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Safety and health in mines research advisory board

Annual Review 1997


Contents

INTRODUCTION

The Safety in Mines Research Advisory Board (SMRAB) is appointed by the Health and Safety Commission (HSC) as one of its independent advisory bodies. It is chaired by HM Chief Inspector of Mines and has eight members representing employers and employees in the British coal mining industry and eight adviser members with relevant expertise in research or mining. Current members are listed in the appendix along with others who contributed to SMRAB's work during 1997.

As well as advising HSC, SMRAB's remit includes the providing of information to the deep mined coal industry on research matters relevant to improving standards of health and safety in the industry. This review summarises progress made during 1997 on research within SMRAB's remit.fs


FIRE AND EXPLOSION

Ignition Control in Rapid Drivages

International Mining Consultants Ltd (IMCL) continued its work, supported by the European Coal and Steel Community (ECSC), to study the more effective prevention of gas ignitions in rapidly advancing headings driven by continuous miners.

Having confirmed that frictional ignition probability reduces as water droplet size reduces, experiments were conducted to investigate the possibility of reducing the water consumption and pressure requirement through the use of air atomising nozzles on continuous miner heads. Results suggest that the low spray density, inherent with air atomising nozzles, is insufficient to provide effective cooling and negates any benefit provided by the small water droplet size.

In other studies to determine the effect of nozzle position on frictional ignition probability, it was shown that ignition frequency increases with relatively small lateral movements of the nozzle away from the pick centre line. Experiments were also conducted with two remote solid cone nozzles at three separate nozzle spacings to determine the optimum nozzle spacing and hence spray overlap. The results showed that, although the spray width at the pick increases, as the nozzles move further apart, the spray depth along the cutting plane becomes shorter, hence reducing the time that some hot particles remain in the water spray.

Comparative measurements of entrained air, from a standard hollow cone nozzle and the solid cone nozzles used in the ignition trials, showed that the current standard 1.4mm hollow cone nozzle induced the highest air quantity for the lowest water flow rate. To induce the same air movement using the most efficient solid cone sprays, at least 12 solid cone sprays would be required to replace a standard system of 8 hollow cone sprays, each passing approximately twice the water quantity - an overall threefold increase in water consumption.

Spontaneous Combustion

IMCL has developed an effective two dimensional computer model and this was used to estimate optimum permeability values for various goaf regions by matching tracer gas transit times in the model with measured transit times in the field, and to model the injection of nitrogen into the goaf for the control of heatings.

Studies of flow through the goaf showed that it was not linear, but that the streamlines curved in arcs across the goaf. The curvature of such arcs varied with distance from the faceline. As a result, nitrogen flow from an injection point is unlikely to remain coincident with a heating during the retreat of the face. Heatings are most likely to be initiated in the goaf regions close behind the face, as velocities rapidly reduce in deeper regions of the goaf. However, it was shown that sufficient air flows are available in the deeper regions to feed an existing heating.

The nitrogen release simulations showed that injection into the abandoned intake would be effective where a fire was located in, or adjacent to, the old ventilation circuit. For a heating deeper within the goaf, it was shown that, to be effective, injection was required at positions away from the abandoned intake.

A three dimensional version of the model has been used to run a number of simulations with methane ingress from adjacent seams and a range of return-end ventilation arrangements. (See under `Mine Environment')

Explosion proof stoppings

The Health and Safety Laboratory (HSL) completed large scale experimental work to establish whether there is an adequate factor of safety in formula
(width+height)  
2 + 0.6m
specified in the Institution of Mining Engineers publication 'Sealing-off Fires Underground' for calculating the length of an explosion proof stopping. Previous small-scale experiments compared various cement-based materials and the traditional gypsum materials. The large scale experiments were carried out in a tunnel with an arched profile giving an explosion test chamber with a cross sectional area of 5.7 m2 and a length of 17 m. The chamber was concrete lined and had a permanent stopping of Hardstop material at one end and test stoppings constructed at the other end.

The first test failed when large lumps of 'Hardstop' were ejected from the test stopping and the bulkhead stopping was dislodged. This was attributed to poor construction as wet slurry was found on the remains of the damaged stopping fragments. In the next phase the permanent 3.1 m bulkhead stopping was reinstalled along with a 1.5 m thick test stopping under the supervision of the Mines Rescue Service and the Hardstop manufacturer, British Gypsum Ltd. The test stopping was subjected to four tests at pressures of 3.22 bar, 3.41 bar, 2.61 bar and finally the full test pressure of 5.5 bar. The stopping withstood the test pressure without any signs of damage. This stopping was removed and a 1.0 m thick Hardstop stopping was installed which was then subjected to eight tests culminating in a final and maximum explosion pressure of 4.22 bar. The 1.0 m stopping was examined and found to be undamaged.

The 1.5 m stopping had withstood the full test pressure. Adding a safety factor of 100% would give a stopping length of 3.0 m, which is the calculated length according to the empirical formula. In the case of the 1.0 m stopping, the test pressure was not achieved but it did withstand eight separate explosion pressures up to the maximum pressure of 4.22 bar. The permanent 3.1 m bulkhead stopping withstood twelve tests until failure. The failure was due in part to the use of the rectangular access tube which gradually collapsed and allowed the explosion pressure to weaken the stopping.

