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By R. Kaund

"Quantifying the rockburst consequence is of critical importance to reduce the hazards with preventative measures in underground mines and deep tunnels. Contours of energy components within a pillar model are plotted at different rockmass damage stages, and plastic strain work and released energy are proposed as indicators of rockmass damage consequence. One pillar model under different loading stiffness is simulated to assess indicators of pillar burst and the resulting damages. The results show the rockmass damage under soft loading stiffness has larger magnitude of plastic strain work and released energy than that which is under stiff loading stiffness, indicating the rockburst consequence can be quantified with plastic strain work and released energy in numerical models. With the quantified rockburst consequence, preventative measures can be taken to avoid severe hazards to mine safety.INTRODUCTIONWith the increasing demand of mineral resources and depletion of near-surface ores, the depths of underground mines have made the mining activities one of mankind’s most dangerous types of work (Ortlepp, 2005). The Mine Safety and Health Administration (MSHA) defines rockburst as “a sudden and violent failure of overstressed rock resulting in the instantaneous release of large amounts of accumulated energy.”The types of rockburst can be classified into three types: (1) strain burst, (2) pillar burst, and (3) fault slip (Müller, 1991). Strain burst is the most common type of unstable rock failure in underground openings, where the intensity and scale are usually smaller than pillar burst and fault slip (White and Whyatt, 1999; White, Williams and Whyatt, 2002; Ortlepp, 2005). The cause of strain burst can range from shattering of rock under high stress concentration to buckling of discontinuities parallel to underground openings. The occurrence of pillar burst also depends on the stress and discontinuity conditions in the rockmass, but pillar burst usually involves the sudden loss of strength in the core or foundation of a pillar (Whyatt and Blake, 2002; Kias and Ozbay, 2013; Khademian et al., 2016). Fault slip or slip burst can be either slip along pre-existing discontinuities or shear failure in the rockmass, when the shear stress is larger than shear strength in the fault (Rice, 1983; White and Whyatt, 1999; Sainoki and Mitri, 2014). The consequence of fault slip can vary from very small rockmass damage to consequential rockmass damage, but large seismic magnitude does not always result in large rockmass damage (White and Whyatt, 1999; Whyatt and Blake, 2002)."

Jan 1, 2018

By David Miller

Longwall gateroad support can be supplied by a variety of methods. The system of support also needs to be flexible as conditions change. This is especially true in the complex geological conditions of the west Kentucky # 13 seam. Methods of gateroad support used in this seam will be presented along with the successes and failures of the systems as conditions changed Lodestar Energy's gateroad support progression through the seven longwall panels retreated to date will provide an insight into planning for future panels

Jan 1, 1998

By W. J. Guo

Overburden movement as a result of longwall mining can be classified into, in ascending order, caving zone, fractured zone, and bending zone. Operation experience and laboratory research have demonstrated that there are bed separations in the bending zone due to differential movement and rock mechanics properties between adjacent strata. In china several coal mines have injected flyash into bed separations to prevent strata movement and control surface subsidence. This method is very effective because it can reduce surface subsidence by 35-55%. Combining the field survey results and laboratory simulation research, the author has investigated bed separation and its mechanisms and effect of backfilling the bed separation and its mechanisms. Finally backfilling application in three mines are introduced, including grout flow chart, grouting pressure, and radius.

Jan 1, 1994

By Peter Zhang

Room and pillar mining under shallow depth is usually conducted with a high extraction ratio, which leaves small pillars with a low safety factor in some old workings in abandoned mines. Pillar sloughage, squeezing or even collapse may have occurred in those old workings with small pillars, either at the time of mining or later. The conditions of the pillars in abandoned mines can affect mining above them. Subsidence fractures and abutment pressure are indications of multi-seam interaction caused by squeezing or collapse of small pillars. To mine over areas with potentially unstable small pillars, the panel layout in the upper seam should be such that multi-seam interaction can be minimized and proper roof and rib support should be installed in the influence zones. This case study presents an example of multi-seam mining above small pillars under a shallow depth of cover. Though the interburden is relatively thick and competent, interaction is still present over the influence zones. Pillar stability and multi-seam interaction analysis, numerical modeling and in-situ monitoring are presented to demonstrate how the upper seam can be mined safely over the small pillars.

