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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 Gary Buchan

"Mine-wide ground control stability is essential to any mining operation to prevent catastrophic failure of underground structures. The ability to recognize or indicate the potential for ground control hazards in underground mine operations is highly desirable to reduce or eliminate ground-control-related injuries and fatalities. The acoustic emissions (AE) form of non-destructive testing has been used for decades to determine material or machine deterioration or potential failure. This paper presents the findings of research in AE with the results of laboratory bolt pull tests and comparisons to field experimentation, including AE sensors and bolt load data in an underground stone mine. The goal of the project is to compare laboratory results of AE sensors on roof bolt pull tests with field experimentation to determine the capability of predicting bolt failure or roof deterioration.INTRODUCTIONMine-wide ground control stability is essential to any mining operation to prevent catastrophic failure of underground structures. The ability to recognize or indicate the potential for ground control hazards in underground mining operations is highly desirable to reduce or eliminate ground-control-related injuries and fatalities. Typical ground control monitoring devices involve drilling into roof or rib, grouting or gluing of sensors, and dedicated cabling for data collection. One of the original goals of this research was to develop a methodology for a simple wireless device that could provide an indication or warning of potential roof bolt or roof failure. Various types of nondestructive testing (NDT) have been used for decades to determine material or machine deterioration or the potential failure of either. One type of NDT is acoustic emission (AE) monitoring, which has been studied for use in mining to predict rock bursts and mine failures since the 1940s (Obert, 1941; Obert and Duvall, 1942). The primary goal of the work was to compare laboratory results of AE sensors on roof bolt pull tests with field experimentation to determine the capability of evaluating bolt loading or roof deterioration in an underground stone mine. Bolt pull tests were conducted in the laboratory to establish a repeatable trend of AE activity versus bolt load. The second part of the project was to then install bolt load cells in an underground stone mine and monitor AE activity in the vicinity of mining."

Jan 1, 2018

By Erdal Unal

In underground mining today, safety and economical aspects demand a better understanding of the rock-mass conditions, particularly for design of underground mine openings excavated in weak and stratified rock mass. The rock mass classification systems, in essence, are empirical approaches utilized during preliminary design stage. These systems have been developed for specific purposes and rock mass types, therefore, direct utilization of the classification systems in their original form, for characterization of complex rock-mass conditions is not always possible. This is probably one of the main reasons why designers continue to originate new systems, or modify and extend the ones already existing. RMR and Q systems, for example, although widely used in mining and tunnelling, can not fully describe the specifications of the weak, stratified and flay bearing rock mass. Consequently, the engineering applications that would be carried out based on original RMR and Q ratings could be inadequate for making design decisions even during the preliminary design stage.

Jan 1, 1990

By Stephen Tadolini

Standing crib supports have been applied in underground mining programs to resist large roof movements and sustain high¬loads. The strength and deformation capability of these systems has been documented under both laboratory and field conditions. The parameter that has not been examined and is not well understood is the effect that a crib or other types of standing support has on the primary and secondary bolting systems. A crib support system with low system stiffness may allow large amounts of closure and deformation and cause the bolting systems to yield or even fail. Conversely, cribs or standing support that are too stiff may experience brittle or buckling failures, negating the advantage of the intrinsic supports previously installed. Utilizing a combination of field measurements and 3-dimensional finite element modeling techniques, the relationship between system stiffness and the subsequent performance of the installed bolting system is evaluated. Additionally, a simple method for calculating the combined system stiffness for standing supports, when using materials with different strengths such as steel, concrete, wood, etc., is presented.

Jan 1, 2003

By Alexandra Garcia

A measurement-based ground control management system, commonly employed in coal mines, has been successfully implemented in a UK potash mine operating at a depth of over 800 m. This system includes the routine deployment of telltales in roadways throughout the mine to mitigate against the risk of large-scale roof falls and the deployment of detailed ?design monitoring? instrumentation for ongoing design verification. The routine monitoring systems used in coal mines have not been, to date, commonly applied in potash mines, despite the similar, variable weak geology and associated ground control hazards. Monitoring information from the site indicates that telltales can be deployed to monitor roof conditions, with action levels based on differential rates of strain, as opposed to absolute movement.

