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By Anthony Lannacchione

There has been a persistent need to forecast roof falls so that miner's exposure to hazardous underground environments can be minimized. Several monitoring techniques have been developed and are used today with varying levels of acceptance in the mining industry. This paper examines the potential for monitoring microseismic emissions activity as a means of forecasting roof falls. The use of this activity to forecast roof falls has drawn only limited attention, resulting in a lack of published field performance data. This deficiency is being partially addressed by analyzing data obtained during longwall mining at Moonee Colliery in 1998. The Moonee data base contains a wealth of information concerning roof fall forecast parameters and the seismic alarm criteria used to develop these forecasts. Four seismic alarm criteria were developed and used by Moonee with varying degrees of success. Roof falls were forecasted 73% of the time with average forecast times of 54 minutes. Ninety percent of the predicted roof falls had a warning time, i.e., the time between warning and roof fall event, of greater than 1 minute. The fraction of forecasted roof falls decayed logarithmically as a function of warning time, until only 20% of events were predicted more than 100 minutes prior to roof falls. False alarms occurred in 50% of the warnings. Many of the false alarms were quickly followed by a cessation in mining which could have temporarily halted the on-going failure process. If mining had continued immediately after the false alarms, a roof fall may have occurred soon after. The microseismic activity collected .from Moonee Colliery demonstrates that techniques to forecast roof rock instabilities in underground mines are possible.

Jan 1, 2005

By Paul C. Hagan, Jianhang Chen, Serkan Saydam

"Modified cable bolts are commonly used in underground mines due to their superior performance in preventing bed separation when compared with plain strands. To better test the axial performance of a wide range of cable bolts, a new Laboratory Short Encapsulation Pull Test (LSEPT) facility was developed. The facility simulates the interaction between cable bolts and surrounding rock mass, using artificial rock cylinders with a diameter of 300 mm in which the cable bolt is grouted. Furthermore, the joint where the load is applied is left unconstrained to allow shear slippage at the cable/grout or grout/ rock interface. Based on this apparatus, a series of pull tests were undertaken using the MW9 modified bulb cable bolt. Various parameters including embedment length, test material strength and borehole size were evaluated. It was found that within a limited range of 360 mm, there is a linear relationship between the maximum bearing capacity of the cable bolt and embedment length. Beyond 360 mm, the peak capacity continues to rise but with a much lower slope. When the MW9 cable bolt was grouted in a weak test material, failure always took place along the grout/ rock interface. Interestingly, increasing the borehole diameter from 42 mm to 52 mm in weak test material altered the failure mode from grout/rock interface to cable/grout interface and improved the performance in terms of both peak and residual capacity.INTRODUCTIONFully grouted cable bolts have been used in underground mining industry for many decades. Initially, plain cable bolts manufactured by winding seven flexible steel wires together were commonly used (Thorne and Muller, 1964). Laboratory and field tests lead to a better understanding of the transfer process between grouted cable bolts and the surrounding rock mass, which is beneficial to improve the reinforcing performance of cable bolts in engineering practice (Fuller and Cox, 1975; Cox and Fuller, 1977; Schmuck, 1979; Stillborg, 1984). Nevertheless, as in many cases the initial cable was sourced from discarded steel with a smooth surface along the strand, plain cable bolts did not always meet performance expectations (Windsor, 1992). Over time, it was realised that cable surface geometry plays an important role in determining anchorage performance. Consequently, modified forms of cable bolts were developed. For instance, twin-strands which were made by using spacers to combine two plain cables together improved anchorage capacity two-fold (Matthews, Tillmann and Worotnicki, 1983). However, this required a large borehole to be drilled. Epoxycoated strands had remarkable performance in practice but were difficult to install without damaging the epoxy layer (Dorsten, Hunt and Kent, 1984). In the 1990s, there was a rapid development in the cable bolt design. A typical example was birdcaged cable bolts. They were initially suggested by Hutchins et al. (1990) via unwrapping traditional seven-wire strands and generating a number of nodes together with antinodes along the strand. But it was difficult to make birdcaged cable bolts that could be coiled for transporting."

