Search Documents

By Thomas M. Barczak

The decade of the 90's brought an unprecedented increase in the development of innovative technologies to provide more effective and easier to install roof support in underground mines. To facilitate the application of these technologies to improve mine safety. National Institute for Occupational Safety and Health (NIOSH) developed the Support Technology Optimization Program (STOP). STOP is a windows based software program that provides mine operators with a simple and practical tool to make engineering decisions regarding the selection and placement strategy of these various standing roof support technologies. This program includes a complete data base of the support characteristics and loading profiles obtained through safety performance testing of these supports at the NIOSH Safety Structures Testing Laboratory. Support design criteria in the form of the required support load density at a specified convergence can he established from four options: ( 1 ) a data base of measured ground reaction data from various mines or ground behavior information inputted by the user. (2) determination of loading requirements based on a detached roof block or rock failure height. or (3) criteria based on the current roof support system, and (4) arbitrary criteria set the by user. Using these design criteria, the program will determine the installation requirements for a particular support technology that will provide the necessary support load density and convergence control. Optimization routines are also available to determine the most efficient support design for a user specified support installation- In addition to these performance measures. STOP can be used to compare material handling requirements and installation costs. Comparisons among the various support technologies are easily made including graphical analysis of relevant support parameters. This paper describes the STOP and its application to optimize standing secondary roof support systems.

Jan 1, 2000

By Andre Cezar Zingano

"The empirical approach for pillar design assumes that geological conditions should be perfect, such as horizontal seams, and that pillar strength and behavior will only depend on the coal seam strength and the vertical pressure caused by the overburden thickness on the coal seam; this vertical pressure is based on the tributary area. Empirical methods do not consider the effect of the interface between the roof and pillar or the floor and pillar.The effect of the interface shear strength on final coal pillar strength has been studied since the 1960s. However, few case studies have been generated from that, especially in shallow underground coal mining (less than 100 meters). This case study involves a mining section at the 101 coal mine in the Santa Catarina state of Brazil, which is experiencing pillar rib spalling and yielding because of a 1-to 2-cm-thick clay layer at the area of contact between the coal seam and immediate roof. The depth of the coal seam is 60 m on average and 2.3 m in height. This particular section also has 12-14% (7-8°) of seam inclination. The pillar size varies from l lxll to 13xl2m; thus, the safety factor (SF) based on the ARMPS approach is more than 1.5. The objective of this paper is to determine the cause of the pillar yielding and study the effect of the clay layer at the interface between the roof and pillar, including the inclination of the coal seam. This study conducted empirical and numerical models (2D and 3D) to understand the effect of the thin clay layer. The conclusion is that the clay layer affected the strength of the interface between roof and pillar, causing rib spalling and reducing the pillar strength.INTRODUCTIONPillar stability and deformation depends on the coal seam strength and vertical pressure caused by the overburden thickness on the coal seam, and this vertical pressure is based on the tributary area. The effect of the interface shear strength on the final coal pillar strength has been studied since the 1960s. However, few case studies have been generated from that, especially in shallow underground coal mining (less than 100 m). This interface could be represented by a thin clay layer or an abrupt contact between the coal seam and a rigid sandstone layer of the immediate roof. These two interface conditions are found in the coal mines of Brazil, and the effect of the interface shear strength could increase when the seam's inclination is higher."

Jan 1, 2016

By G. C. Xiao

A database, incorporating historical data from British mines, to study the factors affecting inflows from proximate aquifers into longwall workings, showed that a main casual factor was the immediate roof strata lithology. When this comprised principally stronger sandstone, there was a tendency for periodic inrushes associated with face weighting. This phenomenon is explained by the collection of water in bed sepa¬ration cavities, where free water connected to a high pressure head aquifer source, may be trapped.