Jan 1, 2014

By Alan Bugden

Goedehoop colliery in South Africa has experienced a number of substantial roof falls in roadways and intersections. Many of these have been associated with geological slips and/or horizontal compressive stresses. This paper describes a numerical modelling study conducted after one such fall with the objective of clarifying the roof failure mechanism associated with these falls. The fall was located at an intersection and occurred as the last cut was being made to complete the intersection. One bounding surface of the fall was formed by the geological slip; the others were formed by fresh failure surfaces. The fall took place despite the slip being identified as a hazard and additional bolts being placed by the workforce. In conjunction with the modelling work tests were conducted on samples from the roof in which the fall occurred, these gave a U.C.S. of 80 -100MPa. Several in-situ stress tests had already been conducted at the mine; a further test was conducted close to the fall site. The tests show that the largest stresses are horizontal. However, the maximum stresses of 6-12MPa are small compared to the compressive strength of the root. Computer modelling highlighted a mechanism arising from the conjunction of a geological slip and horizontal compressive stresses that could result in tensile failure of the roof leading to the fall. Sensitivity studies were conducted to identify risk factors associated with this mechanism and the modelling results used to assist in formulating a support strategy to combat falls of this nature.

Jan 1, 2003

By Susan Robertson

Many injuries are caused each year by rock falls in coal mines. Most of these injuries are not caused by major roof collapses, but from falls of smaller rocks from the immediate top or roof skin. Various surface controls are used in mines to control this surface rock. One that has been found to be very effective is roof screening. Depending on the size of the screen, roof coverage approaching 100% can be achieved. Many mines are reluctant to use screen for primary skin control because of additional costs of time and materials, but others are having great success at both controlling costs and surface rock. Data is presented from two mines that show a dramatic reduction in roof skin injuries when screening is used. Much of this success is due to innovations in roof bolting machines. Four case studies of roof screen experience are presented along with associated costs of materials, impact on bolting advance rates, and potential ergonomic risks. The effects of roof screening on skin control and safety are also included. Finally, this paper will provide information about features of different roof bolting machines that affect production and safety.

Jan 1, 2002

By Alan Bugden

Goedehoop Colliery produces 8 million tonnes of coal a year, principally from room and pillar mining, and is situated in the Witbank Coalfield in the Republic of South Africa. The mine has a long and successful record of producing export quality coal from the 2,4 and 5 seams at depths ranging from 20 to 100 metres using modem mining equipment. Following a number of fatalities caused by large and unexplained roof falls in roadways and intersections of supported ground, a review of the roof control system at Goedehoop was carried out. This led in turn to a programme of underground measurements and surveys. This included detailed investigation of the roof falls, horizontal stress mapping, stress measurement and short encapsulation pull testing on roofbolts. These investigations led to a number of important conclusions in relation to the state of stress in the ground, the roof failure mechanisms, standard of rootbolt installation and performance of the roofbolt system. As a result of the work Goedehoop Colliery has made a number of substantive changes to the roof control management system at the mine. These include: 1. A recognition of the importance of horizontal stress as the main factor responsible for roof falls. 2 The forward planning of roof support based on known features which cause the stresses to be elevated. 3. The introduction of a more effective roofbolt system, which can be more easily installed to a high standard. 4. A response system based on risk assessment and monitoring roof movement on a routine basis. The paper describes the knowledge acquired, the conclusions drawn, the changes made and progress to date.

Jan 1, 2001

By Hai Rong

EH-4 Electrical conductivity imaging system has been playing an important role in prospecting, looking for water resource, and determining coal mine gob boundary by virtue of its significant advantages such as collection of large volume of data, light weight, fast analysis, high-precision, and low cost. However, it has little application in determining the overburden failure zone and development of water flowing fractured zone and bed separation space in complex extra thick coal seam conditions. In order to confirm the overburden failure range and evolution of water flowing fractured zone and bed separations in complex extra thick coal seam conditions the panel 63005 of Laohutai coal mine in China was selected for study in this paper,. EH-4 magnetotelluric method was used to determine the overburden failure zones and bed separations in the overburden. Theoretical analysis and numerical simulation were employed to verify the EH4 survey results. The results show that the failure height in the overburden of the three coal splits was 120m to 170m before panel mining, and increased to 150m to 220m after panel mining. The overburden failure height was on average 7.4 times the equivalent mining height in complex extra thick coal seam condition of Laohutai coal mine when the sand backfilling and fully mechanized top coal caving methods were employed. After mining of panel 63005 started, bed separations were found on the hanging wall and footwall of the F18 fault where water bodies existed. The results obtained in this research not only provide a theoretical guidance for safe mining at Laohutai coal mine but also serve as a reference for predicting and preventing water inrush accidents for coal mines under similar condition in China.

