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By M. W. Fabjanczyk

Cement and resin encapsulated tendons are widely used in the stabilisation of mine roadways. One of the main factors in controlling the efficiency of the reinforcement is the ability of the tendon to generate load in response to rock deformation. The analysis of field data from instrumented reinforcing tendons complimented with extensive laboratory investigations have more accurately defined the mechanisms by which load generation occurs. The role of reinforcing tendon profiles, hole geometry and encapsulation medium properties in controlling the reinforcement stiffness and capacity are discussed. The results of the investigation have implications in both the design of rock reinforcing systems and the optimisation of encapsulation mediums and tendons.

Jan 1, 1992

By Y. Luo

This paper presents a number of mathematical models for predicting the different phases of the complete surface subsidence process associated with underground longwall mining. A computer program developed based on the mathematical models is briefly introduced.

Jan 1, 1992

One of the most important parameters in rock/resin/bolt load transfer mechanism is the surface property of the bolt. Although there are various bolt surface configurations available in the market, very little work is reported on the bolt/resin interface failure mechanism. This paper examines the behaviour of bolt surface roughness under constant normal stiffness conditions which is considered as being a realistic way of evaluating bolt surface roughness. A series of shear tests have been conducted on the flattened bolt surface to study the shear behaviour of bolt/resin interface. Based on the experimental results, a mathematical model has been developed to predict the shear resistance of the bolt/resin interface at various normal stress levels for the bolt types used in the study.

Jan 1, 1999

By R. Fraher

Hazard analysis utilizes interaction mechanisms in combination with geologic, structural and mining conditions to delineate areas of potential damage in seams subject to interaction. Using characterizations of conditions peculiar to multi-seam mining, such as innerburden thickness, geology and structures, initial interaction contour maps can be constructed. Geologic hazard analysis is then carried out on roof, pillar and floor conditions in the affected seam using lower seam rock properties and layout geometries in the overlying seam. Either safety factors or damage ratings can then be developed representing degree of hazard in the lower seam. The final hazard map shows both the aerial extent and magnitude of potential damage from interaction.

Jan 1, 1992

By Yongjie Yang

Experimental study on the strength and deformation characteristics of coal under cyclic loading provides guidance for designing coal pillar which is subjected to repeated mining influence during mining. This paper studies the strength, deformation and fatigue process of No.3 coal from Xinhe Colliery under cyclic loading by the MTS815.03 servo-controlled rock mechanics test system. The results indicate that coal is more susceptible to fatigue damage than other hard rocks. Under uniaxial cyclic loading, the fatigue failure ?threshold value? of coal is smaller than 78% of its uniaxial strength, and there is also some fatigue damage when cyclic loading and unloading experiment is carried out below the fatigue failure ?threshold value?. Axial deformation can be divided into three stages: Initial deformation, steady-state deformation and accelerating deformation. Transverse deformation can be divided into two stages: Stable deformation and accelerating deformation. When the transverse deformation begins to increase rapidly and the deformation recovers only slightly upon unloading, it?s a precursor to failure of coal. Under cyclic loading, fatigue failure develops through a process of compaction, strain hardening, and strain softening.

Jan 1, 2006

By E. Yasar

This paper describes the use of the sand and plaster physical modelling technique to investigate the deformation around mine roadways using a variety of support methods in a particular form of stress field. The characteristics and conditions of coal measures rock were considered prior to the preparation of the models and the insertion of support The investigation has clearly shown how floor lift, roof failure and sidewall movements of mine roadways in laminated weak rock conditions gradually develops as a result of the increase in the stress and, also, how it is related to the level of support used.

Jan 1, 1995

Numerical simulation for microseismicity due to mining at one of the collieries in Australia was carried out and the results were compared to the observed microscismicity. 3-D Stress analysis showed stress concentration on the edges of the mined areas and on the coal pillars- The stress concentration ahead of the coal face was biased to the tailgate. This is similar to the observed distribution of microseismic events. Stress reduction above and below the mined area was seen. Tensile stress was observed mainly above the mined area. It was realized that compressive failure due to nearly vertical maximum principal stress and nearly horizontal minimum principal stress in retreat direction (NS) might occur ahead of the face. Tensile failure due to nearly vertical minimum principal stress above the mined area was expected. Compressive failure due to nearly horizontal maximum principal stress in EW direction and nearly vertical minimum principal stress above and below the mined area were also expected.