Jan 1, 2014

By Sanjay Kumar Singh

This paper presents ground control technology associated with a continuous mining operation for mass exploitation of underground coal deposits in the Indian coal mining Industry It shows that competency of roof strata and performances of continuous miners are safer, than other coal mining method as well as productive and economical. Continuous miners keep playing a major role in the underground coal mining industry for their safety and as well as their productivity. Regarding past experiences in Indian coalfields, Chirimiri and Jhanjara mines have achieved exceptional performances, with a potential for over 500,000 tonnes per year. Tandsi mine has difficult roof conditions and does not typically achieve high production and productivity. Even so, the difficult roof conditions have been controlled with modern ?world best practice? roof bolting systems and an output of up to 1,200 tonnes per day has been achieved. It can further be emphasized that if this technology is implemented properly it will bring an unfathomable change to the Indian mining scenario. There are many mines in India that may have potential and economic viability with ground control technology concepts for installing continuous miner systems and beginning mass exploitation of underground coal deposits.

Jan 1, 2011

By Stephen C. Tadolini

The risk of injury caused by minor roof falls continues to increase in underground mining. Most ground control injuries result from small rock falls that occur in a supported area, but do not involve the failure of the support system. In the roof, these are the "skin failures" that occur between previously installed primary or secondary bolting systems. To minimize skin failures and the associated accidents, several devices were designed and tested in the laboratory to maximize stiffness, minimize material thickness and weight, and ease material storage and handling on roof bolting machines. The goal of this project was to design and evaluate devices that can be installed with traditional primary or secondary roof supports, and that arc easy to handle/install under temporary or previously supported roof.

Jan 1, 2001

By Jinsheng Chen

For underground openings, many factors can cause roof failure and thus roof falls. The most common factors related to Mother Nature are weak roof lithology, geological structures, and an abnormal in-situ stress field. Theoretically, any underground opening can be successfully supported under any geological and mining conditions if the opening width is limited and the roof supporting system is well designed. However, roof control cost must be taken into account. In practice, to minimize the roof control cost, different roof control plans have to be considered to take care of the uncertainty or variation in geological and mining conditions. For most underground mines, it is unsafe or economically unrealistic to use one fixed roof control plan for the entire mine. Therefore, local, abnormal geological and mining conditions must be identified and a site- or area-specific roof control plan must be employed. This study tries to identify the major factors that have significant impact on entry roof stability by analyzing the historical roof fall data at Rofomex mine. Finally, the appropriate roof control plans or roof bolting systems for different geological and mining conditions are recommended.

Jan 1, 2011

By S. M. Matthews

The magnitude and orientation of the in situ stressfield has been determined as a key factor in controlling the stability of openings for both coal mine development and extraction. Monitoring of roadway and pillar behaviour in conjunction with numerical analysis has identified specific stress control measures that have been successfully applied to improve productivity in underground coal mining operations. The influence of the major horizontal stressfield on the stability of openings has been identified at a wide range of sites including mines in Australia, Britain, the USA, New Zealand and Japan. Stress mapping, stress measurement and stress monitoring techniques have been applied to define the in situ stress regime and, together with the correlation of measurements of rock deformation and rock reinforcement behaviour, has enabled appropriate stress control measures to be developed. Stress control measures for development headings have required the selection of roadway orientation, roadway sequencing, reinforcement density and pillar size. The monitoring of behaviour at a range of trial sites has confirmed significant improvements in development stability. Stress control measures for longwall extraction have required rational selection of longwall orientation and extraction direction. Where a mine layout design incorporating the optimum longwall layout design has not been possible, the use of sacrificial roadways has been successfully applied to protect key roadways from stress concentration effects. Comprehensive in situ monitoring of behaviour has been carried out to confirm the mechanisms involved.