Jan 1, 2014

By K. Y. Haramy

Rock burst and coal mine bump research using static and dynamic rock mechanics instrumentation has been conducted for several decades. Research efforts typically have been conducted using static instrumentation such as pressure cells, convergence measurements, multipoint borehole extensometers, and shield loading pressure measurements; dynamic rock mechanics instrumentation, for example, microseismic monitoring, has been used to help provide precursors to those violent failures. Unfortunately, these different styles of study have never been used in tandem. We report in this study the unique opportunity to have both static and dynamic instrumentation available for monitoring a longwall panel in a deep western coal mine and show that the two methods used in tandem provide information to understand the causes of a major bump. Several static rock mechanics instrumentation sites were carefully chosen along the longwall panel in the coal seam, packwalls, roof, and floor to provide the stress history during mining, including the stress redistribution that occurred before and after the major failures. Monitoring of the shield pressure on selected shields of the longwall face revealed that the cyclical loading anticipated during normal caving processes was not occurring. In addition, by locating each microseismic event (rock noise), the areas that were not adjusting to the changing stress field were clearly delineated by a lack of microseismic activity. The area of the longwall panel that did not adjust to the changing conditions ultimately failed. The combination of static and dynamic rock mechanics instrumentation data analysis provided the necessary information to determine the interaction of the structural members and the cause of the major failures at this mine.

Jan 1, 1988

By Maleki. Hamid

The U.S. Bureau of Mines implemented an inte¬grated static and geophysical monitoring program in a southern Wyoming mine to study changes in seismic wave propagation properties in roof strata. The purpose was to develop techniques to assess roof stability problems. The work consisted of record¬ing deformation and stress histories and calcu¬lating seismic velocity profiles. In addition. geology and roof conditions were mapped; the in¬struments were initially installed at the develop¬ment face; monitoring continued until the mecha¬nized retreat face retreated within 60 m of the instruments. Mining geometry consisted of a three-¬entry gate system using 5.2-m-wide pillars at depths of 426 m. Results were analyzed using stan¬dard history plots, velocity tomographs, and numer¬ical modeling techniques. Changes in velocity. frequency, amplitude, dynamic elastic modulus, and dynamic Poisson's ratio were related to roof defor¬mation, fracturing, and support requirements. A reduction in seismic velocity of 17 pct occurred after 60 mm of roof deformation following installa¬tion of supplementary support. Static measurements revealed significant dilation in the mine floor, spreading to the pillar and the mine roof. Failure was initiated in the floor as a result of uniform pillar penetration.

Jan 1, 1993

By Ulrich Langosch

In the 1990s the German mining industry introduced a new generation of shield supports. The new design of support has a maximum load capacity of 10,000 kN, making these units as strong as the shields used in Australia and in the USA. DMT took more than 3,100 underground observations in order to verify the roof fall frequency by statistical analysis. The results of this work have led to practical recommendations for roof control and the required shield support system on longwall faces. The underground observations have been correlated to Rock Mass Classification, to stress calculations and to the angle between the direction of the fissures and the direction of longwall mining_ The analysis work yielded the following two sets of results: 1. There is a critical distance between the canopy tip and the coal face (Tip to Face TF,n,). The TF,n, is predictable and relates to: the thickness of the first roof layer and its uniaxial compressive strength. The face support should have a TF that is less than the TFcrtt in every underground longwall situation. Exceeding the TF crit call immediately result in a roof fall. 2. Using the obtained regression equation DMT is able to calculate the probability of the roof fall frequency (FF), which describes the roof fall sensitivity. When TFcr;, is exceeded the predicted roof fall FF relates to: the measured support resistance (MSR) of the shield support the calculated vertical stress (p?) the fissure-direction index (DI = angle between main fissure direction and direction of mining) and the distance by which TFcrit is exceeded (ATF). Armed with these results DMT is now able to predict the critical distance between the tip of the canopy and the coal face (TFcrit), as well as the roof fall frequency, for all shield designs. By applying the new calculation method it is now possible to compare alternative longwall layouts and different shield support types under pre-set geological conditions. Mining engineers on site are therefore in a position to make the necessary roof control preparations required to run the longwall operation to maximum efficiency. The results provide a useful basis for making practical recommendations and for selecting the most effective design of shield support. The paper uses practical examples to demonstrate the R&D results and present the various methods now available for calculation and prediction in longwall roof control.

