The current British Standard on rock reinforcement components used in coal mining (BS7861:1996) employs the axial double embedment tensile (DET) test to determine the main system load transfer performance criteria, in terms of bond strength and stiffness. 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 hole/rock wall into account.
A reliable, non-intrusive means of determining the extent of failure in the near roof would be of great benefit to mine safety worldwide. RMT's Acoustic Energy Meter can detect broken and loose immediate roof but is limited by the need for physical contact with the strata. A non contacting device would allow survey of unsupported ground from a place of greater safety. Applications include during pre-shift inspections and dynamic monitoring of roof condition during extended cut-out operations.
This paper describes recent research by a team from RMT and Camborne School of Mines which investigated physical principles which might be applied to provide such a tool and tested those with the most promise. A practical instrument would have to be capable of machine mounting or remote operation. The principles investigated were electromagnetic emissions, thermal response, ultrasonic/acoustic emissions and induced vibration.
It was concluded that the principles which warranted further development were thermal response and laser vibration measurement. In the latter case there is a need to develop an efficient means of imparting energy to the strata to induce sufficient vibration. A new, SIMRAC funded Project will progress work in the field of thermal response.
Modikwa Platinum Mine, in the Eastern Bushveld, is a new operation employing a hybrid mining method where mechanised methods are used for the main developments and the narrow, 60cm reef is conventionally stoped using a down-dip conventional mining method. Throughout the mine, the hanging wall immediately above the reef comprises 3 jointed and laminated layers of varying thickness, commonly known as the triplets, below a non-cohesive leuconorite parting plane.
At North shaft, using pre-stressed standing support and mechanically anchored rockbolts, it was found difficult and hazardous to hold all three triplets in the hanging wall, however bringing them down caused a significant increase in dilution and therefore cost of mining.
A novel solution was conceived whereby two of the triplets are kept up using short, stiff resin anchored rockbolts installed systematically within 0.5 m of the face. This resulted in both safety and productivity benefits. The rockbolts create a stable and safe hanging wall and allow the mine to extract UG2 reef more economically. New drilling equipment, developed locally from available drilling technology, can be operated from a safe position, and in situ tests and measurements are made to confirm effective support.
The technology and underlying concepts were successfully tested in a trial stope prior to rolling out the new support system across all the stopes at North Shaft. This has involved an extensive training programme to ensure that the rockbolt system is installed correctly and that the conditions are appropriate for this technology.
The paper describes the motivation behind the project, the underlying technology and its application to Modikwa, and the benefits now being realised.
Numerical modelling techniques have been applied to roof bolted roadways in the UK for several years. It is now possible to model such roadways in considerable detail; measurement techniques to obtain data on rock properties, in situ stress and roadway performance are available to improve model reliability and increase confidence in the results.
Interaction between workings in different seams is common in the coal mines of Europe and can have a major influence on the conditions encountered in bolted roadways. Methods are available to investigate the influence of interaction due to both over-working, in which case the effects are mainly areas of increased or decreased stress, and due to under-working, where potential damage to the fabric of the strata need to be taken into account.
The application of these methods has
They are now applied frequently in UK Coal mines with the results used to assist in planning mine layouts so as to minimise adverse interaction effects and to select appropriate roadway geometries and support systems to cater for the expected conditions.
Goedehoop Colliery in the Witbank Coalfield of South Africa produces some 8 million tonnes of coal from fully mechanised room and pillar mining methods, using resin anchored bolts as roof support. Investigation of the quality of the installed bolts resulted in the finding that it was possible to pull out fully encapsulated roofbolts. This led to a drive to improve the quality of installation by making it easier to install a roofbolt correctly and more difficult to install incorrectly, whilst at the same time achieving high bond strength. The method of installation which was investigated, developed and implemented is known as 'spin to stall.' The roofbolt is pushed through the resin capsule to the top of the hole and then spun to mix the resin, until the increasing resistance to rotation exceeds the breakout limit on the bolt torque nut. The nut then runs up the bolt thread and is tightened to the roof as the drill stalls. This procedure is simple and avoids human factors such as non adherence to 'spin' and 'wait' times which contribute to poor installation with conventional methods. However spin to stall installation has been avoided in the past because of concerns over damage to the resin bond. This paper reports on the efforts made to investigate and understand the factors involved, and the progress made in overcoming the problem of resin damage so that the advantages of this installation method can be realised.
