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).
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)