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Safety and health in mines research advisory board

Annual Review 1998


GROUND CONTROL

Work on three ECSC projects continued during 1998: 'Application of Geotechnical Techniques for Planning and Support of Roadways, Galleries and Face Ends'; 'Stability of Long Term Roadways'; and 'Targeted Rockbolting 2'. Rock Mechanics Technology Ltd (RMT) is working on all three; HSL is a partner for the third of these projects. These projects also received financial support from HSE, UK mine operators and equipment manufacturers. HSE also funded two smaller research projects with RMT entitled, 'Lifting and Suspension from Rockbolts' and 'Ground Control for Small Mines' and one with the University of Nottingham on 'Rock Mass Classification'.

Application of Geotechnical Techniques for Planning and Support of Roadways, Galleries and Face Ends

RMT's work on this project over the last year has concentrated on the application of computer modelling to improving understanding of stress distributions and strata behaviour around goafs. A number of modelling techniques have been applied to investigating the mining geometry associated with the Bilsthorpe fall of ground. The results to date appear to confirm that very unusual circumstances need to combine in order to recreate the type of fall which occurred. This work has highlighted the inherent difficulties in modelling goaf behaviour, on both small and large scales, where it is necessary to model the failure and post-failure characteristics and behaviour of large rock masses.

This has led to a reassessment of basic goaf modelling techniques and development of a more general large-scale goaf formation model using two-dimensional finite difference methods. This modelling approach appears to have considerable potential for application to small pillar problems and problems associated with mining above goafs. However, this modelling approach only developed towards the end of this project and requires further work. A number of mines are currently formulating plans to overwork existing goafs, a potentially unstable situation, and are likely to benefit from modelling the proposed gate road layouts if the technique can be refined.

A combination of large scale three-dimensional boundary element and small scale two-dimensional finite difference modelling was used to investigate stress concentration effects around retreat longwall face ends as a result of goaf formation. This methodology was applied to investigating potential gate-road/face-end support problems at two UK mines. Field work and computer modelling on this project are now complete and the final reports are expected next year.

Stability of Long Term Roadways

A remote reading telltale system is being developed by RMT in order to assist mine operators to monitor the condition of rockbolted long-term roadways and to provide early information on remedial work required. Electronic and embedded software design has been completed and formal application for intrinsic safety approval was made in July 1998. Considerable testing of the system components and software has been undertaken. Work has also begun on the user interface software. Certification is expected in early 1999 and a full underground system demonstration is currently being planned for a large UK mine.

Development of the remote reading telltale system has also resulted in development of a new 'manual reading' dual height telltale. The new telltale has significantly improved water diverting and anchorage features and has now been adopted as the standard UK instrument.

Following an initial feasibility study of early telltale movement at a mine in early 1998, a second, more detailed study was completed. The purpose of the study was to develop a roof support strategy based on the trends of early telltale movement. This work will be a major contribution for the future monitoring of long term reinforced roadways, particularly where remote reading systems are used.

A FLAC 2D numerical modelling assessment of the behaviour of the roof, floor and ribs, of long life gate roads (2200m) supported by steel arches and rockbolts was carried out. This represents the first work carried out under the project by RMT to model the behaviour of roadways supported by a mixed system of rockbolts, steel flat topped arches and remedial standing support (cribs). The analysis also paid particular attention to the behaviour of the floor of the roadway. This involved analysis of the effects of the timing of dinting in the maingate in relation to the face abutment stresses.

A series of computer modelling studies was also completed to investigate various roadway deformation mechanisms and mining operations which may have implications for long term stability of gate roadways at a second mine. These included loss of rib integrity resulting in increased effective roadway width and the effect of dinting on roadway stability. The potential for long tendon rib reinforcement on retreat to control these effects was also investigated.

Two- and three-dimensional modelling and analysis was undertaken to examine the potential stability problems confronting a mine which is proposing to drive new long term trunk roadways to access an unworked area of coal. The area is both underworked and overworked.

Following last year's initial assessment of likely long term roadway conditions for a proposed new area of a mine, which involved model construction and examination of the likely sensitivity of a rockbolted roadway to stress levels, bolt length, bolt density, roadway width and interval to an overlying limestone bed, further models have been constructed and run. These investigated, in particular, the effectiveness of incorporating longer tendon reinforcement as part of the bolting pattern in the form of flexible bolts. Monitoring and a system of periodic condition surveys or risk assessments have been recommended.