These tests, taken in conjunction with the small scale tests, demonstrated that the safety factor in the accepted formula is at least 100%, provided manufacturer's recommendations are followed.

Practical Requirements for General Stonedusting in Mines

It is many years since the percentage of incombustible dust, which has to be added to the coal dust in mine roadways to render it incapable of sustaining an explosion, was derived experimentally and specified in Regulations. The aim of this project at HSL is to assess if modern mining methods have altered the stonedusting requirements for British collieries.

Six representative mines have been visited and dust samples collected from particular areas of each mine, including intake and return roadways, transfer points and at the coal face. The dust samples were then dried, screened and sieved to determine their particle size distribution.

The non-combustible content of the coal collected from the coal faces of the six collieries was found to be within the range 1.8% - 7.4%. The moisture content and volatile content of the coal samples from five collieries were within the range 1.57% - 5.73% and 33.1% - 35.1% respectively (one, since closed, did not give consistent results).

Representative samples for testing were produced at HSL, with the appropriate particle size distributions, using lump coal and stonedust supplied from collieries. Explosibility tests in the 20 litre sphere are expected to be completed early in 1998 and will allow HSC to review the standards set in the Coal Mines (Precautions against Inflammable Dust) Regulations 1956.


HAULAGE AND TRANSPORT

Full Scale Dynamic Tests on Detaching Hooks

HSL has now completed an experimental programme to obtain information about the energy required to open a detaching hook by impact loading, and to compare the resistance to accidental opening offered by several hook designs. This work was prompted by an incident in a South African mine shaft when the detaching hook, provided as a safe-guard in the event of an overwind, was struck by falling debris and released the cage in mid-wind. It was jointly sponsored by the Safety in Mines Research Advisory Committee of South Africa and HM Inspectorate of Mines (HMIM).

The work, using facilities described in last year's report, has compared the resistance to accidental release of the three main existing designs, and some modified designs, by impact on one side with a falling steel billet. For some hooks the energy required to cause a partial release (open the struck scissor plate only) was also assessed. Although not immediately as catastrophic as a full release, a partial release has serious safety implications which will persist until the state of the hook is noticed and rectified.

Experimentally, the programme has shown that a 750kg mass falling from 22m is capable of fully opening some hook designs, although only with a static load on the hook of up to half the full rated load. For the same hook designs, a 250kg mass falling from 12m cause a partial release (open one scissor plate). The masses used were deliberately made fairly large to obtain the required energy with the available drop height, but the results imply that a smaller mass say 50kg, may gain enough energy to cause a full release as a result of falling through a distance of several hundred metres.

Arising from this work, the industry has been made fully aware of the possible consequences of dropping even a fairly small mass down a shaft; that some hook designs need to be inspected frequently in case of a partial release; and that the few users of the more vulnerable designs should move to improved designs in future or modify existing designs to increase safety.

Resin Cappings for Wire Ropes in Mines

Resin cappings were introduced into service in British mines gradually via a series of trials which started in the 1960s. During these trials, their performance was assessed by removing them for examination after progressively longer periods. By the end of these trials, HSL had examined 855 resin cappings (plus 322 white metal cappings) and had written over 250 reports. In addition, research programmes to evaluate the effect of static, dynamic and fatigue loading on the performance on resin cappings had also been carried out. Thus HSL has built up unrivalled knowledge of this type of capping in modern mine shaft applications.

A report summarising HSL's work on the development of resin cappings has been completed this year and submitted to the Institution of Mining Engineers for publication in 'International Mining and Metallurgy'. It takes an in-depth look at the problems which arose when resin cappings were first used on winding ropes and describes how these were overcome. It explains the reasons for some of the recommendations given in British Coal's 'The Ropeman's Handbook' and 'Notes of Guidance for the use of resin cappings'.

The report concludes that laboratory tests and field trials show that correctly made "Wirelock" polyester resin/sand cappings in BCC 465 long basket winding rope sockets can survive for at least two years in mine shaft hoisting installations with very little deterioration. In light duty men and materials only installations, this period can be extended to three years and possibly up to five years if the main body of the rope does not deteriorate.

Non destructive testing and residual fatigue life of locked coil winding ropes

This project at HSL, jointly funded by HMIM and RJB Mining (UK) Ltd, is to investigate the reliability of non-destructive testing (NDT) information obtained on locked coil winding ropes. Fatigued rope samples, together with other samples shown to have defects by NDT methods, have been compared with the final NDT traces obtained from the mine. Ropescan instruments have been examined and tested and recommendations discussed with their manufacturer. A report is expected next year.

MINE ENVIRONMENT

Environmental Prediction of Hydrocarbon Outbursts in Mines

As described in the 1996 report, following a sudden inrush of coal and oil at Thoresby mine, research was commissioned jointly by HMIM and RJB Mining (UK) Ltd with the aim of identifying suitable precursors to allow safe working in this colliery and others with similar geology. HSL in collaboration with TES Bretby Ltd focused on an environmental monitoring system to predict outbursts.

