Barrier Pillars Research Articles

User-friendly finite element design of main entries, barrier pillars, and bleeder entries

This contribution describes development and application of a user-friendly finite element program, UT3PC, to address three important problems in underground coal mine design: (1) safety of main entries, (2) barrier pillar size needed for entry protection, and (3) safety of bleeder entries during the advance of an adjacent longwall panel. While the finite element method is by far the most popular engineering design tool of the digital age, widespread use by the mining community has been impeded by the relatively high cost of and the need for lengthy specialized training in numerical methods. Implementation of UT3PC overcomes these impediments in three easy steps. First, a material properties file is prepared for the considered site. Next, mesh generation is automatic through an interactive process. A third and last step is simply execution of the program. Examples using data from several western coal mines illustrate the ease of using the application for analysis of main entries, barrier pillars, and bleeder entry safety.

Numerical modelling-based pillar strength estimation for an increased height of extraction

Underground extraction of total thickness of a thick coal seam in single lift by bord and pillar method increases pillar height during retreat. Field studies found that the increase in pillar height affects the depillaring operation adversely, especially, during caving of the strong/massive roof strata. Dilution in strength due to the increased pillar height caused catastrophic failure of barrier pillars and goaf overriding. This warrants a systematic study of pillar strength variation for the different heights of pillar. A review of different pillar strength estimation approaches for an analysis of the dilution in strength of the heightened pillar suggested that numerical modelling provides a better option for such a systematic study. Accordingly, investigations are conducted on simulated models in laboratory adopting a recognised numerical modelling procedure. The observed nature of variations in pillar strengths with the increase in its height in the numerical models and empirical formula is found to be matched. But a mismatch is found between the strength values of the two approaches with an increase in height of the pillar. Considering validity of the empirical formulation in Indian coalfields, a relationship is developed to incorporate a correction in the strength values of the numerical models. The suggested correction on the basis of this simple study of the pillar strength variation would be helpful for the use of the established simulation tool during the depillaring of a thick coal seam.

Geotechnical design of underground slate mines

Slate is one of the most important natural materials in Spain, with a potent extractive and processing industry concentrated in the autonomous communities of Galicia, Castile and León. Thanks to its resistance to external agents, its impermeability and its excellent cleavability, slate is used as for roofing and tiling. Almost all the active exploitations in our country where this resource is extracted are open pit mines, where the exploitation ratios have nearly reached their economic limit, making it necessary to look for alternatives that will allow the mining works to be continued. Underground mining is a solution that offers low exploitation ratios, with low spoil generation. The room-and-pillar method with barrier pillars is usually applied for the exploitation of slate deposits. There are several factors to be taken into account when designing a mine (economic, logistical, geotechnical, technical, environmental…), especially for an underground mine. This study focuses on the geotechnical design process of a room-and-pillar underground mine, based on the tributary area theory, the analysis of the tensions in the ground with numerical methods and the choice of an appropriate reinforcement in view of the expected instabilities. This explanation is completed with an example of a design that includes the estimate exploitation rates and production.

Результаты опытно-промышленных работ по повторной разработке на месторождении Жаман-Айбат (Республика Казахстан)

This article describes the features of the transition to the redevelopment of the Zhaman-Aibat field. The author presents the results of experimental-industrial works for the redevelopment of the field, shows the various schemes of interchamber and barrier pillars, adopted in the experimental-industrial development. The conclusions are the following: there is a need for further survey work, using computer modeling, to determine the optimal options for redevelopment.