Jan 1, 1992

By Olayemi Akinkugbe

As coal production in the United States continues to increase, the availability of unexploited or virgin fields continue to diminish. As a result, mine operators are forced to mine in unfavorable or multiple seam conditions. The safe, productive exploitation of coal in multiple-scam conditions requires specific design technology. At present, there are two general design approaches that the mining engineer can use to analyze the multiple-seam problem in question, either simple empirical relationships or complex numerical models. Often, the empirical approaches are too general for the specific problem and the numerical models require too much time and expertise for the practicing engineer. This paper introduces a new computer program, Lam2D, designed specifically to enhance multiple seam mining operation by quickly and easily calculating, stress, displacement, subsidence, horizontal strain and safety factors associated with multiple seam mining situations. The program implements a simplified, two-dimensional boundary-element method in order to model the complex multiple-seam stress and displacement interactions. Most importantly, the program is greatly simplified by incorporating automatic overburden, interburden, coal and gob property generation thereby reducing typical modeling and solution time to a few minutes.

Jan 1, 2004

By Axel Preusse

A high advance rate of the longwall face increases inevitably the mining dynamics within the rock and on the surface. Measures to limit the mining dynamics are the adjustment of the face advance rate to stretch the influences of mining over time and continuous winning to maintain a uniform influence of mining. This paper deals with the dynamics of mining subsidence within the range of a transmission overhead line for transportation of gasoline, diesel, kerosene, heating oil, mineral oil and other hydrocarbons. In a certain segment of the 24"-pipeline it was possible to predict, due to technical, operational and laying conditions, that undermining the pipeline would cause local stress maxima within the pipeline string. To limit these expected local stress maxima, the face advance rate of panel 497 was adjusted to the sensitivity of the pipeline towards the influences of mining. Comparison of the predicted mining dynamics, using the theory of stochastic media, with the measured ground movements led to the conclusion that the effects of dynamical influences which occurred within the range of the transmission overhead line and acted on it were low. This confirmed that the decision to adjust the face advance rate to the sensitivity of the transmission overhead line was correct (Kateloe et a)., 2002). A later relaxation cut in range of the pipeline siphon scarcely produced relaxation movements of the pipeline. The appointed sensitivity data of the undermined pipeline and the according maximum admissible face advance rate were thus correctly measured.

Jan 1, 2003

By W. G. Pariseau

A new approach to the direct inclusion of the effects of joints and geologic structure in rock mechanics analyses is applied to an elastic analysis of stability of a Lucky Friday undercut and 4111 longwall stope (WL) using the finite element method (FW). The approach is base+ on the concept of equivalent properties of a heterogeneous test volume, but does not depend on the assumption that a test volume is a representative volume element. Comparison with conventional FW results show greater variability in fields of stress, strain, displacement, safety factor and energy densities. Stability requires a knowledge of rock mass strengths as well as elastic moduli.

Jan 1, 1989

By Christopher Mark

Despite more than half a century of experience with roof bolting, no design method has received wide acceptance. To begin to improve this situation, National Institute for Occupational Safety and Health (NIOSH) conducted a statistical study of roof bolt performance at a number of mines throughout the U.S. Case histories were collected from 37 mines with a variety of roof bolt types and patterns in a wide range of geologic environments. Performance was measured in terms of the number of roof falls that occurred per 10,000 ft of drivage. The study found that roof falls are rare when the roof is strong and the stress is low, even with light roof bolting patterns. The focus of this paper is on the more difficult conditions, where the roof is weaker and/or the stress is higher. Analysis of the results led to guidelines that can be used to make preliminary estimates of the required bolt length, capacity, and pattern. The guidelines are based on the depth of cover (which correlates with stress) and the roof quality (measured by the Coal Mine Roof Rating (CMRR), and the intersection span. Another contribution is a formula for estimating the horizontal stress level in the eastern U.S. coalfields as a function of the depth of cover. The design guidelines are currently being implemented into a computer program called Analysis of Roof Bolt Systems (ARBS).