The available historical data was insufficient to allow firm conclusions to be made as to the best predictors, however the presence of higher hydrocarbons increased in the presence of oil. A broad range of higher hydrocarbons were identified behind the stopping built in the incident heading, of which n-hexane was measured at the highest concentration suggesting that this compound could be suitable as a marker.

The colliery tube bundle system was shown to be inappropriate for monitoring higher hydrocarbons due to losses in the sampling line during sample transport to the surface. Gresham tube samples were less effective for higher hydrocarbons monitoring than for C1 to C4 hydrocarbons but would still be useful as an early warning indicator of increasing hydrocarbon production. Diffusive sampling was shown to be the most useful tool for monitoring the mine air for higher hydrocarbons. The hydrocarbon monitor based on the VQ41TSB catalytic sensor shows some favourable characteristics for detection of higher hydrocarbons although the instrument was operating at close to its limit of detection for the concentrations found in the mine during this study. The most practical solution to the problem seems likely to be to measure the hydrocarbon to methane ratio as a trend parameter as this overcomes the effects of background methane and propane on the hydrocarbon monitor.

For obvious reasons, mine management took a cautious view in one case when monitoring data showed a rising trend and stopped a heading earlier than planned. While no full outburst events occurred during the study period, confirmed by Gresham tube and hydrocarbon monitor measurements, the research suggests that environmental monitoring can provide the basis for an early warning system.

Seismic Prediction of Hydrocarbon Outburst in Mines

The parallel project to study the use of seismic listening techniques at Thoresby was conducted by IMC Geophysics Ltd. Their equipment was shown to pick up shots that had been fired and located a number of seismic events which were all consistent with movement in the overburden. They did not have the characteristic attributes of events associated with outbursts as investigated previously at Cynheidre Colliery. Development of a recorder more suited to long-term underground use and further investigations into reducing system noise and increasing system robustness and reliability are being pursued.

A similar system to monitor microseismic noises including possible precursors of outbursts, has recently been installed at Boulby Potash mine. The digital system is operating in parallel with an existing analogue system, and is currently recording events which will be located when the geophone orientations have been measured. No outbursts have been recorded as yet.

Methane Prediction

The IMCL computerised fluid dynamics (CFD) model of a longwall retreat face was modified to allow for the ingress of methane from adjacent seams above the working district. The intervening region was modelled as a collection of variably permeable sub-regions to represent the fractured state of this strata. Experimental data from the field and from scale model tests was reviewed to provide information on how best to model the intervening strata between the working district and adjacent seams.

Data collected and examined under previous research studies was re-analysed to clarify the dependence of the height of gas emission above longwall districts (H) on mining parameters. The analysis showed that H is proportional to both the depth of working and the face length. H is also assumed for other reasons to be proportional to the face height. Data was collected on the height of fracture above longwall districts with the intention of verifying, or otherwise, the hypothesis that the zone of emission can be identified with the tensile zone above the face with open fractures. Data was collected from published sources using in-situ anchors, physical models, computer models and microseismic investigations. The results show that the height of fracture determined from these different methods shows a similar dependence on the same mining parameters as the height of emission. There is therefore good evidence that the proposed hypothesis is reasonable.

Ventilation Studies

A current ECSC project being undertaken by IMCL, Nottingham University and a range of other European partners, is investigating several aspects of mine ventilation. The topics include improved efficiency and reduced power costs, ventilation control and mine climate. Work during the period has investigated the effects of shock losses, essentially comparing CFD modelling with the `standard' shock loss calculations at bends and `T' junctions according to the American Society of Heating Refrigeration and Air-conditioning Engineers (ASHRAE).

CFD has shown its capability in illustrating the major areas of shock loss in all the models constructed. Further validation work is necessary to be sure of the modelling techniques and to provide any rules for accurate shock loss prediction. The most significant deviation of CFD values from the ASHRAE calculated values was shown when models were constructed with fine detail such as RSJ supports and covering. The values of pressure drop for all the models were significantly higher than the ASHRAE values. Some modelling work with respect to reducing shock losses has begun and will be further developed. It is intended to evaluate whether a reduction of shock losses at junctions and possible reduction of leakages at various points in the mine, will give a collective total reduction in fan duty, with savings in power costs.

The mine climate work at Nottingham has involved the development of a representative conceptual model of a coal mine which has been subjected to a series of preliminary thermodynamic analyses. These have included determination of the optimal ventilation design and resultant underground psychometric conditions for a range of inlet air and underground thermal load conditions.

Respirable dust regulation review

The Health and Safety Commission is committed to reviewing the Coal Mines (Respirable Dust) Regulations 1975 (RDR) in 1999 and various research projects and other assessments have been set up to prepare for this review.

