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Safety and health in mines research advisory board

Annual Review 2003


CONTENTS

GROUND CONTROL

Introduction

Ground control research continues to be a major part of the portfolio reviewed by SHMRAB. The majority of the work was carried out by Rock Mechanics Technology Ltd (RMT) through ECSC and RFCS collaborative projects. UK Coal Mining Ltd and the University of Nottingham have also contributed to these projects. Progress made during 2003 is reported below.

Improved support systems for highly stressed roadways.

One of RMT's main activities under ECSC Project PR058 was to develop the Laboratory Short Encapsulation Pull (LSEP) test as a suitable test for rockbolt reinforcement system evaluation for inclusion in a revised version of BS 7861:1. This goal was achieved and the draft revision was submitted to the BSI at the beginning of 2003. This draft will now be subject to further consultation prior to adoption.

A number of interesting and important results concerning testing of GRP rockbolts for rib reinforcement were identified at the very end of the research period. This was a result of testing several new GRP bolt designs with "engineered" profiles, formed either by moulding during production or machining of the finished bar.

It was found that the form of the rib profile had a pronounced effect on the level of axial reinforcement provided by the bar, particularly at the resin/rock interface. Bar with a concave profile produced considerably less axial reinforcement than bar with a convex rib profile. This difference was only shown up by the LSEP test and would not have been revealed by the previously used Double Embedment test.

It also became apparent that the machining of a GRP bar could, in some circumstances, lead to premature failure of the bar due to debonding between concentric layers of strand within the bar itself. Retention of the Double Embedment test in the Standard was therefore recommended for GRP bar as a measure of the ultimate tensile strength of a GRP bar/resin system.

A "Handbook of Mine Support Systems" was also produced under the Project and is now available for reference purposes.

Automatic ground hazard warning systems.

Completed in April 2003, RMT's work on ECSC Project PR059 covered the development and demonstration of a Remote Reading Telltale System. The system can incorporate up to 400 telltales with readings every 20 minutes displayed and stored on PCs on the colliery network. The first full system demonstration was successfully operated at DSK's West mine in Germany in a rockbolted roadway which was maintained open behind the longwall for ventilation and access purposes. The system is currently being installed in a second German colliery and is deployed with battery powered, portable readouts in two Indian room and pillar mines.

Several innovative extensometers were also developed which incorporate the basic electronics of the system. These include a "blast proof" extensometer, for use in shot fired headings, a remote reading four point extensometer, for use where more detailed data is required, and a single point remote reading wire extensometer for monitoring convergence and rib dilation. The "blast proof" unit has recently been used as the standard extensometer in the Cliffe Hill hard rock quarry tunnel at Markfield, Leicestershire. Other innovative extensometer designs, including a groutable bay extensometer based on the same components are currently being developed for use in railway tunnels.

Improved understanding of reinforcement behaviour and testing

Completed in October 2003, RMT's work on ECSC Project PR092 involved development of two non destructive methods of determining the in-situ integrity of fully grouted rock bolts. Both methods, Ultrasonics and Radio Frequency, were shown to be applicable in coal mining environments and were successfully applied in UK coal mines. Particular progress was made with the radio frequency system, which had previously only been successful in non-UK coal measure geologies. This success was achieved through improved understanding of the underlying principles and the resulting development of improved instrumentation. The system was used successfully following a fall of ground at Welbeck colliery. It was also employed with excellent results to survey installed rockbolt condition at a major UK pumped storage power station and in a civil engineering road embankment. The system is now a commercially viable non-destructive testing tool available to the UK mining, tunnelling and civil engineering industries.

Another area of work on this Project was the study of the risks associated with the use of mixed support systems (steel and rockbolts) in UK coal mines. Analysis of the statistics showed that mixed support systems were particularly high risk and recommendations were made concerning how the risks could be reduced. In particular it was recommended that mixed systems in rectangular and cambered arch profile roadways should be monitored in a similar manner to rockbolted roadways and, except for very robust steelwork, the assumed contribution of the steel support should be limited to one third of the bolted height.

UK Coal also contributed to this project with studies that led to the satisfactory introduction of pre-tensioned long tendons to reinforce high risk roadways. At Daw Mill Colliery the research focussed on wide roadway drivage and horizon control to change rock mass reinforcement requirements. The work completed at both Wistow and Kellingley concentrated on lateral developments supported by rock bolts.

Advanced geotechnical instrumentation for detecting failed rock and measuring support loads

Also completed in October 2003, RMT's work on ECSC Project PR091 involved evaluation of a number of physical principles which may be able to be exploited in a portable instrument for remote detection of broken rock in mines, particularly loose roof. Of the principles examined, the two which showed most promise were Thermal and Acoustic response.

Thermal response is to be examined further in a new Project, part funded by the South African, Safety in Mines Research Advisory Committee, which is due to start shortly. In this Project, thermal imaging equipment will be purchased and deployed underground in areas where hot rock can be rapidly cooled by application of sprayed cooling water to achieve a large thermal gradient. These were the conditions identified under the ECSC/HSE project for the thermal response method to be most likely to succeed.

