Yoginder P. Chugh

Effects of moisture on strata control in coal mines

Abstract Moisture has been known to influence strata failures in coal mines for a long time. However, it is only in the last decade that researchers have attempted to quantify this effect. In the laboratory, attempts have been made to study the effects of moisture on the strength of coal-bearing rocks, moisture gain in shales as a function of time and pressures generated due to swelling, slaking and swelling indices and their possible correlation to strata control problems, and effect of humidity variations on anchor creep and bearing plate performance in conjunction with conventional bolts. In the field, researchers have attempted to correlate humidity variations with the incidence and frequency of roof falls and roof convergence, and effects of air tempering on reducing humidity variations in the mine. A concise review of what is known about effects of moisture on strata control with emphasis on the Illinois basin coal mines is presented in this paper. Some approaches to mitigate the effects of moisture—leaving coal in the roof, guniting and shotcreting, application of sealants—are then briefly discussed.

Dynamic subsidence simulation and topsoil removal strategy in high groundwater table and underground coal mining area: a case study in Shandong Province

Coal mining in high groundwater table areas causes many environmental issues. In China, such regions have three unique and significant characteristics, (1) multiple coal seams; (2) thick coal seams and (3) high groundwater, which lead to mine subsidence ponds. In the past, reclamation of disturbed land was carried out after subsidence stabilised. Such reclamation is a low-percentage of reclaimed farmland, and has high reclamation cost, difficult construction conditions, because of relatively flat terrain and the shortage of backfilling materials in flat topography. Therefore, salvaging the topsoil before it is submerged in water and reusing it while subsidence becomes stable would be a good way to improve the reclamation efficiency. This paper considers a longwall panel in Shandong province as a case study. Dynamic mining subsidence was simulated using mining subsidence prediction software. To illustrate subsidence development and its effects on farmland, the ground was first divided into cell size of 40 m along with pre-mining topography. The mining subsidence and water logging time for each cell were then calculated. Based on the simulations, the topsoil removal time, scope and depth of topsoil was determined. The salvaged topsoil could be reused subsequently, which would provide enough filling material to raise the percentage of reclaimed farmland, and reduce reclamation costs. The research benefits the development of concurrent mining and reclamation technology in underground mining sites, and promotes the synchronisation of exploitations and treatments.

Estimation of in situ viscoelastic parameters of weak floor strata by plate-loading tests

SummaryThis paper explores an approach to estimation of the viscoelastic parameters of weak floor strata using plate-loading tests. Mathematical equations are derived to describe deformation-time history for the standard Burgers model under three-dimensional stress-strain conditions, which are verified with a finite element model. A number of time-dependent in-mine plate-loading tests were conducted and the values of the viscoelastic parameters were estimated based on the derived equation. It is shown that large discrepancies between the parameter values estimated from the plate-loading tests and those acquired from the in-mine convergence data are the results of neglecting adjacent pillar interaction. Finite element modelling was conducted to investigate the effects of adjacent pillar interactions upon the pillar settlement. Based on the finite element analyses, a correction curve was developed to adjust the viscoelastic parameter values to take into account the interaction between adjacent pillars. With the correction, the parameter values estimated from the plate-loading tests compared favourably with those acquired from the in-mine convergence results.

Noise Variability in Underground Room and Pillar Coal Mines

Noise in an underground coal mine has dominant components generated mainly from 3 sources: (a) continuous mining machines, (b) roof bolters, and (c) cars/vehicles used to transport personnel and/or coal. Each of these 3 noise sources also has a number of well-defined sub-sources with their own noise characteristics. Sound level meters were used to collect noise data in the form of instantaneous readings and also to check calibration of other sound measuring instruments. The most useful information was obtained from a spectrum analysis of continuous digital recordings of noise over time. This paper discusses the variability or dynamics of generated noise in both frequency and time domains in relation to several independent variables related to coal extraction and transportation processes.

A numerical approach to subsidence prediction and stress analysis in coal mining using a laminated model

Subsidence due to underground mining may cause damage to surface structures and farmland and change surface and subsurface drainage patterns. During the past few decades, a number of subsidence prediction techniques have been proposed. Currently, the influence function method is the most widely used method for prediction of longwall coal mining subsidence. The influence function method may provide accurate results if a suitable function can be established for a specific site or region based on field observations. The method, however, suffers from generality, and cannot be used in virgin areas. Furthermore, there is a need to develop a model which can simultaneously analyze in-mine stability of mine workings including roof-pillar-floor interaction and associated surface and subsurface subsidence. Since the rock mass surrounding coal seams usually consists of distinct layers/beds and field observations suggest relative movements along bed interfaces, a homogeneous isotropie continuum model cannot effectively describe ground movements due to coal mining. Therefore, in finite element modeling some investigators reduce the values of the Youngs modulus to very small, say one percent of the original laboratory value, to obtain acceptable subsidence results, e.g. Bowders et al [1]. Valuable attempts have also been made to model the sliding between beds in finite element modeling [2], but results have been presented only for two-dimensional problems due to large memory and computation time requirements for three-dimensional problems. A three-dimensional numerical model based on a laminated medium used in conjunction with boundary element technique was developed to simulate the ground movements due to coal mining, and results obtained were reasonable [3]. Recently, this model has been further modified to simultaneously analyze both subsidence and in-mine stability of mine workings including roof-pillar-