HMIM commissioned the Institute of Occupational medicine (IOM) to review existing relevant information on dust concentration and exposures to ensure that the review of standards includes an assessment of what can reasonably be achieved in mines and whether the assumptions which underpinned the standards in RDR are still valid with current mining practices. This review has concluded that many of the assumptions about the relationships between cumulative personal exposure and fixed point measurements which were behind RDR no longer hold. It should be possible for mines to meet the same timeweighted occupational exposure standard (OES) for respirable dust and the maximum exposure limit for quartz as are set for other industries under the Control of Substances Hazardous to Health Regulations sampling. However, the OES for coal dust could not be met.

The review also identified a lack of current data on the size distribution and mineral composition of airborne dust in coal mines and an extension project to gather representative samples has been commissioned.

The need to review the performance of sampling instruments has also been recognised. A project has been placed with HSL to assess instruments in use in mines overseas and in other industries against European sampling conventions, benchmarked to the MRE 113A instrument approved for RDR.

Passive dust sampler

The previous project on this work, mentioned in the 1996 report, was a feasibility study. It showed that good correlation existed between measurements made with the 'electret' passive dust sampler, developed at HSL, and the MRE sampler at the statutory sampling point in two coal mines, establishing the general feasibility of using the device. The results were presented in 1997 at the Safety in Mines Research Institute Conference in New Delhi.

An extension project is aimed at trials in other coal mines to examine the general applicability of the device. During the trials, external conditions such as temperature and relative humidity, are being monitored to ascertain whether any of these has a systematic effect on the results. If this is the case correction factors can be included and will lead to an improved accuracy of the device. The field trials will be supported by laboratory trials carried out in more precisely controlled conditions.

Diesel fumes/particulates in mines

HSL continues work to quantify the hazard arising from diesel exhaust emissions underground. The carbon surrogate used to assess the hazard in surface industries cannot be used in coal mines because coal dust confuses the measurements.

Measurements on the Buxton diesel engine test rig have been completed. A great deal of data were generated and are being examined in detail but so far there is nothing to suggest that any of the gases in diesel exhaust fume could be used as a surrogate measure of exposure to carbon particulates. A colorimeter used to measure the 'blackness' of the filter showed very good correlation with carbon particles.

The trial was extended to a salt mine where again much data have yet to be fully assessed. The colorimeter again showed good correlation with carbon particles and could provide an alternative measurement method in miscellaneous mines where there is no background of carbon. Additional measurements at roof height, carried out at the request of the mine operator, showed an accumulation of diesel fume there. This measurement was important to the mine as work is frequently carried out on the roof from raised platforms and the large mine galleries are poorly ventilated relative to coal mines.

At the request of HMIM, a new block of work was introduced into the project. There was concern that the original tests done at the Buxton diesel rig did not accurately model the situation in a coal mine since the air drawn into an engine under 'real' conditions contains some coal dust. Consequently further tests were carried out where coal dust was fed into the air intake of the engine to establish the effect on gas and particulate concentrations in the exhaust. This work is still in progress.

The final part of the project will extend the work into a coal mine where measurements will be taken under typical working conditions.

Separation of Coal Dust and Diesel Fumes

Precise measurements of diesel fume in coal mines are frustrated by the presence of airborne coal dust. Simple inertial methods do not sufficiently separate the dust and fume. HSL carried out a small feasibility study which looked into three alternative methods from which instruments might be developed.

If the electrical mobility of diesel fume is significantly different from that of coal dust then the two can be separated with an electrostatic classifier. Direct measurements of the electrical mobility of a diesel engine exhaust were compared to values of electrical mobility of coal mine dust found from the literature. The electrical mobility distribution of diesel exhaust was found to have two peaks, one of which partially coincides with that found for coal dust. Therefore, separation due to electrical mobility is unlikely to be successful.

If diesel particles are significantly more polarizable than coal dust then the two can be separated with an aerosol neutraliser and electrostatic classifier. A theoretical study backed with measurements on model diesel agglomerates and dust particles is in progress

If the rate at which a filter becomes clogged with diesel fume is significantly different from the rate at which it becomes clogged with coal dust then a simple measurement of the resistance to air flow by a filter could be used to estimate the concentration of diesel fume it was exposed to. Measurements of the change of resistance to air flow through filters as they became clogged with coal dust or diesel fume were made for a variety of filters. Glass fibre filters were found to have a clogging factor (the change in resistance to air flow per milligram of dust deposited on the filter) approximately 100 times greater for diesel fume than for coal dust. This suggests that filter clogging is a suitable means of measuring diesel fume and that a technique based on this principle could be developed.

Biomarkers for diesel emission exposure

HSL's Biomedical Sciences staff have looked into the potential use of biomarkers for assessing exposure to diesel fume. A brief literature review suggests that urinary 1-hydroxypyrene might be a suitable indicator of diesel fume exposure.

OCCUPATIONAL MEDICINE

The Institute of Occupational Medicine (IOM) reported at the end of 1997 on the results of three projects which had been initiated by British Coal and were funded to completion as part of the privatisation transitional arrangements.