“"Tensile Roof Failure Arising From Horizontal Compressive Stress And Geological Slips", Alan Bugden, John Cassie. (22nd International Conference on Ground Control in Mining, Morgantown USA, 2003).
Goedehoop colliery in South Africa has experienced a number of substantial roof falls in roadways and intersections. Many of these have been associated with geological slips and/or horizontal compressive stresses. This paper describes a numerical modelling study conducted after one such fall with the objective of clarifying the roof failure mechanism associated with these falls.
The fall was located at an intersection and occurred as the last cut was being made to complete the intersection. One bounding surface of the fall was formed by the geological slip; the others were formed by fresh failure surfaces. The fall took place despite the slip being identified as a hazard and additional bolts being placed by the workforce.
In conjunction with the modelling work tests were conducted on samples from the roof in which the fall occurred, these gave a U.C.S. of 80 - 100MPa. Several in-situ stress tests had already been conducted at the mine; a further test was conducted close to the fall site. The tests show that the largest stresses are horizontal. However, the maximum stresses of 6-12MPa are small compared to the compressive strength of the roof.
Computer modelling highlighted a mechanism arising from the conjunction of a geological slip and horizontal compressive stresses that could result in tensile failure of the roof leading to the fall. Sensitivity studies were conducted to identify risk factors associated with this mechanism and the modelling results used to assist in formulating a support strategy to combat falls of this nature.
Rock Mechanics Technology has developed an intrinsically safe remote reading telltale system which allows up to 4 sets of 100 dual height, water diverting telltales in a mine roadway to be remotely monitored from a surface PC. The system has US MSHA approval and European ATEX approval for use in gassy mines.
A successful full-scale application of the system employing 67 telltales was completed in a 1300m long maingate roadway during retreat of 183.0 longwall panel at West Mine in Germany between June 2001 and October 2002. During panel retreat a number of "teething" problems associated with the system were identified and resolved, the chief problem being associated with cable connection corrosion due to the unforeseen use of sprayed calcium chloride in the roadway.
This paper describes the experience of employing the system at the colliery from the perspective of both the user and the manufacturer, and the improvements and developments which have resulted. The paper also presents examples of roof movement data recorded by the system, including data from behind the retreating longwall. The high resolution and data recording rate possible has resulted in particularly interesting information on roof behaviour within the longwall front abutment area and behind the face.
The paper also describes a series of new transducers and extensometers for the mining industry which employ the same physical principles and similar electronics to the remote reading telltale system. These have been applied in a variety of mining and tunnelling environments worldwide over the last two years.
Comparison of in-situ stress overcore measurement results with those determined from laboratory evaluation of Acoustic Emission (AE) testing on oriented sub-core samples has been carried out for a variety of mining environments. These include UK and German Coal mines at moderate depth, shallow-level South African Coal mines and deep-level South African gold mines. Results show that the ‘Kaiser Effect’ or ‘Stress Memory-Damage’ is readily determinable throughout the brittle deformation process, although particular care is required in interpretation of results during the ‘bedding-in’ phase of testing and the region associated with unstable crack growth prior to failure. The results suggest that AE provides a useful complimentary method for comparison with existing methods of stress determination. The success of the method can, however, be dependent on both rock type and the likely magnitude of in-situ stress in comparison to the strength of the rock.
This paper describes some of the major achievements of ECSC funded Strata Control research over the last 15 years. Whilst it inevitably has a British perspective, it concentrates on the work undertaken under Collaborative Targeted RTD Projects since 1994 with German, French, Spanish and British partners. 4 such Projects have been completed at a total cost of 13.5m€, 3 were due for completion in 2002, 3 are ongoing and 2 more were due to commence in July 2002.
