Underground Longwall Mining Research Articles

Estimation of active rockburst prevention effectiveness during longwall mining under disadvantageous geological and mining conditions

Abstract Underground longwall mining of coal seams in the Upper Silesian Coal Basin is currently being carried out under increasingly difficult geological and mining conditions. Mining depth, dislocations and mining remnants are the main factors responsible for the most significant rockburst hazard, which can be minimized via the use of active and passive rockburst prevention. Active rockburst prevention in longwalls is usually based on blasting, in order to either destress local stress concentrations in the rock mass or to fracture the thick layers of strong roof rocks to prevent or minimize the impact of high energy tremors on excavations. The accurate estimation of active rockburst prevention effectiveness is particularly important when mining under disadvantageous geological and mining conditions, which are associated with high levels of this hazard. The efficiency of blasting applied for this purpose is typically evaluated from the seismic effect, which is calculated based on seismic monitoring data and the weight of the charged explosive. This method, as used previously in the Czech Republic, was adopted in the present study to analyze conditions occurring in a Polish hard coal mine in the Upper Silesian Coal Basin. Parameters of long hole destress blastings in roof rocks (torpedo blastings) from the face of the assigned longwall in coal seam no. 507 were correct a success according to the seismic effect method and corresponded to observations made in situ. The analytical method presented enables the rapid estimation of destress blasting effectiveness and could also be useful when determining appropriate active rockburst prevention.

New Methods of Mining Technological System’s Performance Analysis

The mining production systems, both for underground and open pit extraction consist mainly in a string of equipment starting with the winning equipment (shearer loader, in case of underground longwall mining or bucket wheel excavator in case of open pit mining), hauling equipment (armored face conveyor in longwall mining or the on-board belt conveyor in case of excavators), main conveying equipment (belt conveyor in both cases), transfer devices, stock pile or bunker feeding equipment. This system of mainly serially connected elements is characterized by the throughput (overall amount of bulk coal respectively overburden rock produced), which is dependent on the functioning state of each involved equipment, and is strongly affected also by the process inherent variability due to the randomness of the involved processes (e.g. the cutting properties of the rock). In order to model and simulate such production systems, some probabilistic methods are applied arising from the artificial intelligence approach, involving unit operations and equipment, as the overall system as a whole, namely the Monte Carlo simulation, neural networks, fuzzy systems, and the Load Strength Interference methods. The results obtained are convergent and offer the opportunity for further developments of their application in the study of mining production systems.

Groundwater system for the periods of pre- and post-longwall mining over thin overburden

The groundwater loss is one of the primary issues for environmental destruction. Underground longwall mining methods can cause rapid subsidence of the overburden strata. Especially in shallow coal seam mining. Since the subsidence through the overburden to ground surface, the groundwater may be affected and drawdown into the lower levels above the longwall panels. This paper analysed the potential impact of longwall mining on groundwater over thin overburden. Four water wells were drilled down to the proposed deformation zone above two longwall panels to record the different water levels’ variations throughout the mining period. A numerical groundwater flow model (GGU-SS-FLOW3D) was performed to predict the water levels in the periods of pre- and post-mining conditions. To set-up model, local mining conditions and regional geology characteristics were ascertained in this study. The measurement data from the four water wells were adopted to calibrate the flow3D model. The field measurement and GGU-SS-FLOW3D model can help to better understand the variation of the groundwater system related to the ground subsidence from longwall mining over thin overburden.