Using a Fluid–Solid Coupled Numerical Simulation to Determine a Suitable Size for Barrier Pillars When Mining Shallow Coal Seams Beneath an Unconsolidated, Confined Aquifer

Barrier pillars are an effective and fundamental measure to prevent water inrush when mining shallow coal seams under an unconsolidated, confined aquifer. Based on the complex geological and hydrogeological conditions in the southern area of the Qidong coal mine, the no. 61 coal seam there was selected for a research demonstration. A fluid–solid coupled numerical simulation was carried out using the universal distinct element code. The hydraulic pressures and seepage rates in overlying strata were analyzed for two mining cases, near the aquifer and near the fault. The results showed that the degree of interconnection between the bed-separated and vertical fractures, and increases in hydraulic pressures and seepage rates in overlying strata were key factors in predicting potential water inrush when mining shallow coal seams under an unconsolidated, confined aquifer. Combining the numerical simulation results with China’s coal mining requirements, the no. 61 coal seam can be mined up to 90 m beneath the unconsolidated, confined aquifer, which limits mining to an altitude of −509.36 m. The width of the barrier pillar should be 30.7 m near the fault.

Occurrence, predication, and control of coal burst events in the U.S.

Coal burst represented a major hazard for some U.S. mining operations. This paper provides an historical review of the coal burst hazards, identifies the fundamental geological factors associated with these events, and discusses mechanisms that can be used to avoid their occurrences. Coal burst are not common in most underground mines. Their occurrence almost always has such dramatic consequences to a mining operation that changes in practice are required. Fundamental factors influencing coal burst events include strong strata, abnormal strata caving, elevated stresses, critical size pillars and the lack of sufficiently sized barrier pillars during extraction. These factors interact to produce excessive stress, seismic shock and loss of confinement mechanisms. Over the 90years of dealing with these hazards, many novel prevention controls have been developed including novel mine designs and extraction sequences, most of which are site specific in their application. Without an accurate assessment of the fundamental factors that influence coal burst and knowledge of their mechanisms of occurrence, control techniques may be misapplied and risk inadequately mitigated.

Stress Distribution on Blasting Gallery Barrier Pillar due to Goaf Formation During Extraction

Semi-mechanised blasting gallery mining is a sustainable option to achieve higher production and productivity from underground thick coal seams. Judicious design of underground blasting gallery panel requires understanding of stress distribution on barrier pillars during different stages of extraction. This paper presents a study of stress distribution in and around barrier pillar for the different stages of extraction in the blasting gallery panel. Finite difference analysis taken up for final excavation (depillaring) in the panel with different stages of extraction. Analysis revealed that the stress transferred on barrier pillar increased as progress of excavation increased. Maximum stress was observed at a distance of 10 and 12 m from the pillar edge for virgin and goaved out panel sideby respectively, which gradually decreased towards centre of the pillar.

A Proposed Method of Coal Pillar Design, Goaf Filling, and Grouting of Steeply Inclined Coal Seams Under Water-Filled Strata

The key to safely and efficiently mining steeply inclined coal seams under water-filled strata is to size barrier pillars appropriately and to control the structural integrity of the roof. We propose a method for goaf filling and grouting of steeply inclined coal seams and for appropriately sizing barrier and sectional coal pillars for mining in such conditions. Through theoretical analysis, an expression for a reasonable barrier pillar size was derived, based on the time span of mining, the hydraulic characteristics of the coal seam, the permeability characteristic of the grout in the goaf, and the water pressure. Based on the degree of saturation of the overlying strata and the mining conditions of the steeply inclined no. 48 coal seam of the Longhu mine in Qitaihe, China, we determined through theoretical analysis that the barrier pillar should be 80 m, the width of the sectional coal pillar should be 15 m, the length of the goaf filling should be 80 m, and the grouting length should be 40 m, to ensure safe production in that mine without geotechnical failures or injuries. These recommendations are being implemented.