The project identified that a Digital Laser Vibrometer is a suitable instrument remotely to detect induced vibration in rock and that the signal from the instrument can be analysed using similar algorithms to those used in the currently available Acoustic Energy Meter to determine the condition of the rock surface. The biggest problem associated with developing a practical instrument to utilise this principle is difficulty in remotely transmitting sufficient vibrational energy into the rock mass.

Improved roadway and face end support techniques

RMT's work on the ongoing ECSC Project PR115 is examining the properties of the main cribbing and standing support systems currently used in the UK with a view to incorporating their load/displacement characteristics and other data into a computer program (STOP) developed by NIOSH in the USA. This will allow mine operators to design/select their cribbing systems with greater precision and due regard to comparative cost and performance.

During 2003 a container load of UK crib material (hard wood, soft wood and Link-n-Lock) was despatched to the USA and tested in NIOSH's mine roof simulator in Pittsburgh. The results of these tests are currently being examined and compared with the predictions of the current version of the STOP program, with a view to amendment of the models in the program where necessary. To date the softwood data has been analysed and shows agreement to within 10% with the STOP model. An interesting result of note is the achievement of double the strength from a 9 point crib when compared with a 4 point crib, for only 50% extra timber. Thus, deployment of 9 point cribs at half the density of 4 point cribs could provide the same support resistance with 25% less timber

Tests have also been conducted on a range of popular South African free standing support systems to examine whether any may have application in Europe. This data is also being analysed currently.

A field measurement program is underway to measure the performance of Link-n-Lock and Starblock cribs deployed in a UK Coal mine tailgate under severe loading conditions. This work will allow comparison of laboratory and in-situ performance, assist in identifying the ground reaction curve in this environment and assist the mine in determining how the cribbing system can be improved to achieve safer face end and methane drainage drilling conditions. The crib load measurements are being made with vibrating wire stress meters and crib displacements are being measured using an RMT developed wire/visual indicator system.

The University of Nottingham's (UoN) contribution to this project aims to investigate and help optimise the design of 'mixed' support systems where free-standing supports, such as steel arches, are used in conjunction with rock bolts.

The main body of work undertaken has been concerned with computer modelling of a conceptual coal mine roadway. The modelling was undertaken to assess the effects and interactions of the different components of a mixed support system. The models were developed to assess a range of stress and geological conditions for three different support systems, selected to allow comparison between using a mixed support system and conventional support systems.

The modelling showed that the substantially different mechanisms of strata reinforcement and restraint provided by the steel supports and rock bolts introduce a level of complexity when attempting to understand and predict the reinforcing action of a mixed support system and the relative interactions and contributions during the life of a roadway. The outputs of the models were analysed in the context of the mechanism of load transfer from the rock strata onto the rock bolt support system and onto the steel freestanding supports and were also related to the modelled roof displacements.

The research has involved three dimensional computer simulations using the software code FLAC 3D (Itasca). The three dimensional modelling simulates the development of a rectangular coal mine gate road, the installation of the support system behind the tunnel heading and the progressive loading of the support system as the heading advances. The outputs of the model are being analysed to provide a better understanding of the mechanism and contributions of the different elements of the support system. Support loads in terms of axial loading of the rock bolts and bending moments generated in the steel supports are being evaluated to understand the effectiveness of a mixed support in comparison to a rock bolted support system only.

Improved roadway drivage and ground control under high stress conditions.

RMT's work on ECSC Project PR132 is concentrating on laboratory testing of reinforcement consumables, particularly flexible systems, with a view to revision of BS7861:2. A sub-committee of the Rockbolt Research Liaison Committee has been set up, chaired by HSE, to prepare a draft revision of the British Standard, which currently only covers birdcaged cable bolts. The aim is to develop a standard which will apply to the range of flexible tendons currently in use in the UK coal mining industry, including flexible bolts spun into resin, bird caged and other cable bolts, which use cementitious grouts, and tensionable systems such as Megastrand which are being employed increasingly.

RMT's work to date has included commissioning a new 500T testing machine for controlled LSEP testing of tendons and initial LSEP tests on flexible bolts to determine a suitable embedment length for the BS system test. Currently it appears that 450mm will be suitable which coincides with the minimum length that can be used with most bulbed cables.

Double embedment tests, in accordance with the current British Standard, have been carried out on a PUR product aimed at being used as a substitute for more conventional cable bolt grouts. Initial results were very poor in comparison with currently used grouts and polyester resins. Alternative formulations are currently being prepared by the manufacturers. This is an example of the potential dangers of not keeping standards and guidelines up to date.

