Pillar Workings Research Articles

Modelling physical controls on mine water heat storage systems

The use of abandoned mines as a heat source and store has been receiving increased attention as a renewable heat source and storage solution in the transition away from traditional gas heating. The hydraulic, thermal and geomechanical processes governing heat storage and extraction are complex and understanding these processes is critical to safe heat extraction and injection into mine water systems. This paper outlines the development of a fully coupled thermo-hydraulic-mechanical (THM) 2D model to understand the mechanical stability of room and pillar workings during heat injection and extraction. It was found that the cyclical injection and extraction of heat does have an impact on both the modelled displacements and mechanical stability of the system. The impact risk reduces with temperature and the operational processes (e.g. injection temperature and water level) have more of an impact than the underlying geological conditions. These results are significant and could be included in a regulatory system to reduce the likelihood of stability impacts in mine water heating and cooling schemes. Thematic collection: This article is part of The Earth as a thermal battery: future directions in subsurface thermal energy storage systems collection available at: https://www.lyellcollection.org/topic/collections/thermal-energy

Risks and challenges affecting opencast pillar mining in previously mined underground bord and pillar workings

South Africa is one of the leading producers and exporters of coal globally. A significant amount of the country's production is obtained from previously mined underground bord and pillar workings. This coal is in the form of pillars and remnants on the roof and floor of the old workings. The good quality coal pillars were left behind as primary support during underground bord and pillar mining operations. Due to the depletion of virgin coal reserves, the pillars and remnant coal are now removed using opencast mining rather than underground methods. However, the secondary extraction of pillars and remnant coal from the old workings using opencast methods entails some serious challenges that have a negative impact on the safety and productivity of the operations, affecting both personnel and machinery. If these risk factors and challenges are managed properly, then the opencast mining operations could remove the pillars safely at recoveries competitive with those of virgin coal operations. In this study we review the recurring challenges affecting opencast pillar mining by means of field investigations and consultations with experts at five opencast pillar mining operations, and evaluate the best practices used to combat these challenges. It was found that each mine has its own unique conditions and challenges.

Estimation of rock load for junctions based on roof failure cases for safe mining operation

Coal mine roadway junctions in bord and pillar mining, in particular, are more vulnerable to roof fall due to the excavation induced stresses as well as general stress loading pattern inherent to the typical cross section achieved in practice. Roof fall incidences are generally more in junctions of development galleries and a cause of concern as they provide access to faces in different directions (dip/rise). The CMRI-ISM rock mass classification system, popularly known as Rock Mass Rating (RMR), is being used for the design of support system in underground coal mines for the last 30 years in India. In spite of the well-defined guidelines, it has been observed that roof failures are still a major concern in many underground coal mines. In this research, an attempt has been made to develop a new rock load prediction model for junctions of bord and pillar workings based on the geoengineering investigations carried out in 63 coal mines. After analysis of failure and stable cases, a best suited rock load predictor equation was developed. Analysis indicated that the maximum roof failure cases occurred for RMR values in between 30 and 45, and this zone is considered as highly unstable zone. Assessment of stable and unstable class is also done with respect to rock load for varied RMR. A general support design has also been suggested as per the RMR and severity of the roof fall. The suggested approach can help designing a rational support system for underestimated and overestimated zones thus minimising the roof fall cases leading to improved safety and productivity in underground coal mines.

Graphical and Numerical Methods for Stability Analysis in Surrounding Rock of Underground Excavations, Example of Boukhadra Iron Mine N.E Algeria

Underground mining continues to progress to difficult, complex and deeper levels in order to tackle ascending demand of minerals. These deep mining occurs in a very hard conditions, in which right practice must be implemented in order to overcome the technical and safety challenges and reap economic gains. The redistributed stresses, large deformations and creeping behavior in such environments worried engineers and geoscientific researchers. The working technic in Boukhadra iron mine is based on the sublevels slaughtered, with room, pillar workings and roof behaviour. Based on rock mechanics and geotechnical engineering methods, our research adopted geo-mechanical and numerical approaches to predict the rock mass behaviour in Boukhadra underground mine and to deal its stability problem. The results show that plastic deformations and stress points increasingly in pillars. In zones with low cover, the tensile zones develop at the roof, leading to a progressive rupture giving rise to a droop. In some areas, fractures develop in a rather remarkable way; in this case the breaking of the pillars and collapse can occur generalized and brutal. The stability of room-and-pillar mines does not depend only pillars. It involves the determination of critical factors and limits of mining. Our study demonstrates that the use of scientific and technical achievements, have become a necessity in mining.