Jan 1, 2015

By Michael Alber

Pullout tests on resin grouted rock bolts have been executed in coal mines and surface test sites. The goal of this study was to provide some qualitative information about the rock mass quality from mandatory tests with a bit little more effort. Movements of rock bolt head and steel plate at different load levels were analyzed and were found to yield information about the rock mass deformability. The tests were interpreted similar to plate bearing tests and a rock mass compressibility index C was assigned. This index reflects the complex interactions between the rock mass, resin grout and bolt under load. Although further research is necessary it may be concluded that those tests may aid in recognizing geological conditions.

Jan 1, 2003

By Winton Gale

"The aim of this paper is to summarise the results and conclusions of Australian Coal Association Research Project (ACARP) Report Cl3013 which relate to water inflows into a mine which occur through the overburden above and adjacent to longwall panels. The study assessed available data of inflows into underground coal mines and utilised computer simulation of water flow through fracture networks. The study concluded that flow into mines is typically via an interconnected network of pre-existing and mining induced fractures. The height above the coal seam that mining induced fractures extend is typically related to the width of the panel. However the potential for those fractures to form a connected network which can facilitate flow, is related to the amount of subsidence and the depth of mining. The study compares model simulations with measured data and provides guidelines to estimate the average hydraulic conductivity of the overburden above extracted longwall panels in Australia.INTRODUCTIONWater inflow into coal mines has been a design issue for many years. Guidelines as to the potential for water inflow have been developed in many countries based on local experience and the form of mining being undertaken. In most instances, the guidelines relate to inflows which would endanger underground personnel and operations.In more recent times, water inflow criteria for mines has been widened to include lesser inflows which may not impact on mine safety or operations, but have the potential to reduce water flow within streams and surface aquifers. For the purpose of this paper the larger inflows which are readily measured in a mine or may impact on mining safety are defined as mine inflow and the lesser inflow relating to aquifer water loss as environmental inflow.The water losses referred to are those which enter the mine from the surface or from near surface aquifers.The empirical relationships developed as mine design guidelines or regulations typically relate to high flow (mine inflow) situations and in this paper, these have been reviewed as to their application to environmental inflow. In many cases environmental inflows were unlikely to have been noted or recorded in many mining operations in the past."

Jan 1, 2010

By Karl H. Brandt

In the Auguste Victoria mine a bolted rectangular roadway has been kept open for the first time in the seam D/C after retreat. The target of the project was to gain a basis for planning and experience for future mining layouts in the same scam in view of the reuse of boated rectangular roadways. For this the roadway was additionally secured in sections with longbolts before the influence of the retreating panel had taken place. On retreat a concrete sidepack was concurrently put in place on the face side. The installed monitoring systems consisted of endoscopic devices, multiple extensometers (Tell-Tales), shear-tales, extensometers and convergence measurements. The experience that was gained was carried over into a physical and numerical model.

Jan 1, 2001

By Weihua Song, Hongwei Zhang, Feng Zhu, Sheng Li, Guoshui Tang

"Rock burst is one of the most serious dynamic disasters in coal mining. Rock burst prevention involves two difficult tasks: systematically evaluating the occurrence conditions and effectively forecast for rock burst. This paper systematically studies the effects of natural geological dynamic conditions and mining engineering on and forecast for rock burst. It establishes the evaluation method and index system of rock burst geological dynamic conditions and proposes a method of forecasting rock bursts by monitoring fault activity with anchor cables. Natural geological dynamic conditions are a necessary condition while mining engineering is a sufficient condition for the occurrence of rock bursts. Rock bursts in mine must have suitable geological dynamic conditions, and mining engineering activity is merely an inducing factor. There is a good correlation between anchor cable stress variation and fault activity, when the anchor cable dynamometer indicates that the fault is active. It can forecast a rock burst a few days in advance. This technique has been successfully applied to Panel 83003 of a coal mine in China, which improved the safety production level of the mine and found a new way of rock burst evaluation and prediction.INTRODUCTIONRock burst is one of the most serious dynamic disasters in coal mining. With the increase of mining depth, rock burst disaster is becoming more and more serious. Systematically evaluation and effective forecasting of the occurrence of rock burst in the coal mines are important and difficult work in the control of rock bursts. Chinese researchers have performed numerous researches on rock bursts, Qi et al., established the rock burst model of frictional sliding instability, explained the occurrence mechanism of rock bursts by stick-slip phenomenon in frictional sliding, gave full consideration to the influence of primary rock stress and mining-induced stress on rock burst occurrence, and proposed the stress controlling theory of rock burst. Pan et al., [3] put forward the criterion of perturbation response, explained and analyzed the problem of rock burst in circular openings, put forward the ratio of coal-rock elastic modulus and reduced modulus is the significant parameters for the determination of stabilization; systematically summarized the rock burst distribution, type, mechanism and control in China. They classified rock bursts into three types: coal compression, roof fracture and fault movement. By applying energy transfer principle and conservation of energy, combined with the analysis of the weakening of rock fabric damage, Zhou and Jiang put forward the theory and equation for the occurrence of coal-rock mass impact effect. They analyzed and discussed the formation mechanism of rock burst by combining impact effect theory and energy equation. Wang et al., recognized that the mechanism of rock burst which induced by fault, fold and phase transition is the superposition of tectonic stress and mining stress and that the axes of syncline and anticline and their flanks are the location where rock bursts mostly occur. To prevent stress superposition is the efficient way to control the structural rock burst [7]. Jiang et al., summarized the research achievement of rock burst mechanism and controlling techniques and existing problems in recent years in China, and pointed out the direction to improve the level of prevention of controlling rock bursts in coal mines."