Ultimate bearing capacity and settlement of coal pillar sub-strata

SummaryThis paper develops a rational approach for design of coal pillars under weak floor strata conditions considering ultimate bearing capacity (UBC) as well as pillar settlement. An approximate solution is presented for estimation of UBC for a shallow foundation on a two-layered rock system with consideration of both cohesion (c) and (ø) for both layers. Similarly, deformability underneath a full-size pillar is estimated from deformability calculated from plate loading tests. The effect of adjacent pillars on UBC and deformability of coal pillars in a panel is considered using foundation engineering analysis techniques. The design of pillars based on limiting settlements considers both differential settlements as well as mean settlement of pillar in a panel. An attempt is made to validate the proposed design approach based on field data and observations at an Illinois mine.

Computer-aided planning and management of surface coal mines

Abstract Mini-, micro- and main-frame computers and their associated software are being utilized to (1) analyze geologic and hydrologic data obtained during exploration, (2) select and replace equipment, (3) develop alternate mining and reclamation plans, (4) establish technical and economic feasibility of various mining alternatives, (5) evaluate hydrologic impacts of mining, (6) develop management information systems for effective and efficient control of surface mining equipment and operations, and (7) train mine personnel. This paper briefly describes a variety of applications of computers in surface mining operations. Specific examples are given of five research projects involving development and use of new computer-based planning and management techniques.

Subsidence prediction in shallow room and pillar mines

SummaryA mathematical model has been developed utilizing the relative flexural strength of the strata overlying a coal seam to predict the vulnerability of shallow room and pillar mined areas to subsidence. The model assumes the failure of the immediate roof as the precursor of a subsidence event in shallow room and pillar mines. After the roof fails, either a sink hole subsidence event develops if the unconsolidated material is thin and dry; or a subsidence trough forms if the unconsolidated material is thick and wet. The model relates the Missavage number (Mn), which is dependent only on the stratigraphy and rock strength, to the extraction number, which is dependent only on the extraction ratio and maximum span of the opening. A high correlation coefficient (r=0.78) betweenMn and the extraction number for 27 subsidence events in a southern Illinois mine showed potential for using this model to delineate areas more vulnerable to subsidence. The developed and validated model was then subjected to a blind test on a 12.9 square kilometer area of an Illinois Coal Basin mine. The model successfully predicted 10 out of 12 subsidence events in the blind half of the study area and two of three additional subsidence events in the known half of the study area.

Mitigating mine subsidence dynamically to minimise impacts on farmland and water resources: a case study

Subsidence related to underground mining can negatively impact farmland and water resources. This paper has developed dynamic subsidence reclamation (DSR-reclamation concurrent with mining) approaches to minimise these impacts and are illustrated using a case study in China. Two reclamation plans using DSR and traditional reclamation (TR-reclamation upon completion of all mining) approaches were designed respectively. Then farmland utilisation, water resources development and cost-benefit analyses were performed for the two sets of plans. The results show that farmland and water resource utilisation by DSR is larger than TR throughout the mining period. Farmland utilisation by DSR increased as much as 12.94%. Similarly, final water volume by DSR was 15.01% more than TR. The total revenue using DSR increased by 24.17% as compared with TR over a ten year period. So, DSR can be a very positive and cost effective approach for protection of farmland and water resources.

A Coal Burst Mitigation Strategy for Tailgate during Deep Mining of Inclined Longwall Top Coal Caving Panels at Huafeng Coal Mine

A coal burst mitigation strategy for tailgate in mining of deep inclined longwall panels with top coal caving at Huafeng Coal Mine is presented in this paper. Field data showed that coal bursts, rib sloughing or slabbing, large convergence, and so forth frequently occurred within the tailgate entries during development and panel retreating employing standard longwall top coal caving (LTCC) layout which resulted in fatal injuries and tremendous profit loss. The contributing factors leading to coal bursts were analyzed. Laboratory tests, in situ measurement, and field observation demonstrate that the intrinsic bursting proneness of the coal seam and immediate roof stratum, deep cover, overlying ultrathick (500–800 m) conglomerate strata, faults, and, most importantly, improper panel layout led to coal bursts. By employing a new strategy, that is, longwall mining with split-level gateroads (LMSG), gateroads on either end of a LMSG panel are located at different levels within a coal seam, adjacent LMSG panels overlap end to end, and the tailgate of the adjacent new LMSG panel can be located below the headgate entry of the previous LMSG panel or may be offset horizontally with respect to it. Numerical modeling was carried out to investigate the stress distribution and yield zone development within surrounding rock mass which was validated by field investigation. The results indicate that standard LTCC system gave rise to high ground pressure around tailgate entries next to the gob, while LMSG tailgate entry below the gob edge was in a destressed environment. Therefore, coal bursts are significantly mitigated. Field practice of LMSG at Huafeng Coal Mine demonstrates how the new strategy effectively dealt with coal burst problems in mining of deep inclined longwall panels with a reduced incidence of ground control problems. The new strategy can potentially be applied in similar settings.