The paper focuses on the major achievement of introducing rock bolting as primary support in rectangular roadways in European mines at depth. This is now the standard support system for longwall gate roadways in the UK and has been used extensively at the Gardanne mine in France. Rockbolting of rectangular roadways is also increasingly being applied in Germany with considerable success.
The paper describes the important and, in some cases, revolutionary tools that have been developed under the research programme which have made the safe and cost effective introduction of rockbolting possible and which have also played an important part in reducing Geotechnical Risk in the European coal industry.
Thoresby Colliery was in the process of recovering an old roadway that was flooded following the closure of an adjacent mine. The aim was to re-use the roadway as a gate road for a longwall retreat panel and it was 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.
Ultrasonics proved successful in obtaining bolt end reflections from 40 of the 50 rockbolts tested confirming their intact state. However, interpretation of the ultrasonic echo trace was not always straightforward and a number of techniques were used to improve the analysis. A classification system was devised for categorising the tested rockbolts in terms of the level of confidence attached to each test result. Where confidence in the result is low further testing is recommended. Fracture defects were not detected in any of the rockbolts tested.
A new NDT method under development by RMT, the RF system, was also applied at Thoresby. The system proved effective in the outbye section of the roadway when testing datum bolts and in-situ bolts in the vicinity of the datum bolts. However, the system failed to detect the resonant frequency of other in-situ bolts further inbye. The reasons for this are not yet clear and further work is needed to develop this system.
In general the condition of the rockbolts recovered by overcoring confirmed the findings of the ultrasonic testing with no broken or fractured bolts found to date.
Goedehoop Colliery produces 8 million tonnes of coal a year, principally from room and pillar mining, and is situated in the Witbank Coalfield in the Republic of South Africa. The mine has a long and successful record of producing export quality coal from the 2, 4 and 5 seams at depths ranging from 20 to 100 metres using modern mining equipment.
Following a number of fatalities caused by large and unexplained roof falls in roadways and intersections of supported ground, a review of the roof control system at Goedehoop was carried out. This led in turn to a programme of underground measurements and surveys. This included detailed investigation of the roof falls, horizontal stress mapping, stress measurement and short encapsulation pull testing on roofbolts.
These investigations led to a number of important conclusions in relation to the state of stress in the ground, the roof failure mechanisms, standard of roofbolt installation and performance of the roofbolt system.
As a result of the work Goedehoop Colliery has made a number of substantive changes to the roof control management system at the mine. These include:
1. A recognition of the importance of horizontal stress as the main factor responsible for roof falls
2. The forward planning of roof support based on known features which cause the stresses to be elevated
3. The introduction of a more effective roofbolt system, which can be more easily installed to a high standard
4. A response system based on risk assessment and monitoring roof movement on a routine basis
The paper describes the knowledge acquired, the conclusions drawn, the changes made and progress to date.
A wide range of different cablebolt systems are available and in use in hard rock and coal mines around the world. The birdcaged cablebolt was initially used in UK coal mines in conjunction with rockbolts in the late 1980’s. More recently other cablebolt configurations have been used. Innovations in cablebolt design in the UK have been driven by a demand to find a system as effective as the birdcaged cable, but which can be installed more rapidly and offer immediate support.
A laboratory short encapsulation pull test (LSEPT) has been developed for both rockbolt and tendon systems which measures bond strength and stiffness in rock and assesses the full range of potential failure modes. This test is now used along with conventional ‘gun barrel’ and shear tests to provide full assessment of bolt and tendon performance.
This test may be incorporated into a new performance based British Standard for rockbolts and long tendon systems. This paper describes the performance of some of these systems as being applied to UK and South African coal mines.
Rapid entry drivage systems are now being applied in European mining conditions with major advantages, not only in terms of drivage rates, costs and longwall productivity, but also with improved safety.
This is being achieved through the introduction of bolter miner systems in which the early installation of high strength rockbolts is fully integrated with the drivage system and all ancillary operations. These new systems are fully described with examples of applications in European conditions and procedures for design of rockbolt patterns.
