only at failed (either in tension or in shear) contacts because moisture is considered to intrude into rock through cracks. When a contact fails, its cohesion and tensile strength are gradually reduced as a function of calculation time. The numerical study aims to realistically capture the squeezing process of the surrounding rock mass of roadway due to strength degradation.
In underground coal mining practice, a majority of rocks are composed of clay minerals, feldspar, quartz clastics, and a small fraction of other silicate and carbonate minerals. Shales probably are the most common and can be composed of 50-80% clay materials (Molinda and Klemetti 2008). Clay materials have a platy structure and can absorb water. Water absorption causes swelling, which may loosen bedding and break apart the flat- bedded material structure, resulting in rock deterioration (Huang et al. 1986). Considerable researches have been carried out to study the swelling characteristics of shale. The propensity for swelling is controlled by the mineralogical composition of the rock, for example the presence of swelling clay minerals and the higher the water absorption, the higher the degree of swelling (Olivertra 1990). Huang et al. (1995) carried out a series of laboratory tests and the results showed that the temperature of the shale had the least influence on swelling of shale, while the air humidity and the moisture activity index had a significant influence. Zhang et al. (2004) found that the low strength of shale is also correlated with low Young's modulus and low shear strength.
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].
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)