The Impact of Underground Longwall Mining on Prime Agricultural Land: A Review and Research Agenda

AbstractCoal mining and agriculture have repeatedly come into conflict when they co‐occur. Although seemingly benign when compared with surface mining, underground coal extraction techniques (including longwall mining) cause subsidence of agricultural land and loss of productivity. Despite growing concerns for global food security and increasing demand for coal resources, there is little peer‐reviewed literature on the impacts of longwall mining in prime agricultural areas. In this paper, we examined the present knowledge of subsidence impacts of longwall mining on agriculture and how this may be interpreted for specific locations such as Australia. The review found that subsidence affects soil properties, hydrology and topography. The main impacts on agriculture are altered soil and groundwater hydrology, modified topography associated with increased erosion or waterlogging risk, and zones of compaction or cracking that cause soil physical and chemical changes. Agricultural productivity is also reduced through altering the types of farming practices that are suited to subsided non‐uniform landscapes, decreasing farming efficiency through increasing paddock heterogeneity and decreasing ease of workability. There is a need to consider these multiple impacts under local conditions, with particular regard to the interaction of mine subsidence‐associated disturbances with farming practices. We conclude by describing future research directions required for Australia and other countries outside of the USA—where most of the research has been conducted. Australia has unique soil and climatic conditions making extrapolation of studies from the USA on subsidence impacts and mitigation problematic. Copyright © 2014 John Wiley & Sons, Ltd.

Investigation of overburden behaviour for grout injection to control mine subsidence

This paper describes a field and numerical investigation of the overburden strata response to underground longwall mining, focusing on overburden strata movements and stress concentrations. Subsidence related high stress concentrations are believed to have caused damage to river beds in the Illawarra region, Australia. In the field study, extensometers, stressmeters and piezometers were installed in the overburden strata of a longwall panel at West Cliff Colliery. During longwall mining, a total of 1000mm tensile deformation was recorded in the overburden strata and as a result bed separation and gaps were formed. Bed separation was observed to start in the roof of the mining seam and gradually propagate toward the surface as the longwall face advanced. A substantial increase in the near-surface horizontal stresses was recorded before the longwall face reached the monitored locations. The stresses continued to increase as mining advanced and they reached a peak at about 200m behind the longwall face. A numerical modelling study identified that the angle of breakage (i.e., the angle of the boundary of caved zone) behind the longwall face and over the goaf was 22–25° from vertical direction. This is consistent with the monitoring results showing the high gradient of stresses and strains on the surface 150–320m behind the mining face.

Post Processing of Zone Budgets to Generate Improved Groundwater Influx Estimates Associated with Longwall Mining

Impacts of underground longwall mining on groundwater systems are commonly assessed using numerical groundwater flow models that are capable of forecasting changes to strata pore pressures and rates of groundwater seepage over the mine life. Groundwater ingress to a mining operation is typically estimated using zone budgets to isolate relevant parts of a model that represent specific mining areas, and to aggregate flows at nominated times within specific model stress periods. These rates can be easily misinterpreted if simplistic averaging of daily flow budgets is adopted. Such misinterpretation has significant implications for design of underground dewatering systems for a new mine site or it may lead to model calibration errors where measured mine water seepage rates are used as a primary calibration constraint. Improved estimates of groundwater ingress can be made by generating a cumulative flow history from zone budget data, then differentiating the cumulative flow history using a low order polynomial convolved through the data set.

US coal geophysics expands and gains more support.

SUMMARY The US coal geophysics program was once the exclusive domain and best kept secret of one major coal company. The company was able to develop a full spectrum of remote-sensing technologies to address their complex upstream and downstream challenges. The program started in the mid-1970s, and greatly expanded a decade later with more robust and diverse applications. Innovative geophysical surveys were routinely conducted to enhance the company’s coal exploration, engineering, and environmental programs. As the largest operator of underground longwall mines in the country, Consol Energy was able to improve their mining productivity and mine safety by detecting and imaging potential geologic anomalies that could create adverse underground mining conditions prior to mine development. High-resolution surface seismic surveys were routinely conducted years in advance of mine development. Other geophysical methods were at times employed as part of internal multi-disciplinary (geology, geophysics, geohydrology, and ground control) studies to address environmental, engineering, and permitting issues.