Finally a new concept, fully integrated development system, the IMM, shortly to be installed in mines in the USA and RSA, is described in which cycle times are reduced by combining operations and running systems simultaneously. The IMM could form the basis of future rapid entry development systems for European mining conditions.
A remote reading dual height telltale system has been developed to provide mine management with early warning of impending roof failure. This system is a logical development from the visual dual height and triple height telltales which are increasingly being used worldwide for routine monitoring of mine roof. The paper describes the principles and history of telltales from their origins in France, their refinement and routine use in the UK and their subsequent spread around the world. They are now being used in various forms in USA, Canada, Germany, Spitzbergen (Norway), Poland, Ukraine, Russia, Australia, South Africa, Mexico, India, Egypt and China.
The remote reading telltale system has coal mine intrinsic safety approval in Europe and the United States (MSHA) and the first commercial system was installed in a German colliery at the beginning of the year. Up to 100 dual height telltales can be connected, in a simple “daisy chain” configuration, to each of 4 underground interrogation units. These are connected through a twisted pair link to a surface computer which displays the readings, updates the database, generates warnings and provides the user interface. The paper describes the features and principles of operation of the system which differs considerably from conventional mine-wide monitoring systems. A European Community funded research programme is currently underway with the aim of developing improved alarm generating algorithms including evaluation of interfacing a neural network to the system.
The paper concludes by exploring other potential applications of the system for ground control monitoring currently in development, including extensometry, stope closure monitoring and support load measurement.
In recent years the major proportion of UK coal mining rockbolting ground control research and development has been undertaken by Rock Mechanics Technology Ltd (RMT) with other work undertaken by the Health and Safety Laboratories (HSL) and mining universities. The research programme has led to development of important new procedures, instrumentation and testing technologies which are playing a significant role in reducing the risks of falls of ground accidents in rockbolted roadways worldwide. The RTD (Research and Technological Development) programme has been funded by the European Coal and Steel Community (ECSC), the British Health and Safety Executive (HSE), the UK coal mine operators and the South African Safety in Mines Research Advisory Committee (SIMRAC).
The Acoustic Energy Meter (AEM) of type RDL 3 is a hand held, non-destructive field testing instrument developed by Rock Mechanics Technology Ltd which measures the rate of decay of surface reverberation following an impact.
The AEM was tested on a range of tunnel lining types and was found to be effective for detecting the lateral extent of voids behind tunnel linings for steel, brick and concrete segmental linings. It also detects local lack of cohesion of shotcrete linings, and surface fractures/looseness in unlined, brick, concrete and shotcrete lined tunnels.
The AEM appears to have great potential as a simple tunnel lining testing device. It is recommended that further experience should be gained, using the device on a trial basis, in addition to conventional methods of inspection and assessment.
The rate of decay of the energy from a hammer blow to a coal mine roof is related to the size and nature of the contacts between the rock being struck and its surroundings. The use of this principle as a basis for testing roof condition in South African coal mines has been investigated under a SIMRAC funded project, with the objective of reducing the number of roof falls.
A prototype instrument having a simple numeric display related to the energy decay rate (the Acoustic Energy Meter) was constructed and used to carry out surveys at mines covering the range of coal mine roof geology and condition. The equipment was simple to use and provided excellent discrimination between visually intact roof and less stable conditions, once the normal/abnormal threshold had been established for a given site. Areas of potential roof failure were identified close to roof slips and igneous dykes, and confirmed in areas of visible deterioration. Directional stress effects on roof condition were also identified at some sites, with consistent differences in readings obtained depending on the direction of drivage. Repeat surveys also indicated areas where roof conditions were deteriorating with time.
Based on the success of the prototype, a hand held version of the instrument, has been developed to a specification from Anglo Coal. This incorporates a simple ‘traffic light’ green / amber/ red display, making the device suitable for routine use by mine operatives. This version is now undergoing trials at an Anglo Coal mine.
It is concluded that the Acoustic Energy Meter shows great potential as a test instrument providing output which relates to the stability of the immediate roof.