Considerable work was undertaken during the first half of the year on development of a risk assessment procedure for ensuring long term stability of roadways at a room and pillar mine. Unfortunately the mine has now closed and so the work cannot be completed or tested until an opportunity arises at another room and pillar mine.

As a continuation of the work on consumables carried out during the early part of this project, to examine the condition of in-situ rock bolts in the field and assess the potential effects of corrosion of bolts installed long term, an AT rockbolt was recovered from the roof of a roadway at a mine which had been flooded. Observations relating to the effects of groundwater on rock weathering processes in coal mine roadways were also made here. The roadway was flooded for approximately 12 months and the roof strata probably saturated. The condition of the recovered rockbolt was excellent.

Work is underway on developing an integrity monitored 'sentinel' bolt in GRP (glass reinforced plastic). This development potentially offers a means of monitoring the condition of reinforced rib sides and therefore has significant implications for the maintenance of long term roadways and for risk reduction. RMT are carrying out the development work in conjunction with MAI Systems.

Targeted Rockbolting 2 - RMT

This 42 month project started at the end of 1997 and RMT's research has addressed four areas: assistance in design of rock bolting systems for a new area of a mine which has been selected as the UK case study for the project; work on ultrasonic non-destructive testing (NDT) of in-situ rockbolts; performance tests for stranded ground reinforcements; and work on assessing and reducing falls of ground risks in coal mine roadways.

Design work using numerical modelling for the UK case study site has indicated that roof control in the new, deeper area of the mine can be achieved by employing a reinforcement system which employs a mixture of 4 m flexible bolts and 3 m AT rockbolts installed at the face of the heading. Successful trials of this pattern, installed with a bolter-miner system, were completed in the current, shallower part of the mine. Further modelling was carried out to examine the effects of increasing strap spacing in the shallower area and the potential effects of loss of roof coal. Work is now planned to model increased strap spacing in the deeper section of the mine. Stress measurements will be undertaken in the deeper area before rockbolted drivage commences to confirm the assumed stress levels used in the modelling. This project has highlighted the current uncertain status of flexible bolts when used as part of a reinforcement pattern. An HSE working party has been set up to draft amended guidance in this area. The work has also highlighted the need for strain gauged flexible bolts for proper application of the 'design by measurement' principle where flexible bolts are used. Strain gauged and 'sentinel' flexible bolts have been developed.

The main aim of RMT's research on ultrasonic NDT is to develop an instrumented method of detecting broken rockbolts in-situ in coal mines. This will also involve developing a standard test procedure and bolt integrity classification system, improving the interpretation of ultrasonic data to differentiate between useful and spurious information, refining bolt end preparation and validating the system.

Work to date has centred around obtaining reflected signals from the far end of various lengths of in-situ rockbolts. For the system to be of practical use it must confidently confirm the intact state of bolts where defects are not present. Work has also been undertaken on detecting saw cuts, necking and incomplete resin encapsulation. During the year appropriate instruments have been obtained and tested in the laboratory and at a limestone mine, which does not present the access and intrinsic safety problems of a coal mine. Three ultrasonic systems, including a number of transducers, have been tested: the Krautkramer USD10NF, a new Krautkramer instrument, the USM20L, and a Wavemaker Duet signal generator with Lecroy digital oscilloscope. Work is progressing well and results have been encouraging.

The aim of RMT's work on stranded ground reinforcements is to improve the understanding of the reinforcement mechanisms they mobilise in mines, through improved load measurement, extensometry and testing, and to research suitable principles for measuring their in-situ integrity and performance.

In addition to the development of 'sentinel' and strain gauged flexible bolts described above, further instrumentation development work has been undertaken. This has concentrated on the radio frequency resonance method for determining the integrity of stranded tendons in-situ. This equipment is now undergoing underground coal mine trials and shows promise. It was recently tested at Dinorwig Pump Storage Power Station and successfully determined the in-situ length of 6m rockbolts.