Dynamic Response of Pillar Workings Induced by Sudden Pillar Recovery

When residual pillars are extracted in a short time in room and pillar mining, the load transferred from the removed pillar acts on neighbouring pillar workings in a dynamic manner. This study aims to investigate the instability of large mined-out areas triggered by dynamic disturbance resulting from residual pillar recovery. A mechanical model combining the pressure arch theory (PAT) based method and structural dynamics was first established to assess the stress state and deformation of a pillar subjected to combined effects of static and dynamic loads. The process of residual pillar recovery and potential induced instability of neighbouring pillar workings in a five-pillar system was further numerically simulated in both static and dynamic modes, and the response of adjacent pillars was investigated. It was found that the induced disturbance to a pillar can be characterised by the dynamic amplification coefficient R, the ratio of the increased vertical pillar load to the transferred load. Rmax can exceed 1.5 or even approach 2 in practical pillar recovery using the blasting method. Modelling results showed that while pillar workings adjacent to a removed pillar remain stable in static analysis, violent and large-scale pillar collapses can be triggered in the event of quick pillar recovery of blasting method. Results indicated that when the dynamic effect of pillar recovery is not considered, the load undertaken by adjacent pillar workings would be largely underestimated. To ensure mining safety, the induced dynamic effect should be accounted for in the design of the pillar recovery method.

Influence of rock mass rating and in situ stress on stability of roof rock in bord and pillar development panels

The re-orientation of bord and pillar layout is dictated by the direction of the horizontal stress. However, the competence of strata expressed in terms of rock mass rating (RMR) and the magnitude of in situ stress have a persuasive influence in deciding the gain out of re-orientation of galleries. However, a few queries like: a) Is it essential to re-orient all bord and pillar workings vis-a-vis the direction of in situ stress?; b) What if the strata are competent and the workings are subjected to low magnitude of horizontal stress?; c) What happens to a bord and pillar workings affected by high horizontal stress and having incompetent strata? have remained unaddressed. This paper, therefore, addresses such queries through a new approach of classification of the bord and pillar development workings using numerical modelling. This classification system will aid field engineers in deciding the need for re-orienting galleries vis-a-vis the RMR and in situ stress.

Stability of the parting between coal pillar workings in level contiguous seams during depillaring

In India, there are a number of cases of parting failures in mines due to instability of the parting between contiguous seam workings. So far, no suitable norm exists for predicting the stability of the parting during simultaneous depillaring of contiguous seams or sections, the latter being in thick seams. The factors affecting parting stability in contiguous depillaring have been identified as horizontal to vertical in situ stress ratio, width of splits/slices; rock mass rating, parting thickness and combination of different face positions in upper and lower workings. A large number of two- and three-dimensional models were run covering all these variables to arrive at suitable predictive equations to define parting stability and support requirements in contiguous seams/sections during depillaring operations in Indian coal fields. Finally, two representative case studies, briefly discussed in this paper, validated the developed equations for parting stability and support requirements at mine sites.

Modelling study for assessment of strata behaviour in bord and pillar working under soft floor condition

Safe and successful mining operation in bord and pillar workings under soft floor conditions requires a proper understanding of strata behaviour for better planning of mining layouts and selection of suitable mining horizon for better strata control and management of conditions arising due to excessive roof–floor convergence in the workings. This paper illustrates a modelling approach to assess the caving behaviour of strata, stress concentration, and roof–floor reactions in a depillaring bord and pillar working of Singareni coalfields in India. Assessment of floor heaving and roof–floor convergence in junctions of advance galleries is conducted with progressive goaf exposure. The calibrated model is used to predict the strata behaviour and roof–floor reactions in another depillaring panel. The modelling study has been helpful in better understanding of strata control issues for a confident decision making to ensure better safety and productivity of mine workings.