Jan 1, 2001

By James M. Tennant

In the mid 1970's, American Electric Power significantly expanded its internal coal production. This involved opening several large mines in seams that had never been mined on a large commercial scale before. One of the mines opened was the Martinka No. 1 Mine which is located approximately five miles southeast of Fairmont. The mine was originally designed for room and pillar mining with continuous miners but was later modified to permit the utilization of longwalls and continuous miners for entry development. The coal seam to be mined was the Lower Kittanning which averages fifty-six inches. In order to successfully mine this seam, it required that the mine design recognize the effect caused by the dip of the seam which averages four percent to the Northwest with a maximum of sixteen percent for short distances and a relatively low cover. In fact, seventy-four percent of the reserves have cover of less than four hundred feet. Also, the core hole data indicated that the immediate roof varied from zero to eighteen feet of shale with two to forty-five feet of sandstone overlying the shale.

Jan 1, 1982

By B. Singh

The use of wire ropes has already been tried in a few countries for roof support In mines. But the use of scrap wire ropes, as a method of support on extensive scale has not been reported from anywhere except India. Extensive trials using freshly discarded wire ropes for roof support were undertaken both in the laboratory and field. In laboratory, model beams supported with different reinforcing techniques were tested to evaluate their comparative load bearing capacity vis-a-vis reinforcing factor. Among all the tests, the model supported with wire rope system at 60° angle in combination with two supplementary vertical bolts offered the best result 1.e. about 95% reinforcement. It also accepted post fracture loading of the beam. In statical analysis too, the best result could be obtained with the rope installed across the gallery at 60° angle. All these findings are in conformity with the field observations obtained during roof stitching practices in coal mines. Of late, roof stitching, an effective and economical method of roof support developed at CMRS, India in 1972, found wide application in over 200 coal mines. In this method, a freshly discarded wire rope is grouted across a gallery in two inclined drill holes supplemented with two middle vertical roof bolts at every 1.0 to 1.2 m interval along the gallery. It is tensioned against the roof using wooden sleepers and thus, the whole system functions like a roof truss.

Jan 1, 1990

By Holger Witthaus

The conditions of great mining depth and multi-seam¬longwalling require a special method of rockbolting, It can be described as a control loop which is based on the elements of planning, drivage and measurement. Different planning elements such as the results of the observation of physical models, numerical programmes for calculation of stress-distribution and support success, lab and underground support testing and the knowledge of strata behaviour underground including geotechnical classification are used to get best results. The increasing geotechnical information collected during drivage of roadways are used to estimate and minimise the risks of support-overload. The repeated plan/acutal comparison during the use of roadway and continuously monitoring of support and roadways provided the opportunity of optimising support design during the process of drivage and if necessary in front and after of the longwall face.

Jan 1, 2000

By Kot F. Unrug

Since the introduction of longwall systems, the prediction of roof behavior and support requirements have become the major issue referring directly to safety as well as productivity. In more recent times, and especially in the USA where longwalls are exclusively equipped with costly hydraulic self- propelled support systems, a decision whether a longwall is applicable in a given condition, and when so, what kind of equipment should be selected, is of primary importance. There are numerous examples where lack of experience, and subsequently wrong selection of supports or winning machines, resulted in considerable losses. It should be emphasized that careful studies of prevailing geological and mining conditions, especially in a new area, are required to assure a successful operation. An evaluation of the conditions can be carried out in different ways utilizing such methods as: - Theoretical approach. applicable when geological conditions comply with the theoretical assumptions. - Empirical relationship which gives satisfactory results where the coefficients could be adequately determined. - Structural geological studies which must be undertaken in all cases, since the frequency of fractures and their orientation have a prevailing influence on roof behavior. - Scale model study on equivalent material for determination of failure conditions in the roof and mechanism of movement (flow) of broken rock matter. Coefficients for empirical formulas can be determined by these types of studies, as can boundary limits for mathematical modeling. - Mathematical modeling, e.g., finite element analysis. - Measurements and observations taken underground during the development period end utilization of experience for previous longwall panels in the area. Usually it is the most practical method where mining and geological conditions are comparable. An application of me or more of the above approaches remains to be a matter of judgement for particular conditions, however, the utilization of a practical experience, when available, is usually the safest way for planning a successful longwall operation.

Jan 1, 1982

By Michael Kelly

Longwall mining is the principal method of underground coal mining in Australia, with 70 million tonnes being produced by longwall mines in 1999. However, the mechanisms that control longwall geomechanics are still not clearly understood resulting in stoppages costing several hundred million dollars per annum The CSIRO has been investigating longwall geomechanics with consultant groups and minesites through several ACARP supported projects since 1994. The research has aimed to improve longwall support design and control methods by defining strata failure mechanisms and their occurrence about underground extraction panels, It also aimed to develop better predictive tools bur a broad range of underground environments. As a result of site studies at more than 8 locations some of the aspects that have been touted to control longwall geomechanics, especially from a 3D aspect are highlighted. These aspects include stress controls, geometry, faulting, depth. longwall width and changes in lithology.