Jan 1, 1990

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 Ruixiang Gu

The de-confinement mechanisms of unstable failures as they occur in deep coal mining conditions are investigated using a distinct element numerical modeling code. The Mohr-Coulomb and continuously yielding joint constitutive models were used to study the compressive failures in coal and slip behavior of coal-rock interfaces, respectively. The effect of strength variations within interfaces on the failure mechanism of mining faces is also studied. The stability of interface slip failures was found to have a significant role in the failure mode of coal sidewalls. When the interfaces experience stable slip failures, the sidewalls are more likely to fail in a stable manner. With occurrence of unstable slips at interfaces, unstable compressive failures of sidewalls are triggered. The numerical modeling results support the proposed de-confinement mechanisms and emphasize the importance of including the coal-rock interface behavior in the analyses of unstable compressive failures. The significance of the proposed mechanisms in underground mining conditions is that rock discontinuities with large cohesion drop can become potential factors, inducing unstable compressive failures at ribs and mining faces. Therefore, the de-confinement mechanisms should be taken into consideration in mine design.

Jan 1, 2014

By S. J. Mitchell

An analysis of bolting reinforcement of several large [approximately 18 m (60 ft) cube] pillars was performed. The many overcoring stress profiles in pillars at the mine were used to produce generalized stress distributions at various stages in pre- and post-failure. A complete stress-deformation curve was estimated from this data. The effect of pillar bolting on the load-deformation behavior was extrapolated from the observed unbolted behavior. Bolting can increase pillar strength by up to 10 percent. The effect of bolting on general mine stability was examined with a shoW computer analysis. The computer model was calibrated against measurements performed at the mine, and produced good agreement between measured and modeled stresses. The analysis indicated that bolting increases the bolted pillar safety factors in the event of additional pillar failure by a small amount (approximately 5 percent). Progressive pillar failure is unlikely, because arching transfers most load from yielding pillars to large unmined abutments rather than to the smaller panel pillars.

Jan 1, 1984

By Yongping Wu

Taking the longwall mining face area in a steeply dipping seam as the research object, the stability of an asymmetrical rock mass structure is analyzed using shell theory and masonry beam theory. The results show that an asymmetrical shell structure is formed at the top strata of the mining stope and its magnitude is mainly determined by the mechanical characteristic of the overburden strata of working face, mining space size and so on. Localized instability of the shell structure is the main factor that leads to the overall structural instability in the stope. The instability modes includes: the shell roof instability dominated by tensile failure, the shell shoulder instability initialized by compression-shear failure, and shell bases composite instability occurs in tensile and compression-shear modes. The shell structure stability dedicates rational strata control measures to choose, and the caving height of overburden strata is the key for shell structure instability.

Jan 1, 2013

By Jonathan Duncan, Tim Burgess, William Kendall, Jens Lange

"Steel wire mesh is the accepted standard for roof and rib skin control at most coal mines in the United States. Wire mesh is delivered to the mine in bundles that are heavy and awkward to handle. The large size, stiffness, and weight of the wire mesh can present a safety hazard in transporting underground and storing on the roof bolter. Installing wire mesh on the roof and ribs with the roof bolter can be a challenge as well.Synthetic mesh is lighter than wire mesh and can be folded and rolled into a safe, manageable size. A single synthetic mesh roll large enough to cover the roof and ribs in a deep cut can be safely handled by two men. The roof bolter is designed to carry and dispense the roll as it advances and installs primary roof support, resulting in a single pass roof and rib ground control solution. This paper focuses on the design and implementation of the roof bolter synthetic mesh handling system developed by J. H. Fletcher & Co., Huesker, Inc., and Oxbow Mining.INTRODUCTIONRoof and rib skin control in some coal mines has been an issue since mining first began. Roof and rib skin problems vary from mine to mine. The type of roof rock material exposed after mining; the mine atmosphere; and stresses from depth of cover, over mining, and/or undermining are some of the variables that can contribute to roof skin problems from mine to mine.Potential roof and rib skin problems can be time dependent as well. Some mines have roof skin rock, sometimes called draw rock, that falls when drilling the hole for the roof bolt as the initial roof support is installed. Some roof skins are more stable but can fail over time. The failure of roof skin can even be between properly installed roof bolts with roof plates (Figure 1).Mines have most commonly used a wire mesh that comes in individual panels anywhere from 5 ft x 5 ft (1.5 m x 1.5 m) all the way up to 5 ft x 16 ft (1.5 m x 4.9 m) in size. They are typically constructed of 8- or 9-gauge wire welded on a 4 in (102 mm) square pattern. A 5 ft x 16 ft (1.5 m x 4.9 m) panel weighs approximately 35 lbs (16 kg). These steel wire mesh panels can make it a difficult and sometimes dangerous task to deliver them from the surface to the roof bolter and then store on the machine. Manipulating the mesh panels from the storage area on the machine to the final installation position on the roof or rib can also be a daunting task (Figure 2)."