Jan 1, 2015

By S. P. Singh

Rockburst is a sudden manifestation of the release of elastic strain energy stored in the rock. Therefore the tendency of the rock to burst should depend upon its ability to accumulate elastic strain energy. The use of Burst Proneness Index as a simple and reliable indicator to recognize the burst prone situations in the mines has been advocated in this study. The Burst Proneness Index exhibited a strong dependence upon the brittleness, compressional and shear wave velocities and the point load strength index. It was also observed that the Burst Proneness Index increases with the increase in the strain energy per unit volume, c ressive strength and the modulus of rigidity of the rocks. In addition, important causes and the measures to alleviate rockbursts have been outlined in this paper.

Jan 1, 1986

By Joseph Hirschi, Naga S. Y. Tirumalaraju, Aditya Mishra, Anthony (Sam) J. S. Spearing

"Primary roof support in underground coal mines in the USA consists mostly of fully grouted passive rebar or grouted (partially or fully) tensioned rebar. Research conducted at two room-and pillar coal mines in the Illinois Basin and one longwall mine in Colorado monitored the performance of these passive and active rock bolts under similar geologic and mining conditions and found no significant initial loads in either passive or active rock bolts (Spearing et al., 2011). Active rock bolts showed significant loss of initial pre-load, although the exact reasons for this were not fully investigated. Subsequent laboratory testing has also shown that in weak rock, resin creep occurs, caused by slippage of the mechanical key between resin column and rock interface. These creep tests were studied in the laboratory in an effort to quantifythis phenomenon.INTRODUCTIONResin-grouted rock bolts are the first and main line of defense to counter roof and rib falls in underground coal mines of the United States. Rock bolts are categorized as active or passive. Active bolts are tensioned during installation whilst no (or limited) tension is applied to passive bolts, which need subsequent strata movement to generate reactive loads. During installation of passive bolts, some reactive pre-load may be generated by a significant up-thrust from the rock bolter boom.The selection of active or passive bolts is critical and is mostly dependent on the conditions and properties of the immediate roof (Unrug, 2009). This decision is frequently made based on personal experience, bias, and most recently cost. Active bolts are generally used in cases of highly laminated roof rock. When active bolts are tensioned, they develop compressive forces between these layers forming a beam, which helps to stabilize weak individual layers thereby increasing frictional forces between layers and decreasing effects of horizontal stresses. The fully grouted tensioned rebar system is high in stiffness as compared to mechanical bolts and provides higher surface area of contact along the length of the bolt due to the grout, which permits it to develop a strong anchorage mechanism."