Cancer mortality in coalminers

This major study, part funded by the European Union, sought to identify and quantify any relationship between coalminers' mortality from lung, stomach and other cancers and their exposure to substances found underground which are thought to present an increased risk of cancer - diesel exhaust, quartz and radon and thoron daughters. It looked at the records of 18,166 men who had taken part in the National Coal Board's Pneumoconiosis Field Research programme (PFR) in the thirty years up to 1992 and for whom data on smoking habits and exposures to respirable dust and quartz were available. Their exposure to diesel exhaust fume, radon and thoron daughters were estimated from records from the collieries which took part in the PFR. Mortality rates for the 7002 who had died were compared to the regional reference rates for all males standardised for age and year of death.

Analysis showed that mortality from all internal causes was lower, at 91%, than expected in the reference population. Lung cancer was also lower at 86% but stomach cancer had increased risks at 124%. No other cancer site showed increased risks but deaths from chronic bronchitis showed a standard mortality rate of 120%.

Extensive analysis to investigate exposure-response relationships for specific causes of death failed to come up with any convincing evidence, other than the well-established link between exposure to respirable dust and pneumoconiosis. This study should have had the power to detect an association between quartz and lung cancer if there was one. The more limited exposure data for diesel fumes, radon and thoron mean that lack of an association with lung cancer is concluded with less certainty. The increased risk of stomach cancer did not appear to be linked with time spent in the mining industry suggesting the cause may not be linked with conditions of work.

A second study focused specifically on any link between diesel exhaust fume and lung cancer. It used records of measurements of oxides of nitrogen at mines using diesel locomotives to derive more accurate estimates of exposure to diesel fume. Only weak evidence of a link could be found and this depended on inclusion of one particular colliery where exposures were estimated to be very much higher than the other collieries. No link could be found among men with different exposures at the same colliery.

Progression of pneumoconiosis in ex-coalminers

This study was designed to ascertain whether progression of pneumoconiosis takes place in men after they have left the mining industry. A sample of 200 pairs of X ray films were selected from men who had shown some progression during their working lives and these were all read by three experienced readers.

Of the 200 men studied, 32 showed some further progression, 164 showed no change and 4 showed some reduction of the disease after occupational exposure to respirable dust ceased. There was no evidence of progression being linked to occupation at particular collieries, to smoking habits, age or length of follow-up. Just over half the cases of progression involved the development of large opacities, which may have clinical implications for the subjects. The results warn against complacency in efforts to control coalminers' pneumoconiosis.

Development of a code of practice for work in hot and humid conditions.

Environments in some mines have become more extreme with the depth, distance of faces from the shafts and the power of machinery all increasing. This project aimed to collect knowledge on the health effects of heat strain and how the problem can be managed into a single guidance document.

The code recommends a first action level at a basic effective temperature (BET) of 25oC when job rotation or mechanisation should be used to minimise physical work and information and training should be given to staff. Above a BET of 27oC, health surveillance is also recommended. Above a BET of 30oC, the use of cool refuges or cooled jackets should be considered and it may be necessary to limit the working shift. Guidance is also given on clothing, diet and fluid intake.


GROUND CONTROL

Rock Mechanics Technology Ltd (RMT) completed and submitted final reports on three major ECSC/British Coal funded research projects during the year. These were undertaken with the aid of small subcontracts to various Universities and other research organisations. These Projects were entitled Coal Pillar Design, Multiple Seam Interaction and Targeted Rockbolting.

RMT continued its programme of ground control research with three other ongoing projects funded by the ECSC and private mine operators with contributions from HMIM. These are part of pan-European collaborative projects with German, French and Spanish partners. HMIM has also funded a number of smaller projects with RMT, the University of Nottingham and HSL.

Coal Pillar Design

The three year Coal Pillar Design project has been aimed at advancing understanding of both sides of the pillar equation - pillar strength/behaviour and pillar loading. It concentrated on:

Pillar strength/behaviour

Pillar Loading

The main conclusions of the project have been that the appropriate approach to pillar design will depend upon the level of geotechnical data available or obtainable by the designer within the available timescale/budget and the class of pillar system involved. The final ECSC report contains a number of case studies which illustrate the range of approaches available. Where data is scarce then the stress measurement studies have shown that approximations of pillar behaviour into two broad categories, yielding and non-yielding roof and floor, as characterised by Wilson, are valid and can be applied. However the actual measured behaviour of pillars in the two categories differs quite considerably from that predicted by Wilson's equations. An alternative to Wilson's equations is suggested.

Three dimensional computer modelling has been shown to be a valuable technique for predicting macro scale vertical and horizontal stress distributions around longwall workings in single seams. More work in this area is likely to be particularly fruitful in improving the ability to achieve rational mine designs for good ground control. The technique has already been very successfully applied at a number of mines where options of maingate positioning either on the pillar side or on a stress concentration side of a panel have had to be decided between.

Finally the Project has shown that the combination of macro scale three dimensional modelling, smaller scale detailed two dimensional modelling of roadway behaviour and in-situ stress measurement can now provide a powerful tool for both room and pillar design and longwall finger pillar design. Theoretical considerations show that there is a wide range of possible pillar strengths for any particular geometry depending upon local factors such as geology and stress levels. This approach allows appropriate site specific pillar designs to be achieved, but requires a relatively high level of input data.