Jan 1, 2010

By Ke Yang

"Engineering practices have shown that the skip-fault longwall mining is frequently used when faults with offsets larger than 1 Om are present. Since the skip-fault longwall mining not only reduces the recovery rate of coal reserves, but also increases the cost due to developing another setup room on the opposite side of the fault and moves the longwall into it. The coal pillar so created induced residual stress in and thus affects the safe mining of adjacent coal seams. In order to investigate the influence on panel layout and retreat by a l 6.5m offset normal fault, the similar material simulation model was constructed based on the geological and engineering conditions of No.13 coal seam in the western district of Liuzhuang coal mine employing the down-dip longwall mining. The characteristics of deformation, movement and failure of the overlying strata and abutment pressure when the face cut through the hangingwall and footwall of a normal fault were obtained. There were significant differences between the hangingwall and footwall as conformed by the equilibrium structure model. The results showed that superposition of the tectonic stress from a normal fault of large offset and that induced by mining is one of key factors that lead the severe pressure difficult to control during panel retreating from the hangingwall to footwall. Accordingly, the technical solutions for a fully mechanized mining face directly cutting through the fault are presented and optimized, and the applications of the technical solution in Panels 171301and171303 of Liuzhuang mine enabled them to complete the mining safely and efficiently. INTRODUCTIONFaults have been one of the important technical factors in underground coal mining l1-2i. Engineering practice has shown that the methods of mining through the faults depend on the seam thickness, seam dip angle, offset of fault throw, equipment height, rock strength, and advancing direction l3-5l. In the fully-mechanized longwall mining, the face normally proceeds to cut through the fault directly if the offset of the fault less than 5m, and to skip Uump) the fault if the offset is larger than 5m. In the former case, the most important problem is to efficiently control or avoid roof fall accidents (i.e., crushing, falling, pushover, and rock burst). In the latter case, the most important problem is very expensive to drive a new setup room and move the face equipment to it. The skipped coal pillar may poses problems for mining in adjacent seams."

Jan 1, 2016

By Johann van Wjk

Mandatory Codes of Practice to Combat Rock Fall Accidents came into force on South African Collieries in 1997. These were drawn up in accordance with guidelines issued by the Chief Inspector of Mines, The Codes of Practice (CoP's) are legal documents in terms of the South African Mine Health and Safety Act 1996. One of the main results Of the introduction of the COP'S from a practitioner's point of view is that they formalized much of what would have constituted a typical visit to an underground section previously. Other important requirements were as Billows: Support rules and Strata Control Standard Procedures were to be drawn tip. Identification of Special Areas (areas requiring a higher level of rock engineering input and with, typically, higher order of support and restrictions on mining geometries), Changing conditions underground to he identified timeously. Implementation of the CoP to he monitored. To comply with these requirements on Ingwe Collieries (a division of BHPO Billiton Energy Coal) it was decided to produce too separate forms to be used in conjunction with each other. The first of these, the "Underground Section Physical Risk Rating" measures the physical conditions in each section whereas the second form, the "Section Perfomance Rating" determines how well the section personnel are Coping with the conditions. This form also pleasures compliance with the Support Rules and Strata Control Standards. Both forms are essentially risk matrices but with various scenarios described, each with a certain weighting. The "Risk and Performance Form" system for bord and pillar operations has by now been used for five years in the Ingwe and Eyesizwe Mining Collieries and has achieved a great deal of acceptance. This system has enabled the quantification of previously subjective observations and has improved the strata control awareness and management of underground mining sections. Amongst other initiatives it has assisted ill reducing the severity and frequency of fall of-ground accidents.