Jan 1, 2015

By C. A. Vining, J. R. Nopola

"Evaporites, such as salt and potash, pose unique problems to ground-support designs not encountered in other rock types. Evaporites are subject to continual creep and no amount of ground support can successfully arrest this process. Common ground-support techniques that are useful for most rock types may reduce the effective life of bolts in evaporite mines and can create unseen hazards. A different approach to ground support must be applied to provide long-term support in areas where high creep rates are present. The goal of effective ground-support designs in these situations is not to contain the rock and prevent it from moving; rather, the ground-support design must be able to move along with the rock and contain any fully detached slabs until additional remediation measures can be undertaken. The most appropriate ground-support designs will consider both the expected movement of the rock and the duration that the area needs to remain accessible.INTRODUCTIONEvaporites, such as salt and potash, pose unique problems to ground-support designs not encountered in other rock types. The factors that must be considered in design stem from the deformation behavior of salt, which can fail in a brittle manner, akin to most rocks, but can also deform in a slow, ductile manner, resembling the stretching and elongation of steel. The deformation behavior of salt has been well established in technical circles where it has been studied for decades (Hardy and Langer, 1984). Much of the research in salt mechanics is related to repository science, specifically at the Waste Isolation Pilot Plant (WIPP) in southwest New Mexico. RESPEC has been continually involved in the WIPP from the initial design considerations (Gnirk, Grams, and Zeller, 1977) to the current day (Keffeler, 2016) and has been able to leverage cutting-edge research of salt mechanics into practical solutions for mining operators. While a detailed description of salt mechanics is not provided in this document, Fossum and Fredrich (2002) include a fine overview of salt mechanics. The following list paraphrases their chapter on basic facts about salt to provide some context for the reader unfamiliar with salt mechanics:"

Jan 1, 2018

By Keith A. Heasley, Jian Yang

"LaModel uses a laminated overburden boundary-element model and can calculate not only seam-level stresses and displacements but also surface subsidence for thin tabular deposit such as coal seams (Heasley, 1998). Up to this point in time, the material property wizards in LaModel have primarily been designed for calculating accurate stress distributions in single- and multipleseam situations, and for investigating and optimizing pillar sizes and layouts in relation to overburden, abutment, and multipleseam stresses. However, the critical input parameters that will give the most accurate seam-level stress distribution do not necessarily produce the best surface subsidence prediction. The objective of this paper is to develop a methodology for calibrating the critical input parameters in LaModel to optimize surface subsidence prediction.For optimum surface subsidence prediction, it was found that the gob convergence (as defined by the final gob modulus) and the overburden stiffness (as defined by the lamination thickness) were the two most critical parameters that needed to be calibrated (Heasley, 2016). By calibrating numerous panels with various widths and depths, a formula that provides the optimum final gob modulus to use for subsidence prediction was developed. Also, three different empirical formulas for optimizing the lamination thickness as a function of overburden depth and/or the panel width to- depth ratio were determined for the three cases: supercritical panels, and subcritical panels with and without offsets. Further, if the user has measured data for subsidence factor and angle-of draw, the optimum final gob modulus and lamination thickness can be determined from the measured data. These new subsidence prediction formulas are being implemented into new material wizards in LaModel.INTRODUCTIONIn the past few decades, with the increased need for highly productive mining techniques, full extraction mining methods have been increasingly employed in the U.S. coal mining industry. These total extraction mining methods, which include both longwall mining and room-and-pillar retreat mining, normally cause immediate roof caving and the associated surface subsidence. These methods can be highly productive and cost-effective, but the associated surface subsidence can cause damage to surface structures and negatively impact the surface environment (Peng, 2008). Therefore, a reliable program for predicting the surface subsidence associated with full extraction mining is required for appropriately engineering and managing the surface subsidence impacts of full extraction mining."

Jan 1, 2016

By Wei Yin, Yu Wu, Xiexing Miao

"For engineering problems of “three under” coal (coal trapped under buildings, waters-bodies and railways) exploitation, environment protection and gangue separation from raw coal underground exist in Tangshan Coal Colliery. By analyzing concrete technical problems, a technical approach of closed cycle “coal mining-gangue washing-backfilling-coal mining”, i.e. integration of coal mining, gangue separation and solid backfilling, is put forward with key technologies including gangue transportation with great depth, gangue separation from raw coal underground and integration of coal mining and backfilling. Based on the specific geological conditions of Tangshan Coal Colliery, 591m vertical feeding transportation system with great depth is designed; meanwhile, for efficient gangue transportation, ancillary wear-resisting pipe size and buffer equipment are optimized. Underground gangue separation system is created to be joined with the existing production system and backfilling system; besides, as per separation characteristics of coal and gangue, buddle jig is adopted for gangue roughing of raw coal underground. By cooperation of backfilling hydraulic support and other key equipment, the designed stope backfilling system completely realizes integration of coal mining and backfilling. Engineering practice shows that, through the research and implementation of the above key technologies, integration of coal exploitation, gangue separation and solid backfilling is successfully implemented in Tangshan Coal Colliery; moreover, it completely solves the three major engineering problems and realizes waste disposal, underground washing and separation, and ground surface subsidence control.INTRODUCTIONSo far, “three under” coal (coal trapped under buildings, watersbodies and railways) and key state-owned coal resources trapped under villages in China have reached 13.7 billion t and 5.22 billion t, respectively. With the development of coal enterprises, China currently has more than 1,600 gangue dumps (Miao et al., 2010a) with accumulated stack amount of 4.5 billion t, occupying a land area of 150,000 km2; moreover, surface subsidence caused by mining activities has damaged the environments of industry, agriculture, transportation and living, while new environmental pollution sources formed by coal related industries also result in damage to ecological environment (Huang et al., 2011a). On the one hand, due to large scale coal mining, some coal collieries suffering from resource exhaustion are faced with survival problems; on the other hand, coal mining has caused and been faced with increasingly prominent environmental problems, such as “three under” coal exploitation, waste disposal and ground surface subsidence control (Miao et al., 2010b; Zhang et al., 2009)."