Continuous monitoring of mining induced strain in a road pavement using fiber Bragg grating sensors

This paper describes the application of fiber Bragg grating (FBG) based sensors for monitoring road pavement strains caused by mining induced ground subsidence as a result of underground longwall coal mining beneath a major highway in New South Wales, Australia. After a lengthy planning period, the risks to the highway pavement were successfully managed by the highway authority and the mining company through a technical committee. The technical committee comprised representatives of the mining company, the highway authority and specialists in the fields of pavement engineering, geotechnical engineering and subsidence. An important component of the management strategy is the installation of a total of 840 strain and temperature sensors in the highway pavement using FBG arrays encapsulated in glass-fiber composite cables. The sensors and associated demodulation equipment provide continuous strain measurements along the pavement, enabling on-going monitoring of the effects of mining subsidence on the pavement and timely implementation of planned mitigation and response measures to ensure the safety and serviceability of the highway throughout the mining period.

HYBRID INDOOR TRACKING OF HUMANS IN HAZARDOUS ENVIRONMENTS

The reliable tracking of humans and materials in indoor scenarios is an ongoing research issue. For example, the monitoring of humans in partially hazardous environments – like the surroundings of an underground longwall mining infrastructure – is crucial to save human lives. A centroid location estimation technique based on received signal strength (RSS) readings offers a well known and low-cost tracking solution in such a rough environment where many other systems with optical, magnetical or ultrasound sensors fail. Due to signal fading the RSS values alone cannot ensure a precise tracking. The sensor fusion of the RSS-based localization with an inertial navigation system (INS) leads to a more precise tracking. The long-term stability of the RSS-based localization and the good short- term accuracy of the INS are combined using a Kalman filter. The experimental results on a motion test track show that a tracking of humans in multipath environments is possible with low infrastructural costs.

Rock mechanical analysis of a Ml= 4.0 seismic event induced by mining in the Saar District, Germany

SUMMARY On 2008 February 23 a 4.0 magnitude seismic event occurred in close proximity to a deep underground longwall coal mine in the German Saar mining district. Preceding this event was a series of smaller seismic events, which can be traced back to geological structures. A seismic network was installed to observe and analyse the seismic events. Underground core drilling was performed to obtain rock samples for rock mechanical testing. Possible causes for the events, failure of rock mass or slip on unknown faults, are evaluated by numerical methods. Failure of rock mass is excluded as a cause since the stresses are well below the strength of the rock mass. Seismic events are of similar fault plane strike and are organized in several clusters, associated with local and regional tectonic elements. It was found from backcalculation that fault planes with friction angles as low as 8 ◦ must have been present. The role of pore pressure on the fault stability is discussed and found to play no role in generating the seismic events. Finally, the question of mining in a rock mass obviously close to failure is addressed and it was concluded that any mining activity would lead to a significant seismic response.

Seismic wave propagation through surface basalts – implications for coal seismic surveys

Seismic reflection surveying is one of the most widely used and effective techniques for coal seam structure delineation and risk mitigation for underground longwall mining. However, the ability of the method can be compromised by the presence of volcanic cover. This problem arises within parts of the Bowen and Sydney Basins of Australia and seismic surveying can be unsuccessful. As a consequence, such areas are less attractive for coal mining. Techniques to improve the success of seismic surveying over basalt flows are needed.In this paper, we use elastic wave-equation-based forward modelling techniques to investigate the effects and characteristics of seismic wave propagation under different settings involving changes in basalt properties, its thickness, lateral extent, relative position to the shot position and various forms of inhomogeneity. The modelling results suggests that: 1) basalts with high impedance contrasts and multiple flows generate strong multiples and weak reflectors; 2) thin basalts have less effect than thick basalts; 3) partial basalt cover has less effect than full basalt cover; 4) low frequency seismic waves (especially at large offsets) have better penetration through the basalt than high frequency waves; and 5) the deeper the coal seams are below basalts of limited extent, the less influence the basalts will have on the wave propagation. In addition to providing insights into the issues that arise when seismic surveying under basalts, these observations suggest that careful management of seismic noise and the acquisition of long-offset seismic data with low-frequency geophones have the potential to improve the seismic results.