Multiseam Interaction

The work on multiseam interaction has comprised a combination of detailed underground measurements of roadway deformation and stress variations at two UK mines together with a study of the applicability of numerical modelling to predicting underground interaction effects. The underground measurements have provided very valuable planning data to the mines concerned and, in particular, have identified the clear presence of vertical stress window phenomena at intervals in excess of 150m as well as showing that, at one of the sites, vertical stress levels above cover load are experienced up to 30m within the edge of an overlying goaf.

The application of numerical modelling to interseam interaction has been shown to have significant potential but requires further development. It has been shown that the application of elastic three dimensional boundary element codes does not properly reproduce the interseam stress redistributions actually experienced underground. The use of more complex codes which can include strain softening and transversely isotropic rock behaviour is much more successful but is currently limited by practical constraints to two dimensional solutions. Advances in computer hardware expected over the next two years should mean that three dimensional solutions will be possible in the relatively near future.

Targeted Rockbolting

The targeted rockbolting project has been a pan European collaborative project aimed at studying the application of rockbolting in UK, Germany, France and Spain. As a result of this collaboration UK rockbolting design techniques and equipment are beginning to be applied in Germany and France. The work has included testing and comparison of all commonly used European rock bolting consumables and a pooling of knowledge on rock bolting design by the partners carrying out designs for each other's sites. None of the European consumable systems tested complied with the performance required under the current British Standard for AT rockbolting consumables. Also during the Project, considerable advances in the application of computer modelling to the design of rockbolt and cablebolt reinforcement systems were made. Modelling research was also undertaken under subcontracts by SCT (Australia) and Herriot Watt University.

Siting and Support of Roadways in the Vicinity of Old Wastes

Work under this ECSC project has concentrated on studies of the Bilsthorpe PG45s supply gate roof fall, investigations of abnormal longwall caving and studies of small pillar behaviour. The Bilsthorpe fall studies, which included face monitoring data analysis and numerical modelling, have not confirmed the original interpretation that the area in which the fall occurred was delineated by areas of high potential goaf block formation. The modelling work is still underway but will be completed in 1998. Comparisons have been made between the Bilsthorpe geometry and standard rockbolted face salvages, the only clear difference being the lack of goaf side support at Bilsthorpe.

Abnormal caving investigations concentrated on powered support monitoring analysis from a 120m longwall face which suffered a major strata weighting followed by water inflow. Recordings of leg pressures and support closures were obtained throughout the weighting. These indicated that the weighting was preceded by two less severe high closure events and was very similar in nature to a weighting previously experienced at the same mine on a 250m longwall. It was concluded that the phenomena on both faces were likely to be associated with the presence of strong igneous sills in the overburden. Future occurrences, where such sills were present, were only likely to be alleviated by working very short faces or employing room and pillar operations.

Numerical modelling is also being employed to investigate the behaviour of roadways driven with small separation pillars from goafs at a mine where detailed measurements of strata behaviour have been obtained for both 10m and 15m pillars. For the particular geology encountered at this site it has been concluded that 15m pillars are likely to lead to a more secure layout geometry.

Stability of Long-term Roadways

Work under this ECSC project has included geotechnical instrumentation development, numerical modelling of long term roadway stability, development of risk assessment techniques and development of new reinforcement consumables testing methods.

Instrumentation development has included further progress on a remote reading telltale system which will shortly be submitted for intrinsic safety (IS) approval. RMT has also completed development of a Mk 2 'Sentinel' integrity rockbolt which is now available to the industry for application in areas at risk from broken rockbolts. Despite considerable publicity and relatively low cost, only a small number of these bolts have so far been installed. Developments of 'Sentinel' and strain gauged flexible bolts are also progressing well and production units will be available shortly. Work on non destructive testing of in-situ rockbolts and cables has also progressed well, with successful modifications carried out to the bolt end preparation system for ultrasonic testing and further underground trials of the radio frequency method for testing cable bolts. In the latter case, modified instrumentation was developed which has provided much improved response in coal measure strata. However neither system is yet at a stage where is can be used with confidence in a coal mine environment.

Numerical modelling is being applied to support design at two mines where new areas of reserves are planned to be exploited. In both cases it is hoped to be able to employ rockbolting to maintain stability in long term roadways. The use of hybrid systems involving a mixture of flexible bolts and rockbolts is being examined as are the effects of long tendon rib reinforcement and floor dinting.

In the field of risk assessment, work has concentrated on development of a suitable approach to the stability of roadways and junctions in a low stress environment. Suitable survey sheets and analysis algorithms have now been developed and are currently being tested in the field.

A more realistic laboratory test method for rockbolting consumables has now been developed which involves pull testing within confined rock samples. Tests are now being undertaken using this method to compare the performance of different reinforcement systems. A similar method is also under development for cable bolts.