A numerical study of stress changes in barrier pillars and a border area in a longwall coal mine

In longwall mining, stope voids are filled by caved-in rock materials. Cave-in and fracturing of roof strata cause severe disturbance to and stress changes in the host rocks. The study site is the Svea Nord coal mine. Longwall mining method was implemented to extract coal at the study site. It is intended that the coal in the border area on one side of the longwall panels will be mined after completion of the longwall mining. There is a concern about how the longwall mining affects the stress state in the border area and how the stress changes would affect future mining in the border area. 3D numerical modelling was conducted to consider the above concerns. Two models were constructed in the study. The first one is a local model that includes only two panels to study how the stresses in the barrier pillars are changed during panel mining. The second one is a mine-scale global model that includes all the panels with the aim of studying the disturbance to the border area by the longwall mining. The simulations show that the stresses in the barrier pillars fluctuate up and down during mining because of periodic cave-in events behind the longwall face. A failure zone of about 12m exists in the wall of the barrier pillar. A large portion of the barrier pillar is still intact and is thus capable of protecting the border area. A disturbed zone of about 20m is found to have developed in the wall of the main tunnels on the side of the border area after all the longwall panels have been mined out. The stress state in the remaining portion of the border area remains unchanged. Therefore, by considering a pillar with width of 25–35m into the border area along the access tunnels, it will be possible to mine the rest of the border area with room-and-pillar method in the future.

Blast-Induced Caving from Surface over Continuous Miner Panel at a 110m Cover in an Indian Mine

Continuous mining is a most popular mining method in USA and Europe. Nearly 60% of coal mining in the West is through continuous mining process in underground mines. Continuous miner technology, which has of late been introduced in India, is able to extract 65–74% of the coal in a panel depending upon the seam parameters as against 50–60% by semi-mechanized Bord and Pillar system of mining with SDL/LHDs, which is popular in India. The rate of extraction by the continuous miner being high is able to create larger panels, and thus, reduces the coal loss in barrier pillars. Presently, efforts are being made to introduce this technology on a larger scale in underground coal mines of Coal India Limited in India. When the first depillaring panel started its operation in November 2009 at Jhanjra, the panel was stopped after extraction of nine pillars by the Indian regulatory body (Directorate General of Mines Safety, DGMS, GOI) as the geological conditions of the overlying strata gave trouble of non-caving. The anticipated roof fall did not take place. The exposed roof area reached 10,400 m2, which was equivalent to 46,800 m3 of the void volume posing a threat of air blast. Since all the attempts to induce caving by underground blasting failed, an attempt was made from surface to induce the caving by destress blasting. This paper deals with the design of blasting parameters and execution of destress blasting keeping in view the safety and sentiments of nearby villages.

Measuring pillar width in trona mines using a body wave based in-seam seismic technique

A body wave based in-seam seismic technique was developed for measuring the width of barrier pillars in trona mines for the purpose of assessing the dissolution rate of barrier pillars when in contact with water, a critical problem faced by the trona industry. The main difference between this technique and the conventional in-seam seismic technique is that this technique makes use of high frequency P- and S-wave signals. The technique was tested at two trona mines in Wyoming. Three mine sites were utilized for the study, where the pillar widths varied from 82 to 107m. The velocity survey showed that the trona seam under study was very stable for the wave propagation with a mean standard deviation of only 1.5% for both P- and S-wave velocities. High frequency signals, in the range of 2500–5000Hz, were utilized for high resolution surveys. Three types of reflected signals were observed, which are P-waves, S-waves and S-waves due to the mode conversion at the seam-void interface. The elliptical mapping method was used to accommodate highly irregular survey lines encountered in underground mines. The method also allowed all reflected signals to be used simultaneously and, as a result, significantly increased the physical data available for the pillar width calculation. The average mapping error was estimated within ±4.6m, or about 5% of the pillar width. This body wave based in-seam seismic technique may also be utilized for other mines, such as limestone mines, where the seam impedance is similar to or higher than the impedance of surrounding rocks.