Jan 1, 2015

By Michael Gauna

In 2010, the number of fatal rib falls in U.S. underground coal mines exceeded fatal roof falls for the first time ever. While recent trends have seen roof fall fatalities reduced by about 62% since the 1990s (from an average of 9.6 per year in the 1990s to 3.6 per year during the last five years), the incidence of rib fall fatalities has remained approximately constant (about 1.5 per year). In addition, injury incidents associated with pillar rib failure have been steady at approximately 100 per year. The persistence of rib fall injuries, underlined by the recent increase in rib fall fatalities, has led to this investigation of the coal rib failure parameters and methods available to mitigate rib failure in U.S. underground coal mines A detailed study of the 21 fatal rib fall incidents that have occurred since 1995 (excluding coal burst incidents) indicates that the two most important risk factors are the mining height and the depth of cover. The depth of cover was 700 ft (213 m) or greater in 81% of the fatalities, and the mining height was at or exceeded 7 ft (2 m) in every instance but one. Other risk factors included multiple seam interactions, rock partings in the seam, and retreat mining. The accident history reveals that joints or slickenside features within the rib increase the hazard. Approximately threequarters of the rib fatality victims were roof bolting machine operators and continuous mining machine operators. This paper includes a brief overview of techniques for rib control; including bolts, standing support, bent steel straps, and rib strapping. The available mining equipment features for installing rib support and protecting operators from rib falls are also discussed. A proactive approach to rib control using available technologies will lower the risk to miners and is an endeavor that must be pursued to reduce rib fall accidents.

Jan 1, 2011

By N. P. Kripakov

This paper presents a brief overview of current bump mechanics theories and pillar design methodologies, and relates these concepts to experiences at two mines located in a north-central Utah coalfield where different pillar designs were used to control mountain bumps. Experiences gained at the first mine demonstrated the successful implementation of a two-entry, 9.8 m (32 ft)-wide, yield-pillar design. A U.S. Bureau of Mines field study quantified the timing of chain pillar yielding and resulting load transfer from the gateroad. In-mine pillar response, although apparently sensitive to site-specific conditions, compared favorably to estimates derived using two yield-pillar design methods. A second study conducted at another mine located in the same district, but subjected to different geologic conditions, documents the unsuccessful attempts to employ progressively narrower three-entry pillar designs based on successes achieved at the first mine site. This second mine never achieved a true yield-pillar design. Use of pillar widths ranging between 9.1 m (30 ft) and 27.4 m (90 ft) always resulted in violent bumps in the tailgate pillars. The pillars were either too large to yield, or too small to support peak operational loads. Hypothetical gateroad systems were evaluated using both analytical and empirical approaches for configurations comprised of two rows of conventional pillars, and systems incorporating both yield and abutment pillars. Analysis concluded that yield pillars less than 6.1 m (20 ft) wide and abutment pillars ranging between 30.5 m (100 ft) and 39.6 m (130 ft) square would be required to achieve a stable gateroad design. However, results of a field study conducted on a two-entry, 36.6 m (120 ft)-wide abutment pillar concluded that abutment pressures from the first panel overrode the pillar, and that a still larger pillar may be required to preclude bumps in the tailgate during second panel mining. Without the benefit of a demonstrated in-mine success, it is not clear which design would ensure elimination of gateroad bumps.