Targeted Rockbolting 2

Work on this ECSC project started towards the end of the year. RMT will work on ultrasonic testing of rockbolts, instrumentation and testing of cable and flexible bolts, and risk assessment associated with rib stability. The latter follows several recent rib falls including one which led to a fatality; an extensive rib stability risk assessment programme has been drawn up.

Under the same ECSC project, HSL has started work on the application of neural network computing techniques to the interpretation of data from ground control monitoring.

Lifting and Suspension from Rock Bolts

HMIM has funded RMT to examine what external loads can be applied to rockbolts without adversely affecting ground control with a view to establishing if existing industry guidance is appropriate. Underground measurements, including bolt loads and strata movement during passage of a monorail suspended pantechnicon, are expected to be completed in 1998.

Performance of rockbolts under shock load

HSL completed this year a project concerned with the performance of rockbolts under shock load. This was done by testing fracture toughness specimens manufactured from rockbolts and also by carrying out a limited number of tests on full section but shortened rockbolts.

The results indicate that the fracture toughness of material from two manufacturers does not alter significantly with increases in test velocity up to 16 m/s. There were some experimental problems with the specimen tests which were overcome by using strain gauged pre-cracked tensile specimens and there is now a high level of confidence in the results. The critical crack sizes, which were calculated for slow rate loading in a previous project, can also be applied to dynamic loading situations as there is no significant change in fracture toughness.

In the presence of a blunt shallow notch the full scale samples exhibited some tearing but did not fail. However, when a sharp, relatively deep pre-crack was introduced, brittle failure occurred in both materials. Thus, in the full scale tests, the material responded in a manner which was predictable from the specimen tests.

Effects of Corrosion on the Mechanical Properties of Rockbolts

Rockbolts can be subject to corrosion when the protection of the grouting resin is disrupted, particularly if the natural strata water is aggressive. Independent Metallurgical Practice completed a study for HMIM by subjecting artificially corroded specimens of AT grade rockbolt material specimens to tensile, bending and stress corrosion tests.

The investigation suggests there are four corrosion processes that may impair the integrity of AT rockbolts:

Further tests on galvanised specimens are planned.

Rock Mass Classification

Rock mass classification has been used extensively as a ground control design tool in tunnelling and hard rock mining. HMIM has sponsored research by the University of Nottingham to investigate its potential for application to coal mines using rockbolts as their primary mode of ground control.

The initial UK coal mine classification is being developed by application to detailed case study sites. The classification method is currently being used to characterise the roof strata from 13 selected roof borehole cores from an RJB mine. This involves the design and implementation of an extensive laboratory rock testing programme that includes, uniaxial compressive strength tests, point load tests to determine the degree of fissilty of the strata, Brazilian disc tensile tests and cone indentor tests of each of the individual strata lithologies identified from the borehole cores. This work is ongoing. A Hoek cell is under construction to accommodate cored samples with a 44mm diameter to allow triaxial strength testing of each of the lithologies.

A preliminary numerical model of one selected rockbolted roadway in the case study mine has been constructed using the detailed available information and the classification and test data and is in the process of assessment and evaluation. The UK coal mine classification has been used to help determine accurate elastic and strength parameters necessary for model input for each of the identified lithologies. The larger scale numerical modelling studies examining caved waste behaviour and accurate simulation of deformation and failure processes as well as abutment stress modelling are progressing.

The widening of the case study information to include operating mines outside the Selby complex is important. Discussions have recently been held to include Annesley Bentinck mine as a case study site in order to widen the scope of the initial classification.


OTHER RESEARCH

Applications of Risk Assessment in the Mining Industry

As reported last year, this HSL project aims to review and comment on the application of risk assessment in the mining industry. Following the underground familiarisation visits and assessments of training in risk assessment, the research entered a more formal data gathering phase. The research team has undertaken some 40 structured interviews during visits to the three mines in the study. The interviews were conducted with a range of staff within each mine and were aimed at assessing an individual's understanding of risk assessment, the manner in which it has been introduced and the extent to which it influences working practices. These interviews allow comparisons and contrasts to be made between the three mines, in terms of the effectiveness of the ways in which they have attempted to introduce risk assessment. Initial findings suggest that differences exist with regard to the extent and manner in which risk assessment is being applied.

Assessment of HMIM's Accident Analysis and Investigation Methodology

This HSL project aims to evaluate HMIM's approach to accident investigation and classification. Having conducted desktop assessments last year of the classification of all reportable accidents for the year 1995/96, the project entered the second planned stage of shadowing inspectors on a number of accident investigations. A questionnaire has been distributed to field inspectors providing them with the opportunity to air their views regarding the utility of the current accident classification scheme. The data gathered, combined with feedback from inspectors, will form the basis of recommendations for revision of the classification scheme.

Egress Assistance Beacon System

This research, intended to provide immediate wayfinding assistance after an underground fire or explosion, has been completed by IMCL during 1997. A proof-of-concept system, comprising 28 prototype beacons, was installed in a training gallery of Mines Rescue Service Limited, County Durham. The system operated well under a variety of simulated rescue conditions.