Jan 1, 2000

By Daniel W. H. Su

Underground roof geology reconnaissance is being conducted by CONSOL Inc. to enhance mine productivity. Often, roof instability within mine entries is the consequence of highly variable geology that cannot always be detected from information provided by widely-spaced surface core holes. Utilizing a combination of roof drilling data, geophysical logs, and borehole camera and fiberscope observations, roof geology mapping is conducted within longwall development entries in areas of specific concern prior to commencement of longwall mining, Depending on the uniformity of the site-specific roof geology, data point spacing along the longwall development generally ranges from one to five crosscuts. Surface core hole data are correlated with the roof geology mapped underground to validate the underground observations. Detailed roof geology maps can then be produced to enable ground control engineers and mine management to select cost-effective ground control techniques. However, this procedure is not necessary or useful in all situations and it is employed only when there is strong geologic evidence to justify a closer inspection.

Jan 1, 1999

By Zhanjing Yu

Longwall ground behavior in the Illinois Coal Basin was studied in the past (1, 2, 3). Most of the past studies, however, were concentrated on either surface subsidence or underground structural behavior. In this study, both surface and in-mine ground behaviors are considered, which include: 1) stress and deformation characteristics in the chain pillars; 2) roof and floor deformations; 3) surface subsidence characteristics, including time effect; 4) relationship between the surface subsidence and underground strata behavior; and 5) the ultimate hearing capacity underneath the chain pillars. Field measurements and tests were conducted and the data were analyzed using both analytical and statistical methods. The findings in this study can he used by mine operators in the Illinois Coal Basin for longwall ground control planning and design.

Jan 1, 1994

By Xun Luo

In this paper we report on the use of microseismic monitoring techniques to investigate the fracturing process associated with longwall mining. The study was carried out at three mines in Australia and concentrated on some major concerns of the mines, such as failure patterns and growth rate associated with mining progress, their implications to water ingress and gas emissions, caving processes in the goaf and precursors of roof falls. The results have shown that a zone of shear fracturing has occurred further ahead of the longwall face than predicted using conventional longwall geomechanics theory. Failure patterns in the roof and floor vary from mine to mine and this appears to be dominated by geological structures. The reaction of faults, the formation of fractures which allow gas emissions and precursors roof falls have also observed. Results from this work have opened up new understanding of longwall caving processes and proved that microseismic monitoring is a very useful technique for studies of longwall geomechanics and hazard controls.

Jan 1, 1998

By Satyendra K. Singh

If the extraction is beneath massive roof strata, the roof tends to 'bridge' across large spans without failing, the caving behaviour is complex and irregular. Understandably, these give rise to many geotechnical problems with safety implications and involving sometime huge direct and indirect costs. Only two important problems, 'feather-edging' in pillar mining and `severe periodic weighting' in longwalling, are discussed in this paper in the background of geomechanics investigations in Australia at Blue Mountain Colliery and in South Bulga Colliery respectively. `Feather edging'- a major source of injury and fatal accidents in Australia and South Africa in such situations, refers to a phenomenon where the roof, at the goat edge, falls as a thin wafer of rock often suddenly, over-riding conventional timber breakerlines. The results of the first comprehensive, monitored trial in an Australian pillar extraction coal mine, suggests how the roof falls into feather-edging caving (generally) because of multiple shear-planes (interpreted) and how it may be alleviated. Because of complex fracturing behaviour of massive roof strata, periodic weighting at a longwall face is certain to manifest itself as 'yielding supports', heavy conditions', rapid convergence and in the worst case, as a cause of face stoppage. After briefly discussing the programme of instrumentation (in a chain-pillar and over the would-be goal) and results, this paper attempts to improve mechanistic understanding of the related geotechnical issues associated with the weighting behaviour and aims at establishing a 'threshold' value of weighting severity index (suggesting intensity of periodic weighting) with an objective to optimise operational controls before mining through potential (future) weighting areas.

Jan 1, 2000

By Robert M. Cox

The successful application of the science of rock mechanics to the practical problems of coal mine design requires the acquisition and reduction to useable form of large amounts of field and laboratory data, and the construction and analyses of numerous geostructural models containing a wide variety of geometric and physical strength variables. Typically, the rime constraints placed on a new mine design project coupled with the inherent complexities of a good rock mechanics study combine to limit the applications of rock mechanics to the most rudimentary level. To facilitate a better application of rock mechanics to mine design problems, an interactive software package has been developed for use on a graphic mini-computer system that serves to aid the engineer in the timely and efficient analyses of rock mechanics data and geostructural models. The first phase of the rock mechanics package solicits the input of the raw geologic and rock strength data, analyzes and arranges this data in a logical manner, produces complete listings and statisitical summaries of the various rock properties, and stores the data on magnetic tapes and disks to serve as a future Rock Mechanics Data Base. The second phase of the software package provides for the analyses of a variety of geostructural models using a wide variety of geometric variables coupled with the rock strength data stored in the Rock Mechanics Data Base. This software package allows for the evaluation of numerous geostructural mine models for the design and selection of mine roof support and pillar systems (see Table l).