Optimizing Wireless LAN for Longwall Coal Mine Automation

A significant development in underground longwall coal mining automation has been achieved with the successful implementation of wireless LAN (WLAN) technology for communication on a longwall shearer. WIreless-FIdelity (Wi-Fi) was selected to meet the bandwidth requirements of the underground data network, and several configurations were installed on operating longwalls to evaluate their performance. Although these efforts demonstrated the feasibility of using WLAN technology in longwall operation, it was clear that new research and development was required in order to establish optimal full-face coverage. By undertaking an accurate characterization of the target environment, it has been possible to achieve great improvements in WLAN performance over a nominal Wi-Fi installation. This paper discusses the impact of Fresnel zone obstructions and multipath effects on radio frequency propagation and reports an optimal antenna and system configuration. Many of the lessons learned in the longwall case are immediately applicable to other underground mining operations, particularly wherever there is a high degree of obstruction from mining equipment

Microseismic observation of fault reactivation at underground coal mines

The movement of faults in a mining area can fracture the rock mass at the working face and cause roof collapse to damage machinery and working environment, which may cost mine operators millions of dollars to fix. Understanding of fault reactivation to the mining processes is thus crucial for the mines to control and prevent the problems. Since 1996 CSIRO has applied the microseismic technique to monitor fault activities at a number of underground longwall coal mines in Australia. The results presented in this paper have demonstrated that the fault reactivation associated with mining can be observed remotely and in real-time using an array of seismic sensors. The microseismic results have provided very useful information to the coal mines in the prediction of the impending hazardous areas.

Coal dust exposures in the longwall mines of New South Wales, Australia: a respiratory risk assessment.

This paper presents an analysis of personal respirable coal dust measurements recorded by the Joint Coal Board in the underground longwall mines of New South Wales from 1985 to 1999. A description of the longwall mining process is given. In the study, 11 829 measurements from 33 mines were analysed and the results given for each occupation, for seven occupational groups, for individual de-identified mines and for each year of study. The mean respirable coal dust concentration for all jobs was 1.51 mg/m(3) (SD 1.08 mg/m(3)). Only 6.9% of the measurements exceeded the Australian exposure standard of 3 mg/m(3). Published exposure-response relationships were used to predict the prevalence of progressive massive fibrosis and the mean loss of FEV(1), after a working lifetime (40 years) of exposure to the mean observed concentration of 1.5 mg/m(3). Prevalences of 1.3 and 2.9% were predicted, based on data from the UK and the USA, respectively. The mean loss of FEV(1) was estimated to be 73.7 ml.

古洞崩落岩塊の固化と岩盤ベッドの沈下に関する研究

In the comprehensive research on the surface subsidence occurred by various underground longwall mining in various countries that was reported previously, the author analyzed relationship between ki (breaking factor of fallen debris) and various mining methods or various physical characteristics of the rock strata. Specifically speaking, the author calculated various ki in the solid-state single seam and multipass mining carried out at Kushiro Colliery, and in various mining applied in England. As a result of the studies, it was found that ki value drastically changes according to the difference of physical characteristics of rock strata, and the disturbance of rock strata above goaf by secondary mining as well. With regard to the outcome of the studies, the author proved that the solidification of fallen debris after 2 years of termination of mining at the top lift has sufficient strength to be supported by the shield supports at lower lift under the circumstance of only 100 m below the surface. And the presentation mentioned above was eventually accepted by the Bureau of Mines, United States Department of Interior, and proposal to the US National Project with an idea to apply the Multi-step slicing method was placed to order as a first occasion for the Japanese Coal Mining Industry.