Intrinsically safe Group I (mining) inductive power transfer is feasible, with 50 beacons attached to an IS charging line. An alternative direct powering approach offering lower cost and the capacity to power approximately 120 beacons from a single IS supply was shown to be equally feasible.

The use of high intensity red and green LEDs was shown to be effective. In `light smoke' conditions the beacons are visible beyond 15 metres. Future improvements in LED technology should permit visible range under heavy smoke conditions to be increased to 10 metres or more.

The use of an acoustic sounder was an important aspect of providing additional directional cues in extremely low visibility conditions.

Appraisal of Earth Fault Protection on 3.3 kV Main Distribution Systems

A previous project, reported in 1996, was carried out on underground coalmine 6.6kV main distribution systems to obtain some idea of the level and duration of out-of-balance current in the neutral to earth conductor of main distribution transformers. Such currents indicate earth fault or leakage currents and may be used to disconnect the supply. This year, a similar study was undertaken on a 3.3kV system.

A computer data logging system monitored out-of-balance current (limited to 100A by a 19.05 ohm earthing resistor) in the system supply transformer neutral to earth conductor. In addition a sensitive earth leakage relay operating at 1A with a time delay of 0.4s was installed to give alarm indication only. The main conclusions are:

Abandoned Mines

An ECSC research project studying the behaviour of gas in abandoned mines was completed by IMCL during the year. A computer based prediction model was developed to characterise the flow of contaminants from underground voids. The range of voids modelled was from 18 million cubic metres to less than one million cubic metres.

It was possible to model each mine site with independent volume and resistance, and the voids coupled to atmosphere could be modelled by an effective volume, resistance and a contaminant inflow. However, the nature of the modelled flow was uncertain and could be due to any combination of such parameters as degassing of the remaining coal, changes in water levels, gas transport in water or expansion from more distant coupled voids.

There was very good agreement in the model for total flow and for the total time the flow from the workings was positive. It was shown that, due to the size of the effective void volumes encountered in the application of the model, there was no simple solution to the problem of controlling the discharge of methane from worked areas in existing collieries under adverse barometric conditions. The determination of a model for a void discharging to the surface would enable a considered choice of vent pipe diameter to prevent a dangerously high pressure difference developing in the void on a falling barometer.

Further studies in this field, associated with surface gas emissions and the influence of minewater recovery are on-going.

Environmental Noise

Detailed environmental noise logging exercises were carried out by IMCL at five coal mine sites - four deep mine sites and one opencast site. At all sites there was a significant low frequency component, with differences of 18 - 27 dB between the measured linear and A-weighted levels during the working day. Additionally, the contribution of intermittent noise to the overall noise climate was quantified. Significant differences between Laeq and LA90 were measured at all sites, indicating a strong intermittent component to the noise climate. These data highlighted the problems of assessing proposed new operations or developments at mineral extraction sites. The results indicate that assessments to existing procedures may be inaccurate or misleading and could cause unjustified problems to mineral extractors.

APPENDIX - MEMBERS OF SMRAB

Chairman
Mr B Langdon, HM Chief Inspector of Mines.
Industry Representative Members
Mr P McNestry, national secretary, National Association of Colliery Overmen, Deputies and Shotfirers;
 
Mr R Young, president, the British Association of Colliery Management;
 
Mr N Greatrex, president, the Union of Democratic Mineworkers;
 
Mr W Rowell, managing director, RJB Mining (UK) Ltd;
 
Mr J Sorbie, managing director, Midlands Mining Ltd;
 
Mr B M Davies, technical support manager, Scottish Coal Company Ltd;
 
Mr J Ainley, vice-chairman, the Federation of Independent Mines;
 
one vacancy, open to a nominee of the Trades Union Congress;
Adviser members
Dr D Buchanan, Chief Executive, Health and Safety Laboratory;
 
Mr I Todd, head of mining policy, HSE;
 
Mr R G Siddall, past president of the Institution of Mining Engineers;
 
Dr S Afacan, medical director of Business Health Care Ltd;
 
Prof N Jones, Department of Mechanical Engineering, University of Liverpool;
 
Prof K Dunham, Department of Mineral Resources Engineering, University of Nottingham;
 
Mr W E Hindmarsh, deputy managing director, International Mining Consultants Ltd;
 
Mr P Deakin, vice president, Association of British Mining Equipment Companies.
Others who contributed to SMRAB meetings during 1997
Dr A Jones, Operations Director, Health and Safety Laboratory;
 
Mr A Dobbs, Union of Democratic Mineworkers;
 
Mr N Pearce, Midlands Mining Ltd;
 
Mr G Clamp, RJB Mining (UK) Ltd;
 
Mr G Gilmour, HM Inspector of Mines, Secretary to SMRAB;
 
Mr N Hill, Administrative Officer, HSE, minutes secretary;

SMRAB Secretariat
HM Inspectorate of Mines
Health and Safety Executive
Daniel House
Trinity Road
BOOTLE
L20 7HE
UK

Updated 2009-05-22