Jan 1, 1992

By David Gearhart, Peter Zhang, Gabriel Esterhuizen

"Researchers from the National Institute for Safety and Health (NIOSH) installed instruments in the track entry and in the pillars of a northern West Virginia longwall coal mine between two planned panels for the purpose of monitoring the behavior of the ground. The instruments included four cable bolt load cells to measure the change in gateroad loading, six borehole pressure cells to measure the loading in the pillars, three six-channel roof extensometers to measure roof sag, four convergence measurements on the cribs to measure the crib loading, and a multipoint borehole extensometer to measure the pillar expansion. The instrumentation was all connected through two multiplexers to a single Campbell Scientific permissible datalogger located about 350 feet outby the instruments. The goal of the project was to record all of the measurements as both longwall faces passed the site of the instrumentation and to use the data to calibrate numerical models.This report describes the recording of all of the measurements taken as the first panel approached and then passed the instrumentation site. This study provided valuable data related to the ground response and support performance that will be used to improve safety in longwall gateroads.INTRODUCTIONA longwall mine in northern West Virginia was selected as a second study site for an ongoing research project into gateroad layout and support design. The area was affected by a transition from limestone to sandstone in the roof. The entries were 16 feet wide, and the height was approximately 80 inches. The layout for the longwall panels was 1,170 feet wide by 4,000 feet long. The panel configuration was right-handed, and a three-entry gateroad was used between panels. The pillars next to the headgate were 137.5 feet by 115 feet center-to-center, and the pillars next to the tailgate were 275 feet by 100 feet center-to-center, with the crosscuts at 60 degrees. The width of the gateroad was a total of 215 feet."

Jan 1, 2018

By Dezhong Kong, Jaichen Wang, Shengli Yang

"The stability control of longwall coal face is the key technology of large-cutting-height mining method. Therefore, a systematic study of the factors that affect coal face stability and its control technology is required in the development of large-cutting-height mining method in China. After the practical field observation and years of study, it was found that the more than 95% of failure in coal face are shear failure. The shear failure analysis model of coal face has been established, that can perform systematic study among factors such as mining height, coal mass strength, roof load, support resistance, and face flipper protecting plate horizontal force. Meanwhile, sensitivity analysis of factors influencing coal face stability showed that improving support capacity, cohesion of coal mass and decreasing roof load of coal face are the key to improve coal face stability. Numerical simulation of the factors affecting coal face stability has been performed using UDEC software and the results are consistent with the theoretical analysis. The coal face reinforcement technology of largecutting- height mining method using the grouting combined with coir rope is presented. Laboratory tests have been carried out to verify its reinforcement effect and practical application has been implemented in several coal mines with good results. It has now become the main technology to reduce longwall coal face failure of large-cutting-height mining method.INTRODUCTIONLarge-cutting-height mining method of more than 3.5m mining height, is one of the main methods for thick-seam mining in China. Currently, the largest mining height is 7.3m and the support capacity is 1,800mt. Shendong and Jincheng mining area have the best use of large-cutting-height mining technology (Wang, 2009). Generally, the mining height ranges from 6.5m to 7m and the support capacity ranges from 1,000mt to 2,000mt, annual output of the panel is between 10 and 12 million tons and the highest are 15 million tons. The main goal of large-cutting-height mining technology is to achieve high yield and high efficiency by increasing the advancing speed of longwall face (Yuan, 2011- 2012). However, in certain geologic conditions, rib spalling and roof falls in the unsupported area often occur. Because of the large tonnage of large-cutting-height mining equipment, once the rib spalling and roof falls cause the equipment to be out of alignment, the face advancing speed could be affected seriously that in turn affects the output (Yang, 2012). Therefore, studying the failure mechanism and control technique of coal face in large-cuttingheight mining method is of great significance."

