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Safety and health in mines research advisory board

Annual Review 2001


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 collaborative projects.   HSL, UK Coal Ltd, and the University of Nottingham have also worked on certain aspects.  Progress made during 2001 is described under various topic headings below.  In some cases, the work programmes described drew on more than one project.

During the year RMT undertook research on five safety related ECSC research contracts under the following 3-year collaborative targeted projects:

The first three projects commenced in November 1999 and will be completed during 2002. The latter two projects commenced in November 2000. HSE are contributing to the funding of these Projects through 4 contracts entitled:

During the year RMT completed a SIMRAC (South African Safety in Mines Research Advisory Committee) research project entitled “Suitable Long Tendon Technology and Practices”.  RMT and the University of Nottingham also began a new 3 year ECSC funded project, at the very end of the year, entitled, “Improved roadway and face end support techniques”.

The main technical advances during the year have been made under the following subject areas, each of which is discussed in more detail below:

Ground Control Instrumentation

Remote Reading Geotechnical Monitoring System

Installation of a full-scale underground remote-reading telltale system was completed during April at a German Colliery. The system did not go live until early July, due to certification discrepancies with the German power supply which had to be replaced with a UK manufactured unit. The system has been operating since then with some interruptions due to lack of maintenance. One particular problem encountered was due to the German mining industry’s use of sprayed calcium chloride solution in their roadways which required improved corrosion protection for the cable connections. The system has provided valuable data on the roadway performance during face retreat and, in particular, on roof behaviour in the front abutment area and behind the face where the roadway is being maintained open by cribbing for ventilation purposes.

Transducers have also been developed for the system to monitor roof to floor closure and to measure the load on standing supports. These are currently being employed in the South African Gold Mining Industry, where 3 systems have been installed, 2 for measuring stope closure and one for monitoring movements in a shaft pillar. These transducers do not currently hold European IS certification, but this could be obtained if the Industry wished to use them.

Acoustic Energy Meter

This instrument, for detecting broken mine roof and voids behind tunnel and shaft linings, has continued to be taken up around the world but is not currently in use in any UK mines. It is currently being evaluated for detection of loose, slabbing roof in the USA (NIOSH) and Australia (JCB) as well as being in everyday use in several South African coal, gold and platinum mines.  Recent tests in coal ribs at an Australian mine produced very promising results and further tests are planned.  A recent trial at a British Gypsum mine indicated that a more sensitive geophone was required for the particular roof conditions at the site. A new geophone has been manufactured and further tests at the mine are planned for the near future.

In-Situ Integrity Testing Of Rock Reinforcement

It was previously reported that the Radio Frequency system developed for measuring the length of reinforcement tendons (bolts, flexible bolts and cables) had been successfully used in a French coal mine with limestone roof, at Dinorwic Pumped Storage power station in a slate roof and at Middleton limestone mine, but was less successful in typical UK shaly roofs.  The aim of this work is to be able to determine whether reinforcement is broken in-situ due to overstressing or corrosion.

Analysis of the measurements in France indicated that the problem was due to the frequency range of the instrument used.  During the year a new instrument was built with a much greater frequency range and the first trials were undertaken at Mount Isa mine in Australia.  Following these trials the method has been used successfully at Boulby Potash mine in a dry salt roof and it is currently being used at Thoresby Colliery in the old Ollerton workings which were flooded for some 6 years. Initial results from Thoresby are very promising. However the method has the disadvantage of needing the reinforcement tendons to be electrically isolated from mesh and straps.

The ultrasonic bolt length measuring system has also been successfully used at Boulby and Thoresby. Its main drawback remains the time and effort needed to prepare the bolt ends.

Neither of these RMT systems measures the integrity of roofbolts before failure or of any of the parameters which could prevent further decay in the support system.  Amec has approached UK Coal with a system which could theoretically measure parameters on installed rock bolts like load, bolt to resin bond, resin to drilled hole bond, corrosion and bolt length.  After successful completion of surface trials, the system is to be assessed underground at Thoresby Colliery, starting early in 2002. 