Evaluation of fully-coupled strata deformation and groundwater flow in response to longwall mining

A finite element poroelastic model is presented to evaluate fully-coupled strata deformation and groundwater flow resulting from underground longwall mining in variably saturated fractured geologic media through an equivalent porous elastic continuum representation. The explicit coupling between the groundwater flow field and the solid skeleton deformation field is achieved by simultaneously solving the governing equations for groundwater flow and solid skeleton deformation. The implicit coupling between these two fields is also accomplished by considering the effects of both unsaturated water flow and solid skeleton deformation on the hydraulic properties such as water content and hydraulic conductivity. In addition to the implicit coupling caused by unsaturated water flow, the other implicit coupling caused by solid skeleton deformation is also important because it may produce hydraulic heterogeneity and anisotropy in a geologic medium affecting the groundwater flow field even when the system is in a fully saturated condition. A physically based constitutive model is proposed to account for the deformation (strain)-dependencies of porosity and saturated hydraulic conductivity of the fractured media. To evaluate the impacts of longwall mining and to understand their hydrogeomechanical nature, the numerical model is applied to a well documented case study with satisfactory results. Although the numerical simulation results presented in this paper will not apply to all underground longwall mines because of differences in site-specific hydrogeological and geomechanical settings, the methodology described herein may find some useful applications in many longwall mining projects.

Topographic Influence of Longwall Mining on Ground‐Water Supplies

AbstractThe extent of potential aquifer dewatering resulting from underground longwall mining is determined through application of a nonlinear finite‐element model. The model represents the form of the body strain field that accompanies mining‐induced subsidence, and uses strain magnitudes to define the modified hydraulic conductivity field. The model is applied to test the sensitivity of the induced strain field to ground surface topography. The location and extent of three characteristic zones of conductivity enhancement are defined, representing gravitational detachment above the panel, shear failure above the abutment, and extensile deformation at the ground surface. Correspondingly, well completion locations are ranked with their potential for dewatering representing relatively high potential in upland areas and relatively low potential in valley base locations. These results amplify and offer a phenomenological explanation of observational data. Modeling results are compared with several documented studies in the Appalachian coal fields and favorable agreement achieved.

Transient poroelastic response of equivalent porous media over a mining panel

A transient poroelastic formulation based on the Biot consolidation theory is presented to study the impact of underground longwall mining of coal on strata deformation and the subsequent modification of hydrogeological properties. A finite-element model is used where the geometric conditions may be complex. The important parameters in this model are identified as pore pressure ratio, α 1 and relative compressibility, α 2 with the latter exerting far greater influence on the resulting fluid pressure field than the former. This study indicates that the fluid pressure distribution is strongly controlled by the stress distribution, or the strata deformation induced by mining has a great impact on the change in hydrogeological environment in the mining region.

Rock strength and overburden changes due to subsidence over a longwall coal mining operation in Illinois : Rock Mechanics Contributions and Challenges: Proc 31st US Symposium, Golden, 18–20 June 1990 P563–570. Publ Rotterdam: A A Balkema, 1990

The overburden above an active, underground longwall coal mining operation was characterized before and after subsidence using core drilling, geotechnical instrumentation and in situ testing. The analysis of mining-induced changes in the overburden provides a better understanding of the mechanisms which lead to the surface expression of subsidence and hydrologic changes.

Application of the RACAL Airstream Helmet in Four Underground Coal Mines

Abstract The Mine Safety and Health Administration (MSHA) conducted field studies on the RACAL Airstream Helmet to determine its suitability as an interim personal protective device in underground coal mines where technology is inadequate to control dust. The studies of the RACAL Airstream Helmet were conducted in four underground longwall mining sections to evaluate user acceptance and the helmet's applicability to the coal mine environment and to determine the life expectancy of its final filter. User acceptance and applicability to the coal mine environment were assessed using responses to a questionnaire by miners participating in the studies who wore the helmet in the mines and by actual use of the helmet by personnel conducting the survey. In general, the miners who participated in the study felt that the helmet afforded them personal protection against dust. However, use of the helmet on a continuing basis did present specific problems that would make its universal acceptance questionable. The life of the final filters was determined by monitoring daily the decrease in air quantity through the helmet. The data obtained showed a correlation to previous work conducted in the laboratory in which there was determined to be a relationship between life of the final filter and total airborne dust exposure. This article summarizes the survey procedure used, outlines specific problems associated with the helmet, and consolidates the results obtained from the four underground evaluations conducted.