Jan 1, 2015

By Bruce K. Hebblewhite

BHP Billiton Illawarra Coal operates Dendrobium Mine in an area 10-20km west-northwest of Wollongong in the Southern Coalfield of New South Wales, Australia. The mine recently completed longwall mining in Area 3A adjacent to an overhanging rock feature known as Sandy Creek Waterfall, which had been identified as a significant, and potentially sensitive natural surface feature of the environment. This waterfall lies within the Sydney drinking water catchment area, in a surface terrain of variable topography where non-conventional subsidence behaviour such as valley closure, and far-field horizontal movements are known to occur, over and above conventional subsidence responses to mining. As a part of the conditions of approval to mine, Illawarra Coal undertook to protect the waterfall and the section of Sandy Creek immediately upstream of the waterfall from the effects of longwall mining using an innovative management process and an array of very high resolution monitoring systems. This paper outlines the type of non-conventional subsidence effects that can occur; the range of different monitoring regimes adopted in response to such effects in order to develop an understanding of the valley closure phenomenon in proximity to the waterfall and provide suitable early warning data. The paper also describes the management processes that were adopted, incorporating the use of these monitoring regimes in order to successfully protect the waterfall from the valley closure effects of mining four adjacent longwall panels in close proximity to the waterfall while continuing to maximise recovery of the coal resource in the area. The management structure adopted involved a technical committee, a steering committee, and an external independent reviewer. The technical committee was responsible for design of the monitoring systems, interpretation of the results, and making timely recommendations to the company steering committee who were then able to make informed decisions on when to cease mining each adjacent panel. The steering committee comprised Illawarra Coal management and technical personnel. Although the steering committee took advice from the technical committee, all decisions relating to mining were made by the steering committee. An external reviewer was engaged by the steering committee at the end of each longwall panel to review the results, interpretation and management decisions. This management structure was effective in successfully protecting the very sensitive structure of Sandy Creek Waterfall from potential impacts of nearby mining in the midst of active ongoing natural erosion processes.

Jan 1, 2014

By Ben Fahrman

The classical approach to engineering design involves the purely deterministic calculation of the factor of safety, the ratio of strength to stress. The factor of safety is an easy calculation to perform in many circumstances, and it is very easy to interpret the results. The simplicity of the method is both a strength and a weakness. The results of the deterministic factor of safety calculation are simple to determine and interpret, but they have no ability to account for the uncertainty present in the inputs or outputs. Uncertainty is prevalent in geotechnical engineering applications. This uncertainty can stem from spatial variability, poor sampling, a dynamically changing environment, etc. Probability analyses allow input parameters to take the form of a distribution rather than a single value, making them uniquely equipped to handle the uncertainty inherent in geotechnical engineering design considerations. A stochastic approach also has the benefit of providing results as a probability of failure, which is more meaningful than a factor of safety. Though stochastic modeling is becoming more common in many geotechnical applications, it is still not widely used for coal pillar design. The objectives of this study are to justify the concept of using a stochastic approach to coal pillar design and to compare the results of a simple stochastic analysis to those of standard industry practice. A simple synthetic study was performed to compare the results of a probabilistic analysis of coal pillar design to the results obtained from running ARMPS, which is published by NIOSH. Normal distributions were assigned to each input parameter that is required to determine the factor of safety of a single coal pillar. The probability of failure resulting from the synthetic study was found to be more conservative than the ARMPS stability factor. A probabilistic approach to coal pillar design is discussed in comparison to the traditional deterministic approach. The greatest strengths of the probabilistic approach were determined to be the effective handling of uncertainty and the increased meaning of the inputs and outputs inherent to a probabilistic approach.

Jan 1, 2013

By Dennis Dolinar

"Tailgate entries in U.S. longwalls can be subjected to high loads and deformations. Therefore, tailgate entries are generally supported with varying types and amounts of standing support. However, even with standing support, roof falls and blockages can occur in the tailgate when the support system is not properly designed. Safe and efficient operation of a longwall face requires the selection of the appropriate type and amount of standing support. Because of the importance of maintaining a safe and functional tailgate entry, the National Institute for Occupational Safety and Health (NIOSH) is currently developing a standing support design methodology based on the ground reaction curve. Using the ground reaction curve for design requires that the ground and support interaction be quantified. This can be accomplished by the installation and analysis of data from instrumented field sites. Although necessary, the field results cannot develop the complete ground reaction curve. Numerical models, calibrated to the field results, are used to establish the complete ground reaction curve. In the present study, data was obtained and evaluated to establish points for calibration of the ground reaction curve for two western longwall operations.One test site, Mine A, was located at a depth of 1,550 ft. At this site, the floor consisted of siltstone, while the roof was an interbedded siltstone and sandstone. Because of the depth, a double row of22-in diameter CANs1 were installed to support the tailgate. Both the first and second longwall panels have mined past the instrumented site.The second site in Mine B is at a depth of only 760 ft. The floor at this mine consists of a weak shale/mudstone and the roof consists of thick, top coal overlain by weak shales. At this site, a double row of pumpable cementitious cribs was installed. The second test site has only been subjected to side abutment loading from first panel longwall mining.The measurements indicate that the support at both mines was heavily loaded when subjected to the side abutment loading from first panel mining. At the deeper site, the support began to yield when the face was more than 300 ft outby during second panel mining. In comparison to results from previous test sites in the Pittsburgh Seam and Illinois Basin, the amount of support loading and roof-to-floor convergence was an order of magnitude higher at the western sites. This has resulted in a significant difference in the position and shape of the ground reaction curves."

