Assessment of Coal Pillar Stability at Great Depth (original) (raw)
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Stress-state monitoring of coal pillars during room and pillar extraction
Journal of Sustainable Mining, 2016
Current mining activities of the OKD mines are primarily focused on coal seams within the Karvina Formation in the Karvina sub-basin. A considerable amount of coal reserves are situated in protection pillars that lie under built-up areas. The longwall mining method is not applicable in these areas because significant deformation of the surface is not permitted. For this reason OKD is considering using alternative methods of mining to minimise subsidence. The room and pillar method has been trialed with specific coal pillars in order to minimise strata convergence. The method was implemented in the shaft protective pillar at the CSM Mine and is the first application of the room and pillar mining method within the Upper Silesian Coal Basin. Mining depth reached up to 900 m and is perhaps the deepest room and pillar panel in the world. To determine pillar stability, vertical stress was measured in two adjacent coal pillars which are diamond in shape and located within a row of pillars forming the panel. Two pillars diamond in shape and slightly irregular sides were approximately 860 m 2 and 1200 m 2 in size and 3.5 m high To measure the increase in vertical stress due to mining, four stress cells were installed in each coal pillar. Four 5-level multipoint rib extensometers measured displacements of all sides within each monitored pillar. The results of stress-state and pillar displacement monitoring allowed pillar loading and yielding characteristics to be described. This data and other analyses are essential to establishing procedures for a safe room and pillar method of mining within the Upper Silesian Coal Basin.
Pillar Abutment Loading – New Concepts for Coal Mining Industry
2015
Chain and barrier pillar design for longwall mining and production pillar design for room-and pillar retreat mining have tended to rely on simplistic abutment angle concepts for the estimation of pillar stress increases during and subsequent to extraction. Historically, the underpinning database of monitored abutment loading has been small and displayed considerable variation, leading to the application of a number of mine site-specific approximations and often necessarily conservative assumptions. Also, over the last decade, the trend towards wider longwall faces and narrower room-and-pillar sections in deeper areas has challenged established design practices. However, in recent years, considerable effort has been made both in the US and Australia with regard to expanding the abutment loading database and developing an improved understanding of the pillar loading environment. This paper examines some of the progress made and the implications thereof, with a focus on the derivation of formula for abutment angle prediction.
Rock Mechanics and Rock Engineering, 2019
The assessment of the strength of the coal pillars is essential for the safe extraction of the coal seam. All the pillar strength formulae used worldwide are developed for the flat coal pillars. Therefore, their adoption in evaluating the strength of the inclined coal pillars may endanger the workings of the inclined coal seams. The strength of an inclined coal pillar should be estimated by considering the inclination of the coal seam and its associated behaviour, because the shearing effect along the true dip aggravates the instability of the inclined coal pillars. In this paper, generalised analytical solutions have been developed to estimate the strength of the coal pillars which can be applied for both the inclined and flat coal pillars. The mathematical models are derived to obtain the confining stress in the coal pillar and the corresponding peak stress at the time of its failure using a rock mass failure criterion. The mathematical expressions are also developed for the stress distribution over the pillar considering the increase of the shearing effect with the dip of the coal seam. The Mohr-Coulomb criterion is considered for the shear characteristics at the interfaces of pillar-floor and pillar-roof. The asymmetrical stress distribution and failure along the dip-rise and the strike directions of the inclined coal pillars are addressed in this study. The concept of the confined core and three-dimensional stress distribution over the coal pillar are used to derive the strength formulae for the square, rectangular, and very long pillars. The performance of the derived strength formulae is assessed by the stable and failed cases of the flat and the inclined coal pillars. It is observed that all the inclined and flat coal pillars cases are correctly predicted by the derived generalised strength formulae. According to the derived strength formulae, the strength of the inclined coal pillar decreases with the increase of the inclination of the coal pillar. This paper also describes the variation of the strength of the inclined coal pillars with respect to the coal seam inclinations and the frictional properties of the contact planes for the different width-to-height ratios. Keywords Yield zone • Rock mass failure criterion • Inclined coal pillar strength • Flat coal pillar strength List of Symbols 1sm and 1bm Strength of the solid coal mass and broken coal under confinement 3sm and 3bm Confining stress of the solid coal mass and broken coal cbm Uniaxial compressive strength (UCS) of the broken coal a and b Constants f sm (3) and f bm (3) Failure envelops of the solid coal mass and the broken coal 1T Maximum peak stress in the pillar 3T Confining stress in the pillar corresponding to 1T bm
3D strain softening modelling of coal pillars in a deep longwall mine
2008
In longwall coal mines, the entries on both sides of the panel play a significant role in production rate and safety of operation. With increasing production amount, the rate of conveying material through such entries increases. Therefore, it is required to design wider entries. Support of these entries, particularly in deep mines is difficult.