“Breaking New Ground”, Peter Altounyan, Alan Bloor, Dave Bigby, (World Coal, 2000).
Instrumentation technology is applied routinely for safety monitoring throughout the world, in such contexts as transport systems and industrial processes. This practice is increasing, because of the greater emphasis on system safety by the modern world, and the subsequent development of new instrumentation technology.
The use of instrumentation in mines to monitor the mine atmosphere and other parameters such as temperature and the functioning of safety critical equipment is now well established. However the mining industry could be accused of being slow to adopt instrumentation to assure ground control safety during the actual mining process.
To be of use in this application, instruments need to be low cost, easy to install and operate, and able to give clear warning of unsafe conditions in good time. A lack of suitable instrumentation systems meeting these criteria has been the main reason for slow progress It is only recently that the routine monitoring of mine opening stability has begun, with a new roof movement monitoring instrument, the dual height telltale. This was developed and applied firstly in UK coalmines and subsequently in coalmines in at least ten countries worldwide. Trials of the technology are now taking place in a range of other mines, including gold, lead/zinc, phosphate, potash and limestone mines.
This first generation of instruments relies on manual reading by mine personnel. However, a second generation of instruments is now available which takes advantage of microprocessor technology to allow remote surface interrogation of up to 400 inexpensive roof monitoring instruments, with signal transmission along a normal telephone line.
As excavation proceeds, roof movement monitoring systems of this type provide data on the deformation of mine openings in which they are installed. Initially at least, these systems are likely to be targeted at locations which are perceived as most critical for safety or production reasons.
There is also a safety role for testing instruments able to provide immediate and local information on mine opening condition and stability, analogous to non-destructive testing instrumentation used in the inspection of structures. In response to demand from the mining industry for a simple inspection instrument of this type, RMT has developed the acoustic energy meter. Initial trials in the UK and South Africa into the application of this easy-to-use instrument to detect areas of loose or fractured rock have been very successful. Several versions have now been produced to individual mining company specifications.
Techniques used to safely control the roof in UK coal mines for some 10 years have been applied to a South African coal mine. This new approach known in the UK as advanced rockbolting technology, is based on applying four fundamental principles:
1: Understanding the roof failure mechanisms
2: Using an effective roof support system
3: Designing this support using measurement
4: Monitoring the performance of the system
The paper summarises the approach in detail and describes how it is being applied by Anglo Coal, giving results obtained to date.
Investigations at a number of South African coal mines, including stress measurements at two Anglo Coal mines, have confirmed that the mode of roof failure (lateral shearing due to horizontal stress) is the same as in other coalfields worldwide, and aspects of current world best practice, including the use of advanced technology rockbolting, are therefore relevant in South Africa.
The stress measurements indicated a high level of stress field anisotropy and further investigation of stress conditions in South African coal mines is recommended.
The most effective bolting system to resist shear failure is one with high bond strength and stiffness. Short encapsulation pull testing of existing South African systems confirmed that they have low bond strength and stiffness. An improved rockbolt system with the required performance, and features allowing rapid installation and installation quality and performance audit, has been developed and is currently under full scale trial.
Design by measurement and routine monitoring procedures including the use of a rotary telltale device are also under trial.
It is anticipated that South African coal mines will be able to obtain significant safety and productivity benefits from the application of this technology.
Design of support pillars in coal mines has traditionally involved empirical techniques based on previous experience, perhaps combined in more recent years with consideration of the theoretical stress distribution across the pillars. Measurement of stresses by overcoring provides another tool for the study of pillar behaviour. Measurements adjacent to coal mine roadways are compared and their implications for pillar behaviour and strength considered. Reduced stresses consistent with yielding of the strata adjacent to the roadways are evident. Two groups of results are identified with significantly different types of behaviour corresponding to different effective pillar strengths. Estimates of strength for these two groups are compared with established pillar design equations. The differing behaviour and strengths are attributed to variations in the roof and floor strata and in the degree of confinement they provide to the coal seam. Numerical modelling results are used to demonstrate this and for comparison.
“Designing for Success”, Peter Altounyan, Ken Hurt, Dave Bigby, (World Coal, July 1999, pp47-52).