Prediction of the height of destressed zone above the mined panel roof in longwall coal mining

Abstract Longwall mining is one of the most widely used underground mining methods most suitable in relatively flat-lying, thick, and uniform coalbeds. Due to extraction of the coal seam, the panel roof strata above the mined zone will be destressed and then the roof loads will be redistributed and transferred to the front abutment and neighboring solid sections with higher load bearing capacity where the adjacent access tunnels and the corresponding barrier pillars are located. The height of destressed zone (HDZ), in this paper is taken as equivalent to the combined height of the caving and fracturing zones above the mined panel roof induced due to longwall mining. The height of destressed zone plays a vital role in accurate determination of the amount of loads being transferred towards front abutment and panel sides. The paper describes the mechanism of development of the height of this zone. Finally, five new simple, yet conclusive, mathematical approaches to estimate the height of destressed zone are presented. The results of the methods proposed are compared with each other and with the comparable methods. The methods proposed are further compared and verified with in-situ measurements reported in the literatures. The comparative results confirm the agreement that exist among the methods and those with the in-situ measurements as well. Finally the methods have shown that, in short term, the height of destressed zone ranges from 6.5 to 24 times the extraction coal seam height; while, in long term, the height of destressed zone ranges from 11.5 to 46.5 times the extraction coal seam height. Therefore, beyond this height the overburden pressure will be transferred towards the front abutment, the adjacent access tunnels, the intervening barrier pillars as well as the panel rib-sides.

Three-dimensional analysis of coal barrier pillars in tailgate area adjacent to the fully mechanized top caving mining face

Three-dimensional analysis of coal barrier pillars in tailgate area adjacent to the fully mechanized top caving mining face

Pilar-barreira entre painéis de lavra para a mina de carvão

A função de um pilar-barreira é proteger o painel de lavra em operação contra a transferência de pressões imposta pelos painéis de lavra adjacentes nos quais os pilares foram recuperados. No caso das minas de carvão em Santa Catarina, o pilar-barreira tem função de proteger o eixo da mina e os painéis em atividade contra a ruptura de pilares inesperada dos painéis de lavra antigos que já encerraram sua operação. O objetivo desse trabalho é verificar a validade do método empírico para o dimensionamento de pilar-barreira apresentado em Peng (1986) por meio da simulação numérica de uma geometria de lavra-padrão, que é aplicada para minas de carvão. Modelos numéricos em duas dimensões foram construídos obedecendo à geometria dos pilares do eixo e do painel de lavra para diferentes profundidades da camada Bonito. Foram consideradas as propriedades das materiais que formam o sistema piso-pilar-teto. Os resultados mostram que o método empírico é válido para a geometria projetada, onde os pilares do eixo não sofrem sobrecarga devido à lavra do painel e nem o teto sofre ruptura devido à redistribuição das tensões.

English

Numerous underground coal mines in the Pittsburgh coal basin of West Virginia and Pennsylvania have closed in the period 1980-2003. Of an estimated 1000+ once-active mines, only 11 are currently active, most relatively deep in the basin. These closures have caused flooding of mines and caused new discharges of mine water, of which all are either currently flooding or pumped and treated by mine operators or state agencies. This newly-created aquifer is a series of semi-interconnected compartments formed by mines separated by barrier pillars of 25->400 feet thickness. The leakage rate through barrier pillars is quite variable spatially and may be either very high (with hydrologically open conditions between adjacent mines) or very low (creating relatively isolated “pools” of one or more mines). Due to this fact and to the various closure dates of mines, the flooding history is complex with mines at a wide variety of water levels and flooding extents. However, experience shows that most mines near the Monongahela River flood within 10 years or less, and thus a new “equilibrium state” is expected to be attained by about 2015. At this time, flooding of most mines will be complete and an estimated 53,000 gpm (85,479 acre-ft/year) of mine discharge – both treated and untreated – will flow into the Monongahela watershed from below-drainage mines. This is in addition to an additional 8000 gpm estimated from abovedrainage free-draining mines. At this time, the flooded Pittsburgh coal will become the largest spatially-continuous high-yield aquifer exclusive of CambroOrdovician karst aquifers in the Northern Appalachian region. The locations of discharge from mines are known for 2003, subject to limitations of data availability, and may be speculatively projected for year 2015, subject to numerous assumptions and future circumstances. It is expected that all of the new discharges will require active treatment for metals removal to prevent discharge to the Monongahela and Ohio river watersheds. This resource of treated water may be utilized for other purposes, such as commercial development and aquaculture. Additional