Another area of investigation under this Project has been a number of innovative reinforcement concepts from South Africa, including a version of the Swellex system which retains the applied water pressure and a pumpable "doughnut" which can be used as a low cost rockbolt plate to apply pre-tension. Initial tests with the latter using strain gauged KT rockbolts in a limestone mine have indicated that high levels of pre-tension can be achieved and retained very easily (in the region of 10 Tonnes).

The objective of the University of Nottingham's input to ECSC project PR 132 is to investigate and research the use of pre-tensioned bolts and cables in a coal mining environment and to investigate the beneficial affects of the use of pre-stressing supports on the immediate strata surrounding a coal mine excavation.

Previous research has determined that the tension force applied to the rock bolt causes a compression zone around the bolt. This concept has formed the basis for various theoretical approaches. Basically it has been identified that the vertical confinement pressure generated by the pre-tensioning of the bolt improves the stability in three different scenarios:

  1. By maximizing the residual strength of the roof;
  2. By increasing the elastic modulus of the host rock;
  3. By utilizing the mechanical advantage inherent within a beam.

The three scenarios depend on the magnitude and distribution of the confinement stress in the rock strata, which the pre-tension load generates. Numerical modelling using the finite difference software code FLAC is being used to quantify the confinement stress associated with pre-tension load and to predict the effect that pre-tensioning rock and cable bolts have on increasing the stiffness of the immediate strata around a coal mine tunnel. It is proposed to develop a mathematical relationship between confining pressure and strata stiffness for each major coal measure rock type tested.

Three different model arrangements have been developed to investigate the confinement in the rock strata around the pre-tensioned bolt or cable. The first was to quantify the confining stress generated from a single pre-tensioned cable in a rock sample. The second was to investigate the interaction between four pre-tensioned cables in a roadway. The third was used to address the effect of the pre-tension load on the stability of the roof. This modelling is continuing.

Improved Rock Stress Measurement Techniques

RMT's work under ECSC Project PR135 has continued to concentrate on development of stress determination through detection of the Kaiser Effect in acoustic emissions from laboratory compression testing of oriented rock samples. Work to date has shown that this is a viable method of measuring stress when there is already a known indication of principle stress directions. It has also shown that it is not suitable in its current stage of development for measuring stresses below 5MPa and that it appears to be better suited to measurements in rocks with a high quartz content.

The Project is being conducted in collaboration with Camborne School of Mines and recent work has involved investigation of a new testing method involving cyclic loading of the samples. This is intended to improve the lower range limit of the method by avoiding "bedding in" effects which are experienced during the first loading cycle. A series of tests has been undertaken with oriented sandstone core from a South African colliery. It is too early to gauge the effectiveness of the new methodology as detailed data analysis is currently underway.

Also under the Project, a data logging system and new orientation tool were designed for use with overcoring of Hollow Inclusion cells. The prototype system was successfully trialled during a stress measurement program at Tara mine in Eire. It is now being improved into a mine worthy unit.

The aim of UoN's work on this project is to develop numerical modelling techniques to accurately predict physical characteristics in a mining environment. It is envisaged that this will aid the development and planning of coal mines.

The work during 2003 consisted of the numerical modelling at two distinctly different UK coal mines, each with their own support systems and environments. The first site chosen was UK Coal's Harworth coal mine which is an underground mine working at a depth of 1000m. The second was at Hay Royds, a small privately owned coal mine which uses a room and pillar support system at an approximate depth of 200m. Work has also been conducted on the uncertainty of geotechnical design in tunnels, by varying a single parameter to identify it's significance and effect. This work continues.

A two stage analysis technique was developed. The first stage involves taking each influencing parameter in turn, varying it across its range with all other parameters set at their mean value. This initial analysis gives the user an idea of those parameters that have the greatest influence. After examination of all the influencing parameters the six most important are selected for the next stage.

Stage two involves selecting representative values for the parameters. The most influential parameter needs to have perhaps 5 or 6 values across its range. The least important perhaps 2 or 3, unimportant parameters have 1 value. A dedicated Visual Basic program has been developed to write and then run the 1215 sets of input data and abstract the key output data from the finite difference package.

Initial findings are that this technique can provide a good prediction of the general trend and extreme value of tunnel deformation by varying each input parameter. This further allows the input variables to be quantified as to their importance to the model.

Geotechnical Classification, Modelling and Exploration for Safe and Efficient Mine Layout and Tunnel Support Design

RMT's work under this new RFCS Project, CR03011, will concentrate on improving abilities to employ numerical modelling to solve mine layout and support design problems in UK coal mines and thus avoid falls of ground. This will incorporate the latest 3 dimensional numerical codes (such as FLAC 3D) where appropriate. The first work under the Project has been to begin to model a very complex mine design problem where a new seam is to be exploited with old workings both above and below. It is expected that this site, which suffered a major roof fall during the year, will require most aspects of geotechnical modelling to be applied, covering seam interaction, sub surface subsidence, pillar design, roadway deformation and both tensioned and non-tensioned reinforcement. A significant amount of rock core testing has already been completed.

Updated 2013-12-23