Jan 1, 2001

By Brian E. Travis

Lightweight polymer grids have recently been introduced to the mining industry. They are currently being used for roof and rib control, longwall shield recovery, safety guarding or barricade, highwall screening, internal reinforcement of gunnite/shotcrete applications, sub-base reinforcement, construction of steepened slopes, retaining walls, mine reclamation (i.e. erosion control, turf reinforcement), and in numerous mining waste related projects. These polymer mining grids utilize a strong, lightweight polymer, usually special grades of polypropylene or polyethylene. High tensile strengths are achieved by molecular orientation of these polymers in the manufacturing process. The first step in the manufacturing process is to extrude the polymers into thick continuous sheets. These sheets are then punched, and drawn into open grid structures. Aperture size and configuration are controlled by the geometry of the punched hole and the degree to which the grids are stretched. The drawing process is the key to the molecular orientation required for the tensile strengths. These long chain polymer molecules in their natural state have a random orientation. When stretched, the molecules are pulled into a parallel alignment, thus creating a lightweight product with tensile strength comparable to steel. The finished polymer mining grid product is a viable alternative to conventional supplemental roof control meshes such as chain link fence and welded wire. Polymer mining grids are a fraction of the weight, have comparable load support qualities, and will not rust or corrode in even acidic or alkaline conditions. Specific flame retardant additives can also be incorporated to make the polymer grid self extinguishing for use in underground "gassy" environments.

Jan 1, 1992

By Graham Daws

State of the Art Room and Pillar mining techniques have been employed at two coal mines in the UK in the last two years. This paper describes the design of pillars and rockbolted support systems for Room and Pillar mining operations in these collieres. Within the last year, rock mass classification systems have been used in the design process of a room and pillar mining layout at one of these collieries. In particular, the roofbolt support system was detailed using these techniques and the support system was linked with operational aspects such as depth of cut. The depth of cut was initially estimated using rock mass classification systems and then verified by a stand up trial which was monitored remotely. The stand up time has a direct influence on the operational aspects of the mine and this is described. The design was verified by performance monitoring and the results are discussed. The importance of understanding the role of rockbolting is emphasised along with some examples of accurately determining in-situ bolt performance. The paper also recommends some modifications to rock mass classification systems when applied to underground coal mining operations.

Jan 1, 1998

By Russell Frith

The paper defines the various operational advantages associated with the use of cribless tailgates during longwall extraction and considers the associated ground control issues that need evaluating to ensure a successful outcome. Reference is made to longwall mines in Australia currently using cribless tailgates and the geotechnical and ground support factors that allow them to do so This includes the nature and behaviour of the roadway roof the stability of the chain pillar and also the type and timing of installed support A specific case history from a colliery in NSW will he referred to whereby heavy standing support was replaced with lower density tensioned cables as routine TV secondary support, which resulted in both improved TG roof conditions and a significant cost reduction. The conclusion will be drawn that cribless tailgate roadways can potentially he used in many environments either due to the existence of highly competent immediate roof conditions or through the appropriate application of active reinforcing support measures in less competent conditions. Use will he made of Strata Engineering?s roof behaviour and stability model in demonstrating the various concepts involved.

Jan 1, 2000

By S. M. Hsiung

In late September 1984, a longwall panel in West Virginia lost 60 powered supports when the face had advanced for 250 ft. from the setup entry. An in-mine investigation showed that it was a first weighting failure resulting from inadequate Support capacity. This paper presents the results of finite element modeling, which have led to a better understanding of the first weighting failure mechanism. The dynamic impact of the first weighting failure on the powered supports is analyzed. The optimum capacity for the powered supports in order to control the roof at face area under the geological condition is discussed.

Jan 1, 1984

By Nielen van der Merwe

The data base of failed pillar cases as used by Salamon and Munro (1967) in their original analysis to determine the strength of coal pillars empirically, has been updated for this study. It is shown that the Vaal Basin and Klip River coal fields exhibit similar behaviour, i.e. failure at high safety factors within a short period of time after their creation. Those collapse cases have been omitted from the new data base. New failures that occurred after 1967 have been added, except for those that occurred in the Vaal Basin and Klip River coal fields. (1967) a technique to minimise the area of overlap between the populations of failed and intact pillar cases, the new data base was re-analysed in conjunction with the original Salamon and Munro (1967) data base of intact pillar cases. It was found that a linear formula reduced the overlap area by 22%. The new formula predicts higher strength for pillars with a width-to-height ratio greater than approximately 2S and a lower strength for smaller pillars. For most current South African coal mining conditions, using the new formula for pillar strength will result in leaving smaller pillars without sacrificing stability. The reason for this is that the Salamon and Munro (1967) formula underestimated the strength of larger pillars and overestimated the strength of smaller pillars. The potential benefit for the South African coal mining industry amounts to saving reserves equal to the total production of two large mines per year.