English

Water quantity and quality conditions are described for a mine-pool aquifer in a fully flooded complex of underground mines in northern West Virginia. Abandoned mines in the Pittsburgh coal bed are contiguous, and separated by coal barrier pillars ranging from as little as 9 to about 60 meters thick. Barrier pillars are transmissive enough to circulate significant quantities of water between mines, yet they control head distribution and flow direction within the aquifer. The mine-pool acts as a partly confined to confined aquifer, and recharge is approximately balanced by withdrawal of about 5700 Liters/minute, leakage to adjacent mines, and unquantified outflow to unmined areas. Resulting drawdown prevents the mine-pool from discharging directly into overlying streams. A centrally located subgroup of mines within the aquifer currently acts as a ground-water sink, but water levels are slowly increasing in the sink, and in some outflow areas. Mine waters are highly reduced, with circumneutral pH, and variable Fe concentrations from 5 to over 100 mg/L. Total alkalinity averages about 200 mg/L with a mixed Ca-Na-HCO3-SO4 composition in recharge areas. End of flow path waters contain up to 600 mg/L alkalinity, and are Na-SO4 type waters with higher dissolved solids and metals concentrations. The shift from Ca to Na dominated waters is attributed mainly to cation exchange. Potentiometric head is increasing in the aquifer, and mine-pool withdrawal may have to be increased to prevent discharge to the surface. Mine-pool quality remains poor, and has shown slow improvement in 6 years of monitoring.

Ground-Water Flow and Quality in A Fully Flooded Underground Complex

Water quantity and quality conditions are described for a mine-pool aquifer in a fully flooded complex of underground mines in northern West Virginia. Abandoned mines in the Pittsburgh coal bed are contiguous, and separated by coal barrier pillars ranging from as little as 9 to about 60 meters thick. Barrier pillars are transmissive enough to circulate significant quantities of water between mines, yet they control head distribution and flow direction within the aquifer. The mine-pool acts as a partly confined to confined aquifer, and recharge is approximately balanced by withdrawal of about 5700 Liters/minute, leakage to adjacent mines, and unquantified outflow to unmined areas. Resulting drawdown prevents the mine-pool from discharging directly into overlying streams. A centrally located subgroup of mines within the aquifer currently acts as a ground-water sink, but water levels are slowly increasing in the sink, and in some outflow areas. Mine waters are highly reduced, with circumneutral pH, and variable Fe concentrations from 5 to over 100 mg/L. Total alkalinity averages about 200 mg/L with a mixed Ca-Na-HCO3-SO4 composition in recharge areas. End of flow path waters contain up to 600 mg/L alkalinity, and are Na-SO4 type waters with higher dissolved solids and metals concentrations. The shift from Ca to Na dominated waters is attributed mainly to cation exchange. Potentiometric head is increasing in the aquifer, and mine-pool withdrawal may have to be increased to prevent discharge to the surface. Mine-pool quality remains poor, and has shown slow improvement in 6 years of monitoring.

Stress State Control of Underground Structures by Method of Oriented Fluid Fracturing of a Rock Mass

The results are presented for physical and mathematical modeling of oriented fluid fracturing as a means of stress redistribution in the zone of influence exerted by mine workings. Technological schemes of deposit mining are shown; this makes it possible to unload interchamber and barrier pillars, as well as edge zones of seams and ore bodies being worked.

Control of arch formation in the room-and-pillar system of mining. Part II: Calculation of displacements near productive workings

Analytical expressions are derived for the displacements of the superincumbent strata above barrier and interchamber pillars. The state of the mass is controlled by regulating the elastic sag of the superincumbent strata.

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