Jan 1, 2002

By Anton Sroka, Antoni Tajdus, Axel Preusse, Krzysztof Tajdus

"In Europe, the magnitude and range of mining deformation predicted on the surface have been, in most cases, determined using methods based on Gauss' influence function (e.g., Knothe's theory, Ruhrkohle method, Stochastic method). The increase in the application of numerical methods (finite element method FEM, finite difference method FDM, etc.) provides an opportunity to offer a different approach in describing mining deformations. However, this phenomenon is very complex, and due to numerical limitations, a two dimensional analysis was made (in plane strain) in most cases. Such 2D models could not accurately describe the mining geometry and geological conditions. The authors of this paper carried out the calculations in 3D models which describe a mining situation and the character of the strata in one Upper Silesian Polish mine. Furthermore, the authors include some tips for three-dimensional modeling.INTRODUCTIONThrough the years, many numerical models have been used to describe a phenomenon of underground mining exploitation influence on the ground surface. However, such rock masses are highly disturbed (cracks, fractures, changes in water conditions, etc.), and it is very difficult to suitably describe, in a mathematical manner, such materials. Therefore, many of the numerical models created have been corrected in various degrees.At first, models widely used to describe the behavior of strata in the region of exploitation appeared as elastic models, where scientests assumed that a whole strata behaved in a linearly elastic manner, according to Hook's law. Those models have been modifed to received suitable VALUES (of settlement and range of subsidence troughs. Scientests created this amongst others: elastic models with contact between the rocks layers (Shippam, 1975; Salamon, Chugh and Yang, 1993), elastic models with no tension (Chrzanowska-Szostak, 1988), and elastic models with a contact on the subsidence limit plane (Tajdus K., Tajdus A., 2005). However, in most cases, those models did not bring the expected results and the differences between shapes of measured and the calculated subsidence trough were substantial."

Jan 1, 2010

By John A. Rusnak

Point load testing is used to determine rock strength indexes in geotechnical practice. The point load test apparatus and procedure enables economical testing of core or lump rock samples in either a field or laboratory setting. In order to estimate uniaxial compressive strength, index-to-strength conversion factors are used. These factors have been proposed by various researchers and are dependent upon rock type. This study involved the extensive load France and point load testing of coal measure rocks in six states. More than 10.000 individual test results. From 908 distinct rock units, sere used in the study. Rock lithologies were classified into general categories and conversion factors were determined For each category. This allows for intact rock strength data to he made available through point load testing for numerical geotechnical analysis and empirical rock mass classification systems such as the Coal Mine Roof Rating (CMRR).