Logistic regression model for prediction of roof fall risks in bord and pillar workings in coal mines: An approach

The roof fall hazards are the major problems in underground coal mines, which are generally unpredictable due to the associated uncertainties arising out of the complexity of geological conditions and variability in mining parameters. During the six year period, 1996–2001, 253 Indian coal miners lost their lives and 401 are seriously injured in 490 different roof fall accidents. In India, 32.7% of the total fatal injuries in coal sectors are due to roof fall in bord and pillar method of workings. This paper attempts to predict the severities of roof fall accidents based on some major contributing parameters using the binary logistic regression model. In total, 128 roof fall accidents for the last few years from five underground coal mines in India having bord and pillar method of working are analyzed for this study. The results revealed that wider gallery width is more prone to major and serious accidents than narrower gallery width. Thin seams are more amenable to major accidents in comparison to thick seams. Unsupported or partially supported roofs have higher risk for contributing major as well as serious accidents. Deep workings are more prone to major accidents as compared to shallow depth workings.

Exploitation of developed coal mine pillars by shortwall mining—a case example

The shortwall mining technique is similar to longwall mining but with shorter face lengths, ranging between 40 and 90 m, with the aim of controlling the caving nature of the overlying upper strata, the load on support and the overall operation of the supports applied at the face. Field observations and three-dimensional numerical modelling studies have been conducted for the longwall panel extraction of the Passang seam at Balrampur Mine of SECL to understand the caving behavior of the overlying upper strata. A large area of the Passang seam adjacent to the longwall panels has already been developed via bord and pillar workings. In this paper, numerical modelling studies have been conducted to assess the cavability of the overlying strata of the Passang seam in the mine over developed bord and pillar workings along with the support requirement at the face and in the advance gallery. The caving nature of the overlying rocks characterized by the main fall is predicted for varying face lengths, strata condition and depths of cover. The support resistance required at the face, the load in the advance gallery and its optimal obliquity were estimated for faster exploitation of the developed pillars in the Balrampur mine by shortwall mining.

Stability of the parting between coal pillar workings in level contiguous seams

The factors affecting parting or innerburden stability in level contiguous pillar workings in coal mines have been identified as depth of cover, parting thickness, lateral eccentricity between lower and upper pillar workings, horizontal in situ stresses , Rock Mass Rating and extraction ratio. A large number of two- and three-dimensional FLAC models were run covering all these variables to arrive at a suitable predictive equation to define parting stability and support requirement in contiguous seam workings. Four unstable and four stable cases of parting from Indian coalfields were modelled using FLAC 3D and the results have been satisfactorily compared with the predictive equation.

The monitoring and prediction of mining subsidence in the Amaga, Angelopolis, Venecia and Bolombolo Regions, Antioquia, Colombia

Amaga, Angelopolis, Venecia and Bolombolo are small towns located in Antioquia, in the Central Cordillera of the Colombian Andes. Mining has been practised in this region for a period of at least 100 years. This mining has mainly been small-scale, poorly mechanised and restricted to shallow room and pillar workings. Recently, the semi-mechanisation of some mines has enabled coal to be extracted using longwall mining methods. However, this has resulted in subsidence that has caused severe damage to structures, residential property, and agricultural land, and also induced landslides. In the British Isles, there are several reliable methods that can be used to predict the likelihood and magnitude of mining subsidence. The British Coal Corporation and the University of Nottingham have developed one such method, the “Subsidence With Influence Function Technique (SWIFT).” Based on mining subsidence observations undertaken in the coalfields of Britain over a period of approximately 50 years. The SWIFT program was used to predict the magnitude of subsidence, above a longwall panel, at the Industrial Hullera mine in Colombia. The results were then compared with subsidence profiles obtained from precise levelling and field monitoring. In each case, the SWIFT program overestimated the magnitude of mining subsidence by 0.17–0.20m. However, the morphology of the subsidence profile, area-of-influence and location of maximum subsidence were similar. This overestimation of the predicted subsidence was attributed to the occurrence of strong, igneous rocks, such as rhyolite sills, in the Colombian coal measures. These strong, competent horizons act as cantilever beams during subsidence, causing bed separation and therefore reducing the magnitude of subsidence. In spite of these differences, mining subsidence can be predicted with a reasonable degree of accuracy and precision using the SWIFT technique, provided the software is calibrated and used in conjunction with local expertise.