Jan 1, 2003

By Zhou Ying

Based on the mining condition and roof lithology of the 2-3 coal seam at Gengcun Mine, the simulated materials modeling method was used to study the movement process and characteristics of overburden strata movement and to analyze the movement laws of overburden strata during top coal caving mining. Based on the modeling results, variation of the peak abutment pressure and its effect on the top coal's crushing and caving in the mining process was discussed.

Jan 1, 2001

By Rocky Wu

Rock bursts are one of the greatest challenges to ground control in the mining industry. With increasing mining depth and mining scale, there are more and more industry requirements on yielding rock support. Since 2008, Jennmar has been conducting large scale research and development works to develop a technically reliable and cost-effective yielding rock bolt. This paper introduces a new yielding rock support: the Yield-Lok bolt. The bolt is characterized by the designed yielding ability to produce 150?250 mm (6?10 in) of deflection at 8?10 metric tons (MT) (7.9?9.8 t) loads for every 16.4kJ (12,096 ft lb) energy input. The bolt?s performance characteristics are consistent through multiple and varying amplitude of impacts. In this paper, the design criteria of the bolt and the principle of performance are described; the dynamic testing results are discussed; and the features and application of the bolt are presented.

Jan 1, 2011

By Thomas M. Barczak

Roof support systems are necessary to provide stable mine openings and much research has been conducted to design a variety of roof support systems that will function in various manners to ensure that stable ground conditions are achieved. Despite these advancements in technology, mistakes continue to be made in the evaluation and/or installation that significantly degrade the support capability or lead to erroneous determinations of support expectations. The purpose of this paper is to discuss misconceptions about how roof supports perform and factors that impact their performance. The goal is to present practical information that will assist mine operators and engineers in selecting, installing, and evaluating roof support systems properly, and help them to avoid mistakes that can lead to erroneous expectations and potentially catastrophic results that may lead to roof falls. The paper is limited to a discussion of secondary roof support systems and powered roof supports such as longwall shields.

Jan 1, 2001

By I. L. Follington

A breaker line support (BLS) monitoring system, BLSmon, has been designed and constructed by the CSIRO and installed and commissioned at Laleham No.1 Colliery, Queensland. This system was designed to record hydraulic leg pressure and canopy position measurements and relay these, in real time, to the surface. The system has shown itself to be capable of operating on the BLSs without any adverse affect on production, Analysis of the results of the monitoring exercise has shown that rate of change of leg pressures on the BLSs could be used as an aid in the prediction of adverse mining conditions. Under favourable mining conditions the supports were observed to initially unload from the set pressure, this unloading reduced with time and by the end of a typical lift cycle had either ceased or reversed slightly. This behaviour was considered to be due to one of four possible causes, these being; the compaction of floor debris, and/or compaction of the floor, and/or crushing of the roof and /or support creep. The actual cause can only be determined through detailed monitoring of both roof and support convergence. However observational data would suggest that compaction of debris and softened floor material beneath the supports was the most likely cause. The few positive loading rates recorded under these conditions were of short duration and occurred towards the end of individual lifts. Where mining conditions were unfavourable the negative rates of change of pressure, unloading of the support, were reduced throughout a lift, in both duration and magnitude, when compared with favourable mining conditions. Over the same period the positive rates of change were observed to be greater. The rate of change of loading and convergence can be utilised to identify the onset of instability in the lift area. However, before this facility can be utilised by the mine for the reliable prediction of instability more monitored data and sophisticated processing techniques are required. A mechanism has been postulated for the deformation behaviour of the roof and pillars in a mechanised pillar extraction operation from the continuously monitored data, observational data and results of previous studies. The 7 m wide fenders were observed to yield under abutment stresses before lifting commenced. The stability of these fenders throughout the lifting sequence was considered to be dependant upon the magnitude of their post peak load carrying capability. This is influenced by the post peak stiffness and the extent of roof lowering in the vicinity of the fender. Difficulties with fender extraction occurred where remnant coal left standing in the goaf caused uncaved goat to bridge to the mined fender. Difficulties were also experienced where sub-vertical jointing occurred in the roof of the split reducing it's ability to bridge effectively. Despite the relatively short monitoring period, the potential role and application of continuous monitoring in advancing understanding of the rock mass response to mechanised pillar extraction operations has been demonstrated. This understanding now requires enhancement and shows great potential for aiding the optimisation of mechanised pillar extraction operations.