The design of underground coal mines has to fulfil many often conflicting criteria. This article concentrates on issues related to the rock mechanics and describes how this impacts on the design with examples of options, opportunities and limitations that rock mechanics place on economic design.
There are many equations or methods available for the design of coal pillars. The approaches adopted include back-analyses of failed and successful case histories, extrapolation from strength tests on small scale coal samples to full size pillar and analytical consideration of the limiting stress distribution across the pillar. The latter approach would nowadays normally involve the use of numerical modelling. In many instances a combination of these approaches is adopted.
The range of methods developed can be accounted for by the wide range of geological conditions encountered underground and the different functions coal pillars have to fulfil in different mining methods. It would be remarkable if a single design equation were to be applicable to the entire range of coal pillar types and conditions encountered. The design approach employed adopted should be relevant to both geological conditions at the site and the function of the coal pillar being considered.
Stress measurements provide a tool which can assist in the study of pillars. Comparison of the results from different sites shows a wide range of potential strata conditions and resulting pillar characteristics. For pillars of moderate widths sufficient to allow the development of confinement within the coal, the stress measurements can be used to obtain estimates of the available pillar strengths or load bearing capacities.
For wider pillars employed in deeper mines and with longwall layouts, characterising pillars simply by their strength is less applicable. Such pillars are unlikely to fail in the sense of collapsing. However, the size of pillar employed can have a major influence on conditions in the surrounding entries. In this case the distribution of stress within the pillars becomes more relevant and the performance of pillars can be assessed by its impact on deformations and support requirements in the surrounding entries.
Mine tunnel support using rockbolts was introduced in UK coal mines over the period 1987-1992 and is now used in over 70% of all mine tunnels. In every case the reinforcement design has been based on the observational approach. After the initial reinforcement design has been established, appropriate action levels are defined for predetermined amounts of movement indicating the onset of instability. If the action levels are exceeded appropriate remedial action is taken in the form of additional support or reinforcement to maintain stability.
This paper outlines the observational design method as used in UK coal mine tunnels. It discusses the low cost instrumentation which has been developed to monitor roof deformation, dual height telltales, how action levels are defined based on the geotechnical environment and the types of remedial measures typically taken.
An alternative approach for dealing with areas of increased deformation involves reacting to early roof movement trends and modifying the support installed at the face of the development accordingly. Installation of additional support at the face of the heading improves ground control in poor geotechnical conditions and has the advantage of potentially reducing later additional support costs. A case study is described to illustrate how ‘early’ action levels can be incorporated into a roof support plan enabling a flexible roof support strategy. Such a system will be site specific and continued analysis of monitored information will be required to confirm the action levels or adjust them according to any change in geotechnical environment. More accurate roof deformation monitoring systems, such as the Remote Reading Telltale system recently developed by Rock Mechanics Technology (RMT) and currently undergoing Mines Safety and Health Administration (MSHA) approval are ideally suited to this application. The Remote Reading Telltales system provides accurate mine wide monitoring of roof condition on a surface PC.
An assortment of bolts and resin cartridges from manufacturer of the partner countries (UK, Germany, France and Spain) was comparatively tested according to the British standard BS7861 and the German DIN 21251. The testing methods developed on the basis of different stress configurations are described. Substantial differences were found with respect to load bearing and deformation behaviour of the individual products.
The rockbolt support system which has been successful down to a depth of 650m is not expected to be sufficient for 800m depth. In order to specify a starting pattern for bolt support at 800m depth in the Thick Coal seam, comprehensive analyses were undertaken by British, French and German engineers, and discussed. Although different methods were applied, relatively similar solutions were derived, which are still to be tested underground.
“Ground Control Research Towards the Millennium”, Dave Bigby, Jim Arthur. (Paper presented to the Yorkshire Branch of the Institution of Mining Engineers, March 1998).
