Stress-state monitoring of coal pillars during room and pillar extraction (original) (raw)
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Structural and stress analysis in mining practice in the Upper silesian coal basin
Acta Geodynamica et Geomaterialia, 2013
The irregular distribution of stress in rock mass is a decisive factor for the origin of rock bursts. Besides, a sound knowledge of stress distribution is very important in the excavation of mine workings. Stress state is affected both by natural stress, including the gravitational, tectonic, hydraulic and residual stress and the stress induced by mining operations. Natural stress fields are defined by their geological structure and rock properties. It is important in mining practice to understand that there is a close relationship between recent and residual tectonic stress, as defined by tectonic evolution and tectonic structure. Since 1994, a large number of horizontal stress measurements have been carried out at a depth of 600 m to 800 m under the surface. The application of the results obtained from the measurements of stress and their comparison with the results of structural analysis and their generalization for the Karviná subbasin can be an important contribution to optimize the timespace designs of the mining activity.
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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
Rib Geomechanics : Its impacts in coal pillar extraction
Recent Advances in Rock Engineering, (RARE-2016), 2016
In Bord & Pillar (B&P) depillaring, there are redistributions of mining-induced stresses on acontinual basis, primarily with apronounced effect on the natural supports-ribs and stooks. A rib need to be stable on temporary time-frame such that (a) it should provide adequate safety to men and machines during depillaring and (b) it should offer no resistance to caving, even in the worst situation when it is not 'judiciously' reduced during theretreat. The stability of ribs can be estimated through a well-known aspect ratio called factor of safety (FOS), which is thestrength of ribs (S) divided by stress (P) coming over it. The strength of ribs plays a pivotal role in estimating thestability of ribs, depending on geo-mining details, sequences of extraction and related technical/managerial issues in a depillaring panel. This paper suggests a modified rib strength formulation in addition to discussing related issues.Estimation of stress on ribs using numerical modeling with rational engineering judgements and pragmatic simulations have also been briefed in this paper.Case-study of depillaring panels in eight mines, having no pronounced geotechnical problems due to rib stability, have been analysed with the formula presented in this paper.
The measurement and monitoring of stress in rock mass are very important tasks in mining geomechanics. With increasing mining depth and worsening of the geological and mining conditions, a suitable method to determine and monitor rock stress and stress changes due to longwall coal mining is needed. Detailed knowledge of the stress state in rock mass is very useful when designing safe mining activity, especially in rockburst areas. The paper presents a brief description of the conical ended borehole monitoring (CCBM) method for rock stress evaluation and the technical details of this innovative technology. The second part of the contribution evaluates and discusses initial results and experience obtained from the use of CCBM equipment for determination and observation of mining-induced stresses during mining of selected longwall panels in the conditions of the deep coal mines of the Upper Silesian Coal Basin.
Evaluation of Current Pillar Design Practice in Ramagundam Coal Belt
2016
Bord and Pillar mining is the oldest and most popular mining method to extract coal from underground. It is simple, easy to operate and reasonably safe. The pillars form the natural support to the overburden roof and transfer the load to floor. In the process a large portion of the coal remain blocked for long period till depillaring is carried out. The underground coal mines in India predominantly follow the Bord and Pillar method of extraction. The stability of the roof and floor depends on the stability of the pillar. The pillar stability depends on its strength, nature of coal, presence of discontinuity, method of extraction, etc. In India DGMS guidelines govern the design of pillars whereas in other parts of world the pillar design is based on strength calculation of coal and other factors. Some of the approaches are given by Bieniawski, CIMFR, Obert – Duvall, Jaiswal – Shrivastava etc. In this investigation an attempt has been made to investigate the current pillar design prac...
The Unpredictable Life Cycle of a Coal Pillar
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A unique circumstance created by monitoring a pre-driven longwall recovery room permitted measuring the stresses of a coal pillar throughout its entire life cycle in less than a week. A fender pillar, created in approximately the middle of a longwall panel at a depth of 650 ft, transformed from a solid barrier pillar to a yielding pillar to a residual pillar as 3 ft slices were methodically removed with the longwall shearer. The complete transformation, or life cycle, took place in less than 12 hours. The stresses were quickly transferred from the pillar onto the standing pumpable concrete supports and into the outby pillars. Roof to floor closure measurements, combined with the timing of the pillar behavior, provides a detailed look at the uncontrollable convergence of underground mine openings. Pillars remain the most important form of “primary support” and understanding these life cycles is vital for safe and efficient mine design, in both room and pillar and longwall panel extra...