Jan 1, 2010

By Bob Coutts, Matt Martin, Dan Lynch, Dan Payne

"The Broadmeadow punch longwall coal mine in Central Queensland Australia has experienced significant highwall movement associated with the effect of longwall subsidence when the longwalls approach their final position close to the open-cut highwall. In response to this movement, Broadmeadow employed two types of broadscale highwall monitoring (radar and laser scanners) to provide full coverage measurement throughout three consecutive longwalls approaching the highwall. This was to attain a better understanding of the mechanism causing the movement and potentially enable prediction of instability. Results from the monitoring found the highwall is displaced to magnitudes unlike those typically measured in open-cut mining and in direct contrast with typical longwall subsidence behaviour.This paper discusses the ground movements measured, monitoring methods used, safety measures established, as well as theorising the failure mechanism. Recommendations are made for mine and pit designs for future punch longwall layouts. The paper shows how the movements measured are more aligned to some measurements made during stream valley closure studies previously presented at the ICGCM and challenge the mechanisms suggested by previous literature.BACKGROUNDThe mining of longwalls under or adjacent to large voids (e.g., stream valleys, escarpments, or cliffs) is commonly associated with heavily vegetated or steep surface areas. In areas of extreme topographic variance, access to traditional survey pegs and stations or even new radar or laser technologies is limited. In addition, seldom have the longwall layouts aligned themselves parallel or perpendicular to the surface feature, making interpretation of any available surface movement data more complicated.The punch longwall layout (Figure 1) is also quite uncommon (only undertaken at a small number of longwall mines in Australia) but creates the perfect configuration to enable a somewhat controlled study of the effect of longwall subsidence on a steeply dipping surface feature (an un-vegetated, evenly excavated open-cut highwall). The relatively recently developed radar and laser scanning technology has also enabled near continuous, real-time, sub-millimetre monitoring of a full 500m wide x 100m high highwall. Because the punch longwall enables access and a clear view, the technology could be easily deployed as compared to the highly vegetated and variable topography of stream valleys."

Jan 1, 2018

By Michael Trevits

The NIOSH Lake Lynn Laboratory (LLL) is a unique research facility, located about 50 miles southeast of Pittsburgh, Pennsylvania, that is designed to provide a full-scale mining environment for testing and evaluation of mine health and safety technologies. The LLL occupies more than 400 acres and is composed of surface test and training areas and an underground mine. The Lake Lynn Experimental Mine (LLEM) was built at an abandoned commercial limestone quarry where surface mining ceased in the late 1960's. Later, new underground development was constructed to simulate modern-day coal mining scenarios; including room-and-pillar and longwall mining layouts. In January 1994, a sinkhole opened southeast of the No. 4 Portal of the LLEM and since then, the area of the underground failure and surface deformation has continued to expand and now includes several interconnected sinkholes. A concern developed that the overburden instability could expand and affect the structural integrity of the nearby highwall and the No. 4 Portal. It was unsafe to conduct a detailed underground survey of the mine in this area because the mine roof conditions near the No. 4 Portal had deteriorated significantly. It was decided to investigate the overburden conditions near the No. 4 Portal using ground penetrating radar (GPR). A GPR survey grid was located along an access road that passes over the top of the portal. To delineate the mine conditions near the portal, antennas whose frequency spectra produced pulses centered near 100- and 200-MHz were used. The results of the GPR survey suggests that the overburden rock units appear to be laterally consistent to a depth of about 15-ft. Below that point, the overburden appears to be significantly disturbed and it was hypothesized that this area contains a large roof fall. A follow-up ground truth survey of the mine roof conditions was conducted from the mouth of the portal. It is concluded that the observations and measurements made from the radar records appear to be correct as a large roof fall was observed and measured from the mine portal area.

Jan 1, 2005