Novel Methods For Detecting Broken Rock

RMT are working with Camborne School of Mines on this Project.  During the year, an extensive literature review was undertaken covering seven candidate physical principles, initial laboratory tests were completed on two of these principles and a feasibility study completed in order to narrow down the candidate principles to a smaller group of target methods for further investigation. The original candidate physical principles were as follows:

The laboratory tests were undertaken on Electromagnetic and Ultrasonic Emissions. Following these initial laboratory tests, a Russian built Electromagnetic Emission detection device (Impuls), designed for use in mines, was obtained in a non-operational state. This instrument is currently being examined to determine its principles of operation.

The feasibility study considered the following parameters for each principle:

The outcome was that the last two principles listed above were dropped from further investigation. The next phase of the Project will be to obtain the necessary equipment and to undertake further laboratory and underground tests/trials of the target methods.

Improved Support Systems For Highly Stressed Roadways

Standards Development

Assistance continues to be provided, under the auspices of the Rockbolt Research Liaison Committee, in drafting a revision of BS7861: Part 1. “Specification for rockbolting”. RMT’s main contribution has been development of the Laboratory Short Encapsulation Pull Test as a replacement for the Double Embedment Test, which is currently recommended in the BS for assessing rockbolting system performance. It was reported previously that it had been decided to reduce the embedment length in the new test to 160mm to ensure bond failure. This has required re-testing of all the main reinforcement consumables currently used in the UK. This work is now complete and this has allowed appropriate acceptance criteria to be recommended to the Committee. These are as follows:

  Steel Rockbolt Systems GRP Rockbolt Systems
Minimum system bond strength 100 kN 70 kN
Minimum system stiffness (between measured loads) 180 kN/mm (30kN-70kN) 60kN/mm (25kN-50kN)

The programme has included further tests on galvanised rockbolts. All load transfer test data on galvanised bolts was brought together in a single report which was presented to the Committee. The conclusions were that the testing programme indicates similar load transfer performance for galvanised AT rockbolts as for standard AT rockbolts and galvanised bolts have been incorporated into the new BS draft.

Double embedment and LSEP tests were also undertaken on new designs of single and double Garford bulb cable bolts. These did not reach the BS7861:Part 2 acceptance levels and the manufacturer has recently modified the bulb spacing and re-submitted for further testing.

Rockbolting Procedures

It has been suggested that, rather than using a ‘spin and hold’ procedure for installing rockbolts, a ‘spin to stall’ procedure could be advantageous in that it could eliminate potential for operator error and also speed rockbolt installation. However, the reason for using the current ‘spin and hold’ procedure is concern that stalling the rockbolt as the resin gels is likely to damage the resin bond. An opportunity to investigate these issues further was found at a South African coal mine which is currently using ‘spin to stall’ rock bolt installation.

Two sets of tests were undertaken to examine bond strength profiles achieved along bolts using a combination of pull and push tests in steel tubes. For the first set of tests, the bolts were installed into steel tubes in the laboratory using a handheld drilling machine. In the second set of tests the steel tubes were placed in holes in the roof of a South African coal mine and the bolts were installed using a Fletcher Bolter. These tests were undertaken using a 20mm diameter rockbolting system with single speed resin, which has been shown by underground pull testing and double embedment testing to have similar load transfer properties to the AT system. Following bolt installation, the tubes were cut into short lengths and pull and push tests were conducted.