Subsidence problems due to abandoned pillar workings in coal seams

Abandoned mine works are a potential cause of ground subsidence and hence are of major concern where development or re-development is to take place. In the United Kingdom, information necessary to locate potential hazards may be available but occurs in numerous scattered locations and it may require considerable time to access the data. In recent years thematic geological maps have been produced for some of the British coal field areas. The paper describes the historical evolution of the mining system, emphasising the pillar workings which began in the sixteenth century. Methods of investigation such as downhole hammer and geophysics are briefly mentioned and the importance of zoning the land discussed. Stabilisation measures may involve occupying the workings with hydraulically-emplaced fill or cheap bulk grout. In areas of less risk, special foundation structures may be used.

Sinkhole subsidence due to mining

This paper reviews the modes of formation of sinkhole subsidence associated with mining activities, drawing on examples in India. Sinkhole (pot-hole) subsidence is an abrupt local depression at the surface which can be hazardous to life and property due to its tendency to occur without warning. Shallow extraction, weak overburden and geological discontinuities are the main factors which cause them. Sinkholes occur due to the failure of a mine roof which migrates through the overlying strata until the failure zone intercepts the unconsolidated overburden. Alternatively they may occur by the creation of cavities in the overburden following the inflow of sand and soil from the overlying weathered and friable strata through faults. Overburden cavities eventually cave in and sinkholes appear at the surface. Sinkholing phenomena can be controlled to some extent by proper design of mining supports and construction of walls to create a barrier around an area prone to sinkholes in bord and pillar workings. Backfilling and grouting can be used to stabilize abandoned underground workings.

Modelling of barrier pillars in bord and pillar workings : Esterhuizen, G S Proc 7th ISRM International Congress on Rock Mechanics, Aachen, 16–20 September 1991V2, P1093–1097. Publ Rotterdam: A A Balkema, 1991

Modelling of barrier pillars in bord and pillar workings : Esterhuizen, G S Proc 7th ISRM International Congress on Rock Mechanics, Aachen, 16–20 September 1991V2, P1093–1097. Publ Rotterdam: A A Balkema, 1991

Statistical prediction of subsidence events above abandoned room and pillar workings in Penn Hills area : Cervantes, J A; Kim, Y C; Farmer, I W Proc 8th Annual Workshop Generic Mineral Technology Center Mine Systems Design and Ground Control, Reno, 5–6 November 1990P145–156. Publ Blacksburg: Virginia Polytechnic Institute and State University, 1990

Statistical prediction of subsidence events above abandoned room and pillar workings in Penn Hills area : Cervantes, J A; Kim, Y C; Farmer, I W Proc 8th Annual Workshop Generic Mineral Technology Center Mine Systems Design and Ground Control, Reno, 5–6 November 1990P145–156. Publ Blacksburg: Virginia Polytechnic Institute and State University, 1990

Estimation of critical convergence and rock load in coal mine roadways ? an approach based on rock mass rating

Based on field instrumentation in eight different coal mines representing varying depths and strata conditions, a relation for obtaining the critical convergence value has been established. In development heading for bord and pillar workings this relation can be used successfully to control the premature collapse of the roof. An empirical relation for rock load has been established. This can be utilized for optimum design of support system. The roofs have been categorized as stable, short-term stable and unstable. Proper attention should be provided for an unstable roof and the support design is to be changed before the convergence reaches the critical value.

Fender design in bord and pillar workings : Ram, A; Singh, T N J Min Metal FuelV36, N12, Dec 1988, P533–538

Fender design in bord and pillar workings : Ram, A; Singh, T N J Min Metal FuelV36, N12, Dec 1988, P533–538

Stability of parting rock between level contiguous coal pillar workings

Stability of parting rock between level contiguous coal pillar workings

Stability of parting rock between level contiguous coal pillar workings

The factors affecting stress distribution in thin parting between contiguous pillar working have been identified as depth of cover, parting thickness, lateral shift between pillars, percentage extraction and modulus ratio. A large number of two-dimensional finite element models covering these variables ultimately give an expression for the maximum tensile stress. Barton's rock classification [1] is next modified considering this stress and three-dimensional finite element analysis of four failed cases. In all, six collapsed cases and four stable cases have been used to estimate the safe roadway junction spans as against the existing spans, using the modified classification equation. Safety factors of parting rock in all case studies have also been calculated using a recently developed failure criterion for jointed rock masses [2] as well as the Hoek and Brown criterion [3], comparing their performance. Design guidelines are presented for contiguous pillar workings.