The Effect of Energy Transmission on Mine Coal Pillars
2016
Random kinetic energy induced from strain energy stored in mining structures can distribute the stresses in rock masses. This physical transformation from potential to kinetic energy can lead to a severe coal burst which can be highly damaging. An efficient tool that can evaluate this stress distribution can play an important role in the design and planning of coal pillars and mine layouts. This paper presents a novel three-dimensional finite element modelling methodology (3D FEM) that has been developed to determine the structural response of a pillar subjected to kinetic energy release. This methodology can be used to determine the areas where a pillar is susceptible to violent, uncontrolled failure as well as to study the structural responses of a coal pillar. As part of the study a parametric study of combination of softening parameters in both coal and coal/rock interface was conducted to determine critical regions in the pillars that may lead to a better design strategy in coa...
New coal pillar strength formulae considering the effect of interface friction
International Journal of Rock Mechanics and Mining Sciences, 2019
Coal pillars perform the vital function of sustaining the weight of the overburden and protecting the entries and crosscuts during mine development and production. The strength of coal pillars is known to be greatly influenced by the friction of coal/roof and coal/floor interfaces. Unfortunately, none of the current empirical formulae for pillar strength has considered the effect of this so-called interface friction. This paper develops new coal pillar strength formulae considering the effect of interface friction. The formulae, which are in linear and power forms, are derived based on a series of UCS tests on coal specimens at three different interfaces (high: c = 124 kPa and μ = 0.40; medium: c = 76 kPa and μ = 0.22; and low: c = 55 kPa and μ = 0.13) and at ten width-to-height (w/h) ratios (w/h = 2-8, 10, 12, and 16). Having compared the formulae against stable and unstable pillar cases from different areas around the world, this paper finds that: (1) the low-friction formula of the linear form = + () S 2.7 0.12 0.88 p w h provides the best representation of the pillar cases database; (2) to improve pillar strength prediction, a safety factor = 1.5 is suggested to be used for all pillar designs using the linear low-friction formula (except for USA coal pillars with w/h ≤ 6 that need a higher SF up to SF = 2.5); (3) when the friction of the coal/roof and coal/floor interfaces is not known, it is beneficial to assume that the interface has a low friction value and thus to use the linear-low friction formula to predict pillar strength.
Influence of surface topography on the loading of pillar workings in near surface and shallow mines
2000
Mining often takes place in areas with steep and variable surface topography. Variable surface topography could be due to natural features such as valleys and hills, or man-made features such as excavations caused by surface mining operations or surcharging by dumping of spoil material, or tailings from the metallurgical plant. The research carried out for this project shows that the stability of underground excavations could be adversely affected by the proximity of such topographical features, especially in near surface mining operations of less than 100 m below surface. Perhaps the biggest hazard concerning the influence of surface topography on the loading of pillar workings in near surface and shallow mines is incorrect pillar design. This includes the identification of critical areas under influence of topographical features, and consideration of failure mechanisms not necessarily taken into account under normal conditions. This study shows that standard pillar design techniques are not applicable in such areas of variable stress and that a rational pillar design method is therefore required for such situations. The main objective of this research is to quantify the influence of surface topography on the stability of pillars and to describe a design methodology for pillars in areas of variable surface topography. Consideration of the critical factors identified in the study will improve the design of stable pillar systems, which are required to alleviate the hazard of catastrophic pillar collapse in areas under influence of varying surface topography. The proposed procedure for the design of pillars in areas of variable topography caused by surcharging is based on the procedure described by Fourie (1987). The design procedure can be summarised as follows: