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By L. Z. Wojno

Tunnels in South African gold mines are developed at depths down to 3 600 m below surface where the virgin rock stress approaches 100 MPa and, on occasions, through rock where the field stresses exceed 150 MPa. In addition, many tunnels are subjected to increases in field stress of over 50 MPa during their useful lifetime. As a consequence, the walls of most tunnels are fractured causing the rock to dilate into the tunnel. Quantitative data on the extent of this fractured zone are presented. The function of support in these circumstances is to reinforce and maintain the integrity of the fractured rock. In 80 per cent of the 800 km of tunnels developed annually, the support methods used are capable of maintaining the stability of the tunnels. However, the remaining 20 per cent of tunnels is subjected to severer conditions where dilation in excess of 250 mm occurs and/or where the probability of strong seismic ground motion is high. In these tunnels, additional support is required. The type and pattern of support for both normal and severe conditions is described. In spite of the improved support used, damage to tunnels may occur. To minimize this damage, the concept of a rock reinforcing tendon that will yield and do work, when subjected to quasi-static or rapid deformation, has been put forward. Specifications for such a tendon are presented and the development of certain yielding tendons are described.

Jan 1, 1987

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 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 G. Herget

To improve quality control of roof conditions, a program was carried out at Elliot Lake uranium nines in Ontario, Canada to test a portable TV camera system. This camera has a 29 mm diameter probe and is capable of identifying crock locations, crack width and approximate dip direction. The extent of fractures to the roof was checked with the aid of a vacuum system by placing packers in various boreholes and checking for communication. The removal of some rock bolts indicated an increase in fracture extent.

Jan 1, 1982

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 D. Bigby

The current British Standard on rock reinforcement components used in coal mining (BS7861:1996) employs the axial double embedment tensile PET) test to determine the main system load transfer performance criteria, in terms of bond strength and stiffbess. This test involves measurement of load/displacement characteristics of a tendon fully grouted into internally threaded thick-walled steel tubes. Part 1 of the Standard covers rock bolting systems and Part 2 covers birdcaged cable bolting systems. Both parts are highly prescriptive, defining a specific design of bar (AT) and cable bolt (birdcaged). In 2000, it was agreed by the UK Rockbolt Research Liaison Committee that Part 1 of the Standard should be revised to allow further industry innovation of bolts and resins. Amongst other aspects, it was recognised that the double embedment test was limited by its inability to examine load transfer between the bolt and the host rock and the UK Health and Safety Executive commissioned an RMT research project to develop a new test to replace the DET test. The new test, the Laboratory Short Encapsulation Pull (LSEP) test has now been incorporated into a draft revision of BS7861 Part1 which has been issued for industry wide consultation. In 2003, it was agreed to begin revision of Part 2 of the Standard to encompass the many types of long tendon reinforcement which were, by then, being used in the UK coal mining industry. These included flexible bolts, deformed strands and tensionable systems encapsulated with resin, cementitious grout and combinations of the two. A second RMT research project was funded by the HSE to examine adapting the Laboratory Short Encapsulation Pull Test for the revised long tendon Standard. This project is nearing completion and a revised Part 2 of the Standard is being drafted. This paper presents some of the results of the two research projects, describes the laboratory tests and acceptance criteria developed and discusses the implications of the research on improving the understanding of reinforcement performance and its characterisation. In particular it shows how the new test can reveal very different load transfer characteristics to the double embedment test, as it takes the load transfer at the holefrock wall into account.

Jan 1, 2005

By Andrew Lewis, Amarit Nitaramorn, Alexander Garcia, Peter Altounyan

"Indonesia is a major coal producer, the vast majority being sourced from opencast operations. Underground production to date has been constrained by unfavorable ground conditions and limited local experience. However, an underground trial in East Kalimantan has now shown that it is possible to successfully extract coal and mine safely, despite the technical challenges.A trial drift mine has been worked to a current maximum depth of 160 m (525 ft) in a dipping coal seam. The 3 m (10 ft) thick coal seam is overlain by very weak mudstone and sandstones. The mudstone forming the immediate seam roof strata proved a particular challenge given its adverse reaction to water and overall friability.Rock bolts were used as roadway support, the system being carefully selected to optimize effectiveness. An innovative method of partial extraction was trialed, which demonstrated the potential for productive mining despite the ground conditions. Roof falls have occurred within the working area, and roadways demonstrate reasonable stability over time.Throughout the course of the trial, strata behavior monitoring has been an important aspect, and a comprehensive ground control monitoring system is in place. This has enhanced the ability to increase the level of support where required and to react to deteriorating conditions. In addition, monitoring information has allowed a quantitative assessment to be made of the technical feasibility of future full-scale mining.INTRODUCTIONWith current production at over 200 million tons, much of this destined for overseas markets, Indonesia is the second largest exporter of coal (Ritschel and Schiffer, 2007). The majority of the coal is sourced from the Kalimantan coalfields. Mining is predominately opencast, yet in recent years there have been attempts made to introduce underground operations. This move would permit continued access to good quality seams that are beyond the reach of viable opencast mining."

Jan 1, 2010

By Hui Chen

The paper describes the use of a physical model technique to investigate the failure modes of mine tunnels with reference to the in situ stress field. The characteristics of stratification commonly encountered within UK coal mining conditions have been taken into consideration. The investigation has indicated that the weak floor strata in coal mining tunnels, under the action of high rock stresses, will be subject to the floor lift problem in both predominantly horizontal and vertical stress fields. However, the failure mode and mechanism varies between the two stress field patterns. In the predominantly horizontal stress field, the tunnel floor tends to fail by gradual bending of the laminations, whilst in the predominantly vertical stress field, the floor tends to fail by a shearing action with the failed material lifting in the form of a block. The failure mechanism is discussed with reference to mechanics theory.

Jan 1, 1993

By Thierry Lavoie, Mark K. Larson

"Engineers at the National Institute for Occupational Safety and Health (NIOSH) and Itasca Consulting Group, Inc., conducted a study to determine the procedure for calibrating a caving model implemented in FLAC3D. The model is calibrated to measured surface subsidence. This sedimentary caving model was developed by Itasca for NIOSH for the purpose of more closely simulating mechanical processes during caving. When a model is considered calibrated to surface subsidence, the load between the extracted panel's roof and floor is a reasonable estimate of gob loading. Such information can also provide an estimate of the amount of overpanel weight that is transferred to the abutment. This load distribution is a parameter needed, for example, to estimate loading of pillars in longwall gate roads. For the current case, a calibrated model resulted in a calculated abutment angle of 30.6°considerably higher than the assumed value used in empirical methods, but reasonable considering the presence of massive stratigraphic members in the overburden. Because of additional information available after this study that the extracted seam height was actually greater in the first panel, a corrected range of abutment angle was estimated to be from 27.7° to 30.6°, still significantly higher than the empirical assumption. The expectation is that better estimates of abutment loading profiles will result in improved evaluations of mine layout design and, therefore, reduce the number of fatalities and injuries resulting from falls of ground.INTRODUCTIONThe safety of miners in underground coal mines depends on careful consideration of the mechanisms involved in failure and transfer of load. The successful design of pillars and panels in longwall and room-and-pillar coal mining requires thorough attention to any information and measurements available that help understand these mechanisms. Although empirical methods, such as Analysis of Pillar Stability (ALPS) Mark (1987); (Mark, 1992) or Analysis of Retreat Mining Pillar Stability (ARMPS) (Mark and Chase, 1997), can provide a sense of success or failure, complex geology and failure in the roof or floor material or complex material behavior of the coal can make evaluation by an appropriate numerical modeling tool necessary. The numerical tool must simulate important mechanisms and appropriate detail that underlie the mechanism. For example, Starfield and Cundall (1988) advised using ""the simplest model that will allow the important mechanisms to occur, and could serve as a laboratory for the experiments you have in mind."" To that end, data gathering about the geology, geometry, and material behavior is very valuable. Such data facilitate selection of an appropriate model and model inputs. Moreover, this data facilitate improved assessment of risk to miners and how to minimize that risk."

Jan 1, 2016

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 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 P. Oliver

Rockburst conditions were first encountered at Inco's Sudbury area operations in the mid 1930's when mining advanced to below the 610 m (2000 ft.) horizon. Now, mining is being carried out at depths down to 2130 m (7000 ft.) at Inco'a Creighton Mine. While rockburst potentials have increased with depth. improvements in mine planning, methods, support systems and developments in destressing have alleviated the condition to where it is less of a problem now than it was in the early years. Hang of the improvements were developed, of necessity. through observation and reasoning because these were the only tools available at the time. Now, we are making use of stress - measurements, numerical modelling and microseismic source location technology to - help further reduce rockburat hazards.

Jan 1, 1987

By L. A. Sandbak, Yan Xing

"The purpose of this study is to evaluate the rock mass stability around the tunnels by three-dimensional (3-D) numerical modeling. Complex geological and tunnel features have been included in the model using the software package 3DEC™. Based on the available information on stratigraphy, geological structures, and mechanical properties of rock masses and discontinuities from the underground mine, 3-D stress analyses were performed on various cases with different K0 (lateral stress ratio) values, constitutive models, as well as support systems. Relations between these factors and the distributions of stress, displacement, and failure zones around the tunnels were obtained and discussed. Finally, comparisons between the field deformation measurements and numerical simulations are used as validations for the simulations, helping to determine the appropriate K0 value and material constitutive models for the underground mine.INTRODUCTIONThe case study site is an underground mine in the USA with a monthly ore production between 20,000 and 30,000 tons. The regional geology of the mine is structurally and stratigraphically complex. Rocks in the area include carbonaceous mudstone and limestone, tuffaceous mudstone and limestone, polylithic megaclastic debris flows, fine-grained debris flows and basalts, all part of the Cambrian-Ordovician Comus formation and related dikes. Both the mineralization and faults have general orientations striking N-S to NW-SE with high dip angles. The ore body in the mine has an incline of about 25---45 degrees located in relatively low quality rock; hence, cut-and-fill mining has been used.This underground mine has two vertical shafts used for ventilation, ore hoisting, and for personnel and material transportation. Development drifts were designed and driven to extract and transport the ore, as shown in Figure I. Bolts, mesh, and shotcrete have been applied in this mine for ground control. In order to reach various locations, the tunnel system is designed with different heading directions and inclines, which will be described in detail later. The area selected for modeling is a cubic region including all the tunnels inside the black square (Figure 1)."

Jan 1, 2016

By Chris Adams

The application of pre-tensioned tendon roof support assisted Harworth Colliery in their revival, from impending closure to lowest cost deep-mined producer in the UK, over just six months last year. The paper describes this success story, how it came about, the reasons "why", and the method "how"

Jan 1, 2003

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 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 Graham Daws

The introduction of UK rock bolting technology has been applied in many large underground coal mines around the world. This paper explains the methods that have been applied for its successful introduction and discusses several case studies. It does not give fine detail but is intended to allow decision makers to gain a basic understanding of the method used to introduce rock bolting to a mine.

Jan 1, 2011

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 Francis S. Kendorski

Rock reinforcement has been in widespread use and generally has been accepted in underground mining and tunneling since the 1950s. The first rock reinforcement technologies employed were mechanical anchors such as split wedges or expansion shells with 5/8-inch-diameter steel bolts. Failures occurred in weak strata which provided poor anchorage or in ground with corrosive waters. Friction rock fixtures consisting of relatively thin-walled tubes have been in use for about 15 to 20 years. While generally performing adequately, longevity problems have developed from corrosion in water bearing ground Longevity of rock reinforcement is much enhanced with grouted bolts. Portland-cement-grouted rock reinforcement has been in use since the mid-1950s, primarily in funneling and other civil engineering underground construction. Tests of decades-old installations have revealed few problems except in ground with aggressive waters. Polyester-resin-grouted rock reinforcement was introduced in the United states to musing in the late 1960s and to tunneling in the early 1970s. Experience from 30 years of resin-grouted bolt installations and field tests have identified longevity problems associated with degradation of steel reinforcing bars, but in generally unusual situations. Improvements continue in resin chemistries, packaging, corrosion protection, grout quantities, and mixing and distribution in the drilled hole to achieve long-term performance. Specific case histories cited with resin- or Portland-cement-grouted rock reinforcement longevity or performance problems, upon close examination, reveal that the causes of the problems were qualify control procedures being inadequate and not in accordance with good practice. Manufacturers' recommendations and engineering specifications, if followed in the field, and with competent inspection and supervision, would have prevented must, if not all, reported longevity or performance difficulties.

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