Jan 1, 2000

By Stankusm John C., Xiaoting Li

"Quantitatively identifying the load on a steel king beam set is a complicated issue in designing the king beam structure at the slope bottom area. From an engineering perspective, understanding the nature and support mechanism of the king beam structure as a standing support is critical in identifying the calculation input parameters for the structure design software. Based on design practices in recent years and successful applications in several slope bottom intersections, the authors present a king beam design methodology, including qualitative analysis, quantitative calculation with a structural design software, and computer Finite Element Analysis (FEA) verification.Especially important is the concept of interaction and the confinement effect between the king beam structure and surrounding rocks, which is essential in designing a proper steel structure in terms of maintaining the roof stability, as well as saving the steel materials and costs of manufacture, labor, and delivery. This paper details the design methodology and applications in several underground slope bottom intersection designs, reaching the goals of improving roof stability and reducing costs.INTRODUCTION AND DESIGN METHODOLOGYCurrently, the load on a square or arch set is typically determined by either past empirical design experiences or known heights of roof falls in current or adjacent mines. These methods work but sometimes are too conservative from an engineering perspective. For example, when designing a king beam structure for an intersection, these methods may lead to a design with extremely large and heavy steel beams, which are not convenient for delivery and installation on-site, not to mention, costly. Therefore, accurately identifying the potential maximum load on the king beam structure is important for designing a safet, effective, and cost-efficient steel structure. Based on past several years’ practices and fundamentals of ground control and structural design engineering, the authors propose a design methodology including the following components:The nature and mechanism of effective roof support (qualitative analysis),Ground reaction curve,Interaction and confinement effect between bolts and steel standing support and surrounding rocks,Factors affecting the load conditions of the king beam set"

Jan 1, 2018

By Gregory Molinda

Groundfalls are much more likely to occur in coal mine intersections than in entries. NIOSH is using the experience of U.S. coal mines to determine the factors which influence intersection instability and provide guidelines for the safe excavation and support of intersections. Detailed field investigations have resulted in a database of U.S. coal mines containing 12 mines and 639 roof falls so far. By using the roof fall rate as the outcome variable, correlations between roof geology (CMRR), intersection span, and roof support have been established. Case studies have indicated that replacing 3 way intersections with 4-way intersections may not reduce the total number of roof falls. Additionally, the size of intersection spans tend to decrease with lower (weaker) CMRR. Protocols have been established for the collections of roof bolt parameters. The performance of individual roof bolts can now be tracked with roof fall rate.

Jan 1, 1998

By Fangting Dong

The broken rock zone (BRZ) and the tunnel support theory are investigated. The relationship between the BRZ and the related factors, the interaction between the BRZ and supports, and the function of supports are analyzed in detail based on the authors' underground observations and model taste. A summary of the application of the BRZ theory in the past is also presented.

Jan 1, 1988

By Hamid Maleki

The U.S. Bureau of Mines implemented a monitoring program in a western U.S. coal mine for the evaluation of both conventional and alternative support systems. The conventional support consisted of 2.5-m-long vertical bolts installed in an 5.5-m-wide entry. The alternative system consisted of 2.5- and 3-m vertical and inclined bolts installed at a higher density during widening of a cut from 3.6 to 5.5 m. Roof deformation, roof stress, rib deformation, and bolt loads were monitored during the 2-month test period. In addition, roof failure patterns and lithology were characterized within the six-entry main test section. Results were analyzed using roof deformation and failure profiles, roof stress profiles. bolt loading and bending moment diagrams, and numerical models. The mechanics of roof deformation, failure, and support reaction in three zones in the mine roof were characterized. Significant improvements were shown in roof stability in the section using the alternative system. Results are encouraging and indicate the effectiveness of alternative support systems and excavation techniques for limiting inelastic deformation of roof in highly stressed underground mines.