Jan 1, 2002

By Brian Clifford

Thoresby Colliery is in the process of recovering an old roadway that was flooded following the closure of an adjacent mine. The aim is to re-use the roadway as a gate road for a longwall retreat panel and it is therefore important to establish the condition of the existing rockbolt support during the recovery operation. Two NDT methods, ultrasonics and RMT's newly developed RF (Radio Frequency) system were used in combination to interrogate the rockbolts to determine their condition. This paper describes the condition of the rockbolt supports, the NDT methods used and the problems that had to be overcome to successfully apply the two methods.

Jan 1, 2002

By Vladimir I. Klishin

Complex method of ruling of rock pressure is considered in conditions of rapid roof subsidence. The purpose of this method is elaboration of emergency devices into hydraulic props. Also method of unloading of solid roofs is worked out. An analysis of means of protecting hydraulic props of powered supports 6om dynamic loads made it possible to divide them into four classes according to principle of operation: emergency, operating hm an increase of pressure; emergency damping; emergency, operating according to the "plastic element" principle; inertial emergency, operating hm dynarmc loads. It was established that the overwhelming 'majority of designs of the first three classes of emergency devices (EDs) are based on !mditional approaches, which exhausted their possibiiities of high-speed response. Delay on their opening is caused mainly by inertia of the moving locking parts. In our opinion, the most prospective are EDs using the inertia of the moving locking parts for depressurizing the piston cavity of the hydraulic prop with a minimum delay (inertial EDs). The essence of the designs of EDs having an inertial principle of action is that into the hydraulic prop containing movable and immovable elements is additionally introduced a spring-operated contracting support, which is installed on the movable element. Under dynamic loading of such a hydraulic prop hm the roof, the support moves relative to the movable element, performing the role of an inertial mass, which leads to instantaneous depresswization of the piston cavity of the hydraulic prop. Technological measures for unloading of solid roofs consists in creating of long oriented cracks in rock massif. Description of method for creating cracks, demanding equipment and means for cone01 the progress of splits are included. Also technological schemes for realization of the method of saving underground mines and rock mass are included. The results of experimental realization of this method on the Kuzbass and Poland coal mines are also included.

Jan 1, 1997

By E. M. Williams

The U.S. Bureau of Mines is investigating the use of in-seam seismic methods to monitor stress-induced changes in coal near the working face of longwall mines. Seismic surveys performed through a longwall panel and pillars near the face can provide valuable information on geologic anomalies, high stress regions, tailgate support performance and other conditions that could endanger miners or interrupt production. This paper describes the details of the equipment and data analysis techniques used, and the significant results obtained and their implications for ground control. Using seismic tomography, it is possible to conduct measurements on the pillar and reconstruct an image representing the internal velocity distribution. Successive measurements over several days identify changes in velocity attributable to changes in stress. Several field measurements were conducted at a western U.S. longwall mine to observe stress induced changes within the panel and pillars during mining. A permissible data acquisition system was designed to meet mine safety requirements for operation in gassy air. Certification from the Mine Safety and Health Administration (MSHA) was obtained, and this approved system was used to monitor stress changes in several pillars as the longwall face passed during first panel mining. Tomographic surveys were completed across the longwall panel to map the apparent abutment stresses ahead of mining. Velocity distributions show an increase in vertical stress across the panel from the headgate to the tailgate. Additionally. a yield zone along the face near the tailgate and the forward abutment approximately 100 feet outby the face in the tailgate side of the panel were detected. These results agree well with data obtained from other instrumentation emplaced in the panel near the tailgate. Using velocities for both compressional and shear waves, the Poisson's ratio was calculated for the panel.

Jan 1, 1994