The support of underground roadways is a fundamental a fundamental requisite of deep coal mining and without knowledge of the mechanics of ground control a mine is unlikely to be operating safely and efficiently. The cross-section of a roadway determines the ventilation, size of equipment that can be used, the possible methods of working and the amount of unwanted material mined. It is important, therefore, for a support system to be designed satisfactorily for the size of excavation required and to make this possible an understanding of the behaviour of the surrounding strata is required.
Mining has been carried out for many centuries in the UK. Historically explosions have often been seen as the major contributor to fatal accidents in coal mines but, as pointed out in an paper by Langdon and Harris, failures of support systems were by far the most common cause of fatalities. In the early part of the nineteenth century, fatalities had reached such an extent that Royal Commissions were established to enquire into conditions in coal mines.
After nationalisation, the industry experienced great technological change with hydraulic support systems replacing the use of wooden props. Longwall advance faces were the most widely used system for mining coal in the UK. The roadways were still being driven and supported conventionally. In the 1980’s, however, retreating faces became widespread. To develop and operate these faces necessitated much more rapid roadway development and, as a result, rock bolting was introduced.
This led to a renewed requirement to understand the behaviour of rock surrounding mine openings. The technique was introduced in stages with close co-operation from Her Majesty’s Inspectorate. A guidance document was introduced and today this forms the basis for using strata reinforcing techniques under new regulations which require geotechnical appraisals to be carried out . Such appraisals clearly imply a high level of understanding of the behaviour of the strata around the excavations.
Recent advances in the science and practice of ground control and strata reinforcement are reviewed here and the authors highlight areas where further research is required to enable the industry to progress into the millennium and mine coal safely and efficiently through improved ground control.
A Code of Practice for use of rockbolts as roadway support was introduced into UK coal mines in the early 1990’s making use of the best technology available from around the world The code includes geotechnical assessment, initial design, design verification and routine monitoring. Roof movement and bolt load monitoring is used as the principal tool for modifying the design to suit changing conditions and to signal the need for remedial action and the action to be taken. The Code of Practice (now Industry Guidance) has allowed rockbolting to be introduced safely into a wide range of mining conditions. Based on the success of this, similar guidance is being developed for other European and former Soviet countries. The paper outlines the origin and the development of the code, the role of the UK Mines Inspectorate, the principles behind the code and provides a case history of how this code is applied in a typical coal mine in the UK.
“Advanced Technology Rockbolting”, Peter Altounyan, Ken Hurt, (World Coal, May 1998).
European coal mine longwall productivity levels generally lag well behind those of the US and Australia. A key reason for this is the use of continuous miners to drive rectangular section rockbolted gate roadways in these countries. This is the most effective method of achieving rapid development, which is needed to achieve high productivity longwall retreat mining.
UK engineers have successfully developed a rockbolting system capable of safely supporting mine rectangular section roadways, even in deep mines in weak rock. This system is now used to support roadways in UK coal mines in a technology change which has greatly increased both mining productivity and safety.
RMT investigations into mining conditions in Germany, Kazahkstan, Poland, the Ukraine and Russia indicate that mining conditions are similar to those found in the UK. It is concluded that high productivity retreat longwall mining using rectangular rockbolted roadways, as now practised in the UK, is highly applicable to these countries, providing the possibility to mine coal more economically and safely.
This article will describe the approach used in the introduction of this technology, its impact on mining operations and the potential for application to the traditional coal mining industries of Europe and Russia.
Retreat longwall mining
is recognised as the most efficient method of underground deep coal mining
worldwide. Experience from the USA and Australia where productivity is
at high levels, indicates that key technical requirements for success
are longwall gateroad designs that:
* Can be driven rapidly;
* Maintain profile and perform reliably throughout their life and
* Minimise gate end delays on retreat.
This is being successfully achieved utilising rockbolting technology for support in rectangular profile roadways with roadway drivage utilising continuous miner-type machines and mine layouts leaving stable pillars between panels.
The successful exploitation of rockbolting in United Kingdom coal mines was facilitated by the use of an observational design method that involved detailed monitoring of rock strata movements and rockbolt loads. Rockbolting as a method of tunnel support was introduced into United Kingdom coal mines in the late 1980's. The technology built on Australian systems and practice, with the addition of systematic measurements of roadway deformation and bolt load for design and safety purposes.