Jan 1, 1992

By Ross Seedsman

Discussion on the design of roof support in tailgates has often been conducted without a clear statement of the stress and failure conditions acting. There is general agreement that in the tailgate the vertical stresses above the chain pillar increase. Within the stress 'bulb' associated with this vertical stress increase there will be an increase in horizontal stress. This does not mean that the horizontal stresses acting across the roof line increase. In retreat longwalling, the proximity of the tailgate to the adjacent goof results in a substantial reduction in the horizontal stress field acting in the immediate roof. Furthermore, at the face/tailgate corner the onset of significant compression of the chain pillar. if it is designed to yield, will result in a further reduction in the horizontal stresses in the immediate roof. It is suggested that this latter mechanism can cause the horizontal stresses in the immediate roof to go to zero. Whatever, the horizontal stress acting across the roof in the face/tailgate corner will be significantly lower than in the face/maingate corner. The implication of the model is that support design in tailgates should be based on the assumption of zero horizontal (in fact tensile) stresses. How the roof behaves in a tensile stress environment is a function of joint spacing and orientation compared to excavation width and direction. It is speculated that the empirical relationship between support density and roof rating results from the relationship between joint spacing and bedding spacing. The onset of stress reductions has major implications to the design of cable and bolt anchorages in the tailgate environment.

Jan 1, 2001

By J. R. Koehler

The failure of yield pillar-based gate mad designs to provide adequate ground control performance is primarily related to the use of "critically" sized chain pillars. A "critical" pillar is one that falls into a range of pillar sizes that are too large to either yield nonviolently or yield before the roof and floor sustain permanent damage, but are too small to support full longwall abutment loads. To directly compare the in-mine performance of critical and yielding pillar designs, the U.S. Bureau of Mines recently completed a field study in a tapering gate road at the Sunnyside No. 1 Mine, Sunnyside, UT. Extreme pillar stresses and associated coal bumps characterize the response to first panel mining of a 16.8-m-wide critical design. Significantly lower pillar stresses, early yielding of the pillar and adjacent panel rib, and an absence of coal bumps suggest that a narrower 12.2-m-wide design more closely approaches proper yield pillar dimensions. Probehole drilling of several 10.6-m-wide pillars revealed low stress levels and substantial pillar and panel rib yielding prior to abutment onset. suggesting a properly functioning yield pillar design.

Jan 1, 1995

By S. S. Peng

In recent years many roof falls have been conveniently attributed to the adverse existence of a high horizontal stress. The normal practice of not conducting a follow-up study in a roof fall investigation usually leads to a wrong conclusion. This paper describes two cases of massive oriented roof falls in two separate room and pillar mines. The investigation in each case involved the analysis of past history and current conditions from which remedial measures were recommended and implemented. A follow-up period of up to three years was performed to evaluate the effectiveness of the recommended remedial measures. In both cases, high horizontal stress was proven positively not the cause, contrary to the original conviction of mine management Rather, a weak immediate roof was the basic cause Under a weak immediate roof, a proper roof bolting plan and adequate entry width and intersection span must be strictly observed.

Jan 1, 1999

By Timothy Ross

The Pittsburg & Midway Coal Mining Co.'s Kemmerer Mine is one of the deepest surface coal operations in the world, with the highwall extending to approximately 1,000 ft above the pit floor. To increase recovery, auger mining is being evaluated beneath the highwalls of several seams representing the current economic limit of pit development. The primary augered seam is over 50 ft thick, can approach 100 ft thick, and dips into the highwall at an average of 17 degrees. The seam thickness and dip present many geotechnical design and operational challenges for multiple Sup to five) lift augering. This paper presents the design criteria and methodology applied to determine appropriate web and septum thicknesses for the various cover depths anticipated, as well as operational measures that can be taken to increase overall coal recovery. The primary tool used in the design was the finite difference code FLAG. Models incorporating typical strata sequences associated with the coal seam were run at several different cover depths corresponding to the maximum anticipated auger penetration. Web and septum thicknesses were varied until the minimum thickness satisfying a given safety factor criterion was determined. Using this procedure, design curves were developed for web and septum thickness versus cover depth. Operationally, several recommendations were developed to ensure that overall coal recovery is maximized. By angering across the scam dip, auger penetration per hole can he increased dramatically, and by carefully planning maximum planned auger penetration versus hole spacing, increased coal recovery can be safely attained.

Jan 1, 1999