The other major work area has been in adaptation and application of the new laboratory short encapsulation pull test for stranded tendons. Tests have been completed on double birdcaged cables and double nutcaged cables in sandstone and double birdcaged cables in coal. With 500 mm of encapsulation, the double birdcaged tendons developed full rated load before strand failure in both rock types. No bond failure occurred. The double nutcaged cables exhibited a much lower bond stiffness with bond failure at 51 tonnes.

The test method has also been applied to study of double birdcaged cables grouted in sandstone and subject to immediate constant strain whilst the grout is curing. These tests, at two different strain rates, showed that, although the bond stiffness was considerably reduced over that obtained with cured grout, the grout still effectively cured despite the continuous strain, and the bolts still developed their full rated load before failure. This has positively informed an area of considerable previous uncertainty. The results suggest that the maximum roof movement prior to cable failure in actively deforming sandstone roof could be less than 50 mm, if strain was concentrated over a limited area. Repeats of these tests in mudstone and coal are planned.

The aims of RMT's research on falls of ground risks in coal mine face roadways are to improve the understanding of ground failure mechanisms in reinforced coal mine roadways as longwall faces retreat along them and to develop techniques to assess the risks of falls of ground from the roof and sides in such roadways.

Work is being undertaken on examining the risks of falls of ground and developing appropriate risk assessment techniques at four mines. It is intended during the project to be able to develop a general falls of ground risk assessment strategy for coal mines which has been proven by wide application throughout the industry.

A range of laboratory tests was carried out on a new Weidmann 25 mm diameter glass reinforced plastic rockbolt which has a continuous thread running over the entire length of the bolt. The results of the test programme indicated that the bolt, in addition to achieving the standards set in the current British Standard for mechanical properties, generated a double embedment test bond strength comparable with or better than current GRP bolts used in coal mine ribs. Compared with GRP bolts in current use, the new bolt showed a significantly higher bond strength, shear strength and end load capacity and therefore potentially offers a substantially improved performance.

Laboratory short encapsulation pull tests were carried out on a range of rib reinforcement consumables in Barnsley seam coal. The rib consumables tested were:

These tests measured the bond strength and system stiffness of the rib consumable / resin or grout / coal system. Of the two standard rib reinforcement systems tested in a nominal 27 mm hole, the AT bolt / AT resin system performed better in terms of peak bond strength and system stiffness than the GRP bolt / AT resin system. Of the long tendon systems typically used for additional reinforcement the Weidmann GRP bolt / HPRG grout system achieved considerably better system stiffness and peak bond strength results compared to the other tendons tested. When normalised for bond length, the results indicated that the systems using a grout encapsulant had higher bond strength than the two resin systems. The Weidmann GRP bolt had the highest overall bond strength. The slimline birdcaged cable showed an unusual characteristic with a slow continuing load build up after initial bond failure, leading eventually to tendon failure at very high strains.

RMT has subcontracted work on three dimensional modelling of ground deformation around rockbolted underground openings to Exeter University (Cambourne School of Mines). The work so far has concentrated on assessment of FLAC 3D for the simulation of the effects of high horizontal stress on mine openings. This has included assessment of implications for face-line/gate intersections and solid headings (elastic). Previous difficulties due to limited calculation data points have been overcome and allowed a greater insight into stress distribution around a typical coal mine roadway. Only simple non-linear modelling has been performed to date. Difficulties have been identified within FLAC 3D in simulating complex constitutive failure criteria and conditioned unloading and these are being further investigated.

Targeted Rockbolting 2 - HSL

HSL is participating in the ECSC research project 'Targeted Rockbolting 2' with work on neural network analysis of coal mine roadway behaviour. The principal aim of the research is to evaluate the effectiveness of using neural computing techniques in the analysis of data from rockbolted roadways. It is anticipated that by analysing information from extensiometers, strain gauges and associated geophysical / geomechanical data a neural computing technique can be found that will complement current methodology to give a better estimate of the stability of the reinforced roadway. Data from both current and exhausted workings at RJB Mining's Prince of Wales colliery has now been analysed. A literature search is underway to identify research elsewhere in the field of applying neural computing techniques to the analysis of rockbolting data. The search is to be extended to cover other, non-neural, computational techniques associated with this field. The work continues.

Rock Mass Classification

The University of Nottingham Department of Mineral Resources Engineering completed a project for HSE on the application of rock mass classification principles, extensively used in tunnelling and hard rock mining, to the design of mining systems using rockbolts as their primary mode of support.