Jan 1, 1994

By Houquan Zhang, Ming Li, Guimin Zhang, Bei Zhang

"The support quality of bolts in deep roadways is one of the important factors for efficient and safe coal production. A new kind of non-destructive testing technique and equipment was used to monitor the axial bolt load in real time in deep roadways. Based on the test results, the working status of bolt support in deep roadways was evaluated. The research results show that the variation characteristics of axial bolt load is consistent with the distribution characteristics of rock pressure no matter whether it is in the initial support stage or mining disturbance stage. The axial load of bolts located 20m outby the development face has an overall rising trend within 3-5 days after installation. Within a certain distance in the front abutment pressure zone in front of the face, the axial bolt load increases with the advance of coal extraction. When coal and rock in the abutment pressure zone yield, the axial bolt load is dropped correspondingly. This kind of non-destructive testing technique provides an advanced means of detection and evaluation on bolt support quality.INTRODUCTIONMany coal seams more than 800m deep are being mined in eastern China. Roadways of large cross-section are being developed in coal seams more than 600m deep in western China (Kang et al. 2014). Moreover, these roadways are commonly supported by roof bolts and have encountered many support problems, including soft rock strata, intense mining disturbance, and abnormal tectonic stress. The stability of its surrounding rocks is difficult to maintain, the roof bolt even fails before mining (Shen 2014; Jiang et al. 2015). Therefore, it is very important to develop an effective method to detect the bolt quality in real time and evaluate the bolt's safety status.During installation of roof bolt, the installation quality is heavily affected by installation equipment and tools, field conditions, and operators' skills (Li et al. 2005; Shan 2009). The anchorage length will be shorter than the designed one if the resin cartridges are pushed to the bottom of excessively long drill hole and stay there without being shredded and mixed. The support function of the bolt would be weakened by the shorter anchorage length."

Jan 1, 2016

By Larry Stolarczyk

A clear vision of the future coal mining industry was developed into a hierarchy of technology needs by the National Mining Association (NMA) and the Chief Executive Officers of the mining industry. Although improvements in safety and environmental factors were the main drivers in the technology roadmapping workshop sponsored by the Department of Energy, the agile mining machine of the future must efficiently extract coal from deeper, thinner, and more geologically complex coal beds. One of the challenges will be to develop sensors and software that enable clean coal cutting in a changing geologic environment. The roadmap identifies imaging ahead of raining and horizon control within the top ten needed technologies. For these technologies to be adopted by the mining industry, they must be proven to be effective while driving down mining cost and risk. This paper describes how three-dimensional imaging ahead of mining and horizon sensors enable improved ground control and safety in the mining of underground coal.

Jan 1, 2003

By Selmar A. Oliveira

"Safe mine closure is an important step in the mining lifecycle. Plans for safe and efficient closure should begin at the inception of mining. Ideally, closure occurs when the reserve has been depleted. However, when the abandonment of a mine occurs prior to the depletion of the reserve, several problems arise which need to be treated. This situation occurred at Verdinho Mine, located in the state of Santa Catarina, southern Brazil. Verdinho Mine is a coal mine that operated for more than 40 years without had backfill technologies applied. Soon before the exhaustion of the reserves, it was abandoned by the owners without proper closure. As a result, the Federal Public Ministry filed a public civil action to promote proper closure. Several parties are currently involved, including the National Department of Mineral Production (DNPM), the mining regulator of the Brazilian Government. To meet the judicial determination, a cooperation agreement was initiated between DNPM and the Federal University of Rio Grande do Sul (UFRGS) to identify and to monitor parameters which could result hazard due flooding mine, to model the various conditions to minimize the impact of flooding caused by total abandonment. Many processes must be planned and planned early in mining operations, and these processes must be performed over the life of the mine. Financial resources must be guaranteed by a specific fund fueled by operating revenues. When these resources are not properly employed, costs and technical difficulties prevent proper closure progress. If the removal of subsoil equipment, treatment and control of groundwater, regeneration of the surface, and closure of openings are not done correctly, there can be environmental, social, and stability impacts over time. To avoid this in the future, a detailed study is being performed at the Verdinho Underground Coal Mine on how to appropriately implement mine closure and monitoring procedures. This study is being conducted in response to a lawsuit filed by the Federal Public Prosecutor’s Office. INTRODUCTION The nature of the Mining Industry is such that all mining is a temporary activity, where the operational life of a mine is defined by the final project scope. The operating life of a mine lasts from a few years to several decades. Mine closures occur once the mineral resource at a working mine is exhausted, or operations are no longer profitable. Mine closure plans are required by most regulatory agencies worldwide, such as the Mineral Production National Department (DNPM) in Brazil."

Jan 1, 2017

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