In circumstances where monitoring indicates that rockbolts alone are insufficient to maintain tunnel stability the Australian practice of installing double birdcaged cable bolts has also been widely adopted. The design of cable-bolt reinforcement systems is also based on measurements of the deformation behaviour of the roadway roof and is therefore an example of design by a measurement or observational approach. Local modifications can be made to reinforcement pattern on the basis of these measurements, allowing an optimum level of reinforcement to be installed.
The in-situ stress regime is recognised by United Kingdom coal mine operators as a significant design parameter related to efficient mine design. The results obtained from recent overcore stress measurements undertaken in Coal Measures strata are analysed and presented. A relationship has been deducted which relates to maximum horizontal stress to the depth and to the elastic properties of the rock. This relationship is considered more suitable for estimating the maximum horizontal stress magnitude in Coal Measures strata than existing methods based solely on the depth of cover. This study also indicates that all in-situ stress determinations in sedimentary strata should be quoted with the elastic properties of the test horizons.
“Developments in British Rockbolting Technology” D.N. Bigby, (Coal International May 1997).
Rockbolting is now firmly established in British coal mines for support of retreat longwall gateroads. The author describes British rockbolting technology and continuing research programmes aimed at design optimisation and improved consumables.
The main technical factors, collectively termed Advanced Technology (AT) rockbolting, were:
* A reliable design
methodology (design by measurement);
* A routine monitoring system to ensure timely installation of remedial
support wherever required;
* An understanding of rock stress and a means to measure it in-situ;
* High strength and stiffness consumables, and
* A unified approach enshrined in a Code of Practice.
The developments have been in improved instrumentation and software for design and routine monitoring; methods for laboratory evaluation of reinforcement performance, and computer modelling for support design optimisation.
Over the last seven years rock bolting has been safely introduced into British coal mines as the main system of support for longwall gate roadways. Prior to 1991 roadways were generally supported by steel arches. The major change in support systems has been accomplished safety due to the development and implementation of low cost, easily installed routine monitoring instrumentation which is systematically installed in all rockbolt supported roadways. The instruments provide an immediate visual indication of stability problems to workmen and can also be read accurately by colliery officials and engineers to provide mine-wide records of the history and stability condition of all roadways. A set of procedures has been developed covering the installation, reading, recording and reporting of the instrument data. The paper details these systems and explains the methodology employed to determine appropriate site specific action levels and appropriate remedial support action. The authors were responsible for development of the instrumentation and procedures and believe that the system has the potential for reducing accidents due to falls of ground in coal mines throughout the world.
Mine Tunnel Reinforcement Design using the Observational Method.
The observational approach to design (ICE 1996) makes use of feedback from monitored data to vary design during construction in a systematic and preplanned way. The New Austrian Tunnelling Method is essentially an example of the observational approach applied to tunnelling in which a sprayed concrete lining forms at least part of the support system. In coal mine tunnels, advance rate and cost considerations, together with high stress field conditions means that a shotcrete lining is not generally appropriate as support. However a very similar approach to NATM for coal mine roadways, developed and applied in the UK from 1990 enables rockbolting to be safely and rapidly introduced as sole support for over 70% of all current UK coal mine tunnels. The authors were responsible for all technical aspects of this change. The same procedures are now being used by RMT Ltd to introduce rockbolting in Germany, Russia and other European and Asian countries and the instrumentation used is being applied to improve safety in coal mines in Australia.
This paper describes the reinforcement system used, the design process, the novel instrumentation and monitoring procedures, the establishment of appropriate trigger levels for contingency actions and the additional support measures employed in adverse circumstances. This highly successful approach provides valuable experience for NATM tunnellers, in terms of the reinforcement techniques employed, the instrumentation used and not least in the systems employed to ensure that monitored data is effectively collected, processed, interpreted and acted upon in a timely manner. Operational advantages, in terms of safety and drivage rate improvements and production cost reductions are quantified in the paper.