Jan 1, 1982

By Jesse L. Craft

Field investigation of surface subsidence events associated with both active and abandoned underground coal mines in the United States has established criteria that enable the prediction and classification of mine-related subsidence. The key was establishing the role of geology in all types of subsidence and the recognition of beam subsidence in specific geologic settings. The classification is based on overburden thickness, variations in stratigraphy, topography and mine plan. Types of subsidence are Pit, Room, Sag, and Beam. Pit subsidence is associated with three geologic settings: shale overburden, seasonally wet unconsolidated overburden, and perched water table in unconsolidated overburden. Room subsidence occurs where the roof across the width of the room fails to the surface and a shallow depression develops. Sag subsidence develops where the pillars are failing in conjunction with roof failure. Overburden thickness is generally greater than 150 feet. Beam subsidence can occur in all terrains. It is associated with retreat mining and room and pillar mining where pillar crushing or pillar compression has taken place and barriers are left inplace. They occur as beam arch or one-sided beam subsidence. Beam arch failure occurs where mining has taken place on both sides of a coal barrier. One-sided beam failure is associated with retreat mining moving from a barrier towards the entry in hilly topography. This type of subsidence occurs where a hard, thick, rock unit is acting as a bridge supporting the overburden and outcrops on the hill side.

Jan 1, 1992

By Thomas Barczak

Hydraulic cylinders are used in several roof support systems, such as longwall shields and mobile roof supports, to provide critical ground control Many state-of-the-art hydraulic support system operate at pressures as high as 7,000 psi to provide the necessary support capacities Pressure in the upper stages of multi-stage cylinders can be intensified by a factor of 2 or 3 providing pressures of 20,000 psi or greater. A proper understanding of the operation of these high pressure systems is necessary to safely maintain and repair these support systems. Likewise, the performmce characteristics need to be fully understood to evaluate the impact of design parameters on the capability of the support to maintain effective ground control This paper examines the basic operating principles of state-of-the- art hydraulic cylinders and discusses relative issues pertaining to. (1) setting loads, (2) support stiffness, (3) yielding behavior, (4) errors in assessing support loading, and (5) hydraulic failure mechanisms and how to detect them

Jan 1, 1998

By Kevin J. Ma, John Stankus, Dakota Faulkner

"Expandable rock bolts are widely used in hard rock mines as an efficient ground control product. However, capacity and service life can be significantly reduced if the metallic body is subjected to corrosion. In some hard rock mines in the U.S., highly corrosive ground conditions exist, and it has been reported that inflatable rock bolts have corroded within a few months after installation. To provide mining industry a cost-effective inflatable bolt and combat the corrosion issues, Jennmar Corporation, Inc., and its subsidiary Keystone Mining Services, LLC (KMS), analyzed corroded bolt samples, identified root causes, evaluated properties of various coating materials, and developed a new inflatable rock bolt, Python M3TM, that is protected with an innovative PyFlexU2TM coating. The new generation Python M3 features improved steel chemistry for reliable performance, modified profile for better inflation, and surface preparation and coating application. The PyFlexU2 is impervious to liquid and air, durable, and UV resistant. With a flexible adhesive, and highly corrosion-resistant undercoating, and a very hard sacrificial surface coating, the PyFlexU2 coating system provides the Python M3 superior protection against chemical corrosion and physical scratch damage. The under-coating has exceptional flexibility and adhesion to prevent coating micro-cracks or fractures after bolt inflation and possesses excellent corrosion resistance to acids (pH <3), alkalis (pH >11), fuels, and salt solvents. The corrosion and scratch resistant PyFlexU2 coating offers very effective bolt protection for extra longevity in highly corrosive environments. The Python M3 coated with PyFlexU2 has been tested in the most challenging conditions, including laboratory corrosion tests in extreme acidic and basic solvents, rock slurry, and borehole scratch insertion tests. With demonstrated corrosion and scratch resistance, the product has been greatly welcomed by hard rock mines in the West and is currently installed in large scale. This paper identifies the root causes of the bolt corrosion, discusses the analysis process, and details laboratory and underground tests carried out on the Python M3 coated with PyFlexU2. The Python M3TM and PyFlexU2TM are subjects covered by pending U.S. Patent Applications assigned to FCI Holdings Delaware, LLC."

Jan 1, 2017