The initial UK Coal Mine Classification has been applied to 13 roof cores from gate-roads at an RJB mine. The selected rock cores have been photographed. Classification of each of the cores was initially undertaken by subdividing each borehole into strata units based upon lithology and / or bedding plane / parting plane frequency. Within the classification process the average strength and spacing of the parting/lamination and bedding horizons have been evaluated for each of the cores. This procedure has been carried out for specific gate-road sites at the mine. Laboratory test samples have been selected from the borehole cores from the investigation sites and an extensive rock test programme has been completed and a substantial database of rock properties established. Other mine sites have been targeted and the classification is being applied to selected investigation sites in specific roadways.

As laboratory data and detailed classification information has been gathered or became available, a detailed numerical modelling programme has developed for the selected rockbolted roadway investigation sites. Evaluation of the optimum representative mechanical properties of each of the rock units using the classification has been carried out. A series of detailed analyses have been conducted, examining the deformation processes of strata surrounding the rockbolted roadways applying the available constitutive models. Therefore the optimum constitutive models for use within the design of UK coal mine rockbolted roadways have been identified. Representative rock mass properties have been determined for Site 1 and each of the other investigation sites using the rock properties database and the UK Coal Mine Classification. Detailed numerical models of each investigation site have been constructed, assessed, evaluated and validated for the initial case study mine. Some detailed case study information from other mine sites has been gathered.

The project has shown the potential for a practical classification system which can be used in the design of ground control systems for UK coal mines. RMT, through a subcontract under the ECSC Targeted Rockbolting 2 project, is providing some funding to allow the work to continue and RJB Mining have agreed to provide further case study data from any of their mines.

Lifting and Suspension from Rockbolts

RMT's final report on this project was submitted to HSE in June 1998. The research was aimed at investigating the effect on coal mine roofs of lifting and suspending from rockbolts. The investigations examined the effect of lifting bolt loads on the surrounding rock and rockbolt support systems and possible long term and cyclic loading effects.

Field tests of anchor bolt systems were undertaken at three sites, using both partially and fully encapsulated anchor bolts. Measurements were also made of operational load variation during face retreat for anchor bolts used to suspend face-end equipment. In addition, an anchor bolt installed some five years previously was retrieved and examined and was found to be in good condition and still suitable for use. The field tests were simulated using computer modelling to confirm agreement between theoretical models and results obtained.

The research confirmed that loads applied to anchor bolts have negligible effects on surrounding roof strata. It was concluded that a roof fall caused by loading of properly installed bolts was most unlikely to occur, except in the extreme circumstance where the roof was on the point of failure in any case.

The most likely failure mode for anchor bolts used for lifting is bond failure due to poor installation, or loss of resin on installation. This is most likely to occur where installation is made into highly deformed roof, particularly if using the partially encapsulated type. Anchor bolt failure due to bolt deformation associated with subsequent roof movement is also possible. This is particularly likely where immediate roof shear results in significant bolt bending. Operational loading of lifting bolts used to suspend face end equipment varied widely. It was concluded that the system of monorail suspension used at the test site needed improvement to allow better distribution of loads between lifting bolts.

It was recommended that current guidance on anchor bolts used for lifting should be updated, based on the results from this work. The new guidance should include a requirement to keep records of bolt installation details and testing and deformation history where reuse is a possibility.

Ground Control for Small Mines

HSE has funded RMT to undertake a short project aimed at assisting small mines to improve support safety by applying new rock mechanics knowledge to traditional working methods, building on experience gained in larger mines. The work programme includes surveying and reviewing small mine working methods in conjunction with the HSE and the Federation of Independent Mines. A number of small mines will be selected to provide a representative range of working methods and conditions and site visits undertaken to assess strata behaviour, confirm layouts, estimate the stress field, identify any particular ground control risks and review the available data on geology, deformation history and support experience. At a sample of these mines additional work will be undertaken including evaluation of layouts through computer modelling, evaluation of hazards and risks, development of a ground control risk assessment for the mine and advice on risk reduction. The main outcome of the work will involve compilation of draft guidance on ground control suitable for small mines. Work on the project started in November 1998 with visits to nine mines, six of which were selected for more detailed study.

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Updated 2009-05-22