In summary, the results indicated the following:

  1. Bond strength differences between 'spin to stall' and 'spin and hold' installation prior to nut tightening, were confined to within the first 350mm of bolt length.  Above this region there was no significant difference between the two installation methods, prior to nut tightening.  It is probable that this difference arises because the resin near the bottom of the bolt is the first to cure sufficiently to stall the bolt, and is damaged in the process.
  2. Mixing through the resin during bolt installation, rather than pushing through, did not result in any measurable increase in bond strength for ‘spin to stall’ or ‘spin and hold’ installation.
  3. The push test results indicated that the mean bond strength decreased by 24% progressively from the first push test position 400mm along the bolt to the last position, 100mm from the top of the bolt.  The effect was not dependant on the installation procedure, suggesting that this decrease arose either from the influence of remnant resin capsule packaging materials or to variation in resin/catalyst distribution following mixing. A reduction in load transfer of 24% in the top section of the rockbolt is a significant finding which could have implications for the European industry.
  4. Tightening the nut following bolt installation had no effect on the results obtained for ‘spin and hold’ installation.  However, for ‘spin to stall’, it had a significant adverse effect on two out of the three installations.  This was almost certainly caused by the action of tightening the nut inducing sufficient tension in the bolt to overcome the resin bond shear strength, and causing permanent bond strength reduction after full hardening of the resin.  This did not happen with ‘spin and hold’, probably because the resin had hardened sufficiently during the hold time to resist the subsequent tension applied.

The results indicated that ‘spin to stall’ installation, as currently practiced at the study mine, has two drawbacks in terms of bond strength achieved:

The second effect was more serious. Technical solutions to reduce or eliminate the problems are thought to be possible and are to be investigated.

It is suggested that European resin manufacturers should consider whether the loss of reinforcement at the top of the bolt measured with South African consumables is likely to be relevant to their products and, if so, whether this effect could be reduced. Further tests including the AT system have been proposed.

New Support Systems

UK Coal’s contribution to this project has included studies of the performance of thread bar and pre-tension long tendon roof and rib bolting systems against current standard roof bolt technology.

Thread bar rock bolts were initially trialled and monitored extensively in the ribs only.  Results in all circumstances indicated improvements in support standards and performance, reducing rib dilations in all cases to lower levels.  Following the finding of these improvements in ribs, a full roof and rib trial is being carried out at Maltby Colliery.

Early results from the installation trials of the pre-tension systems currently under review have been encouraging.  The first monitored trial was part of a supplementary support system in a high stress/deformation drivage at Rossington.  Roof monitoring results recorded a substantial reduction in roof displacement from the levels normally experienced.  It is now planned to extend the use of these systems to additional sites for further research.  Installation trials of a flexible bolt type pre-tensioned anchor have been completed at Harworth Colliery.  The system proved to be very efficient and early indications suggest its effectiveness in improving support characteristics in this difficult environment.   A full research project using this system is due to start in 2002. 

Support Systems Handbook

Also under this Project, RMT is preparing a Support Systems Handbook, designed to summarise the key properties and attributes of the many new support systems which have become available to Mining Engineers over the past decade.  A format has been agreed with the European partners on the Project and a first draft is being prepared with the assistance of the many support manufacturers.

Stability And Support Of The Sides Of Mine Roadways

In addition to the Acoustic Energy Meter surveys described above, a desktop study was undertaken on the possible application of polymer/cement based spray coatings for rib support in UK coal mines. The main system considered was a flexible membrane developed in the USA for mesh replacement. The manufacturers claim that it is designed to “permanently stabilize the integrity of rock structures accommodating the stresses associated with strata movement while providing a barrier to moisture degradation”. It is a water based, polymer modified cementitious spray material with optional fibre reinforcing.

It was concluded that the tensile strength and flexible nature of the system would best suit one of the four common rib deformation categories identified, namely bulging rib. Therefore it is most likely to be suited to an underground site where the ribs are failing in compression or shear and, in particular, where it can be applied before significant rib degradation takes place. Two possible UK mine sites were identified where trials would be most appropriate and a laboratory testing regime was designed. It is not certain whether any further work will be undertaken in this area, as it depends upon finding a willing trial site and commitment from the manufacturer.

A programme of underground rib drilling trials and pull tests, using standard UK rib reinforcement consumables, was planned to be undertaken at Riccall mine in the Stanley Main seam during the fourth quarter. Unfortunately the work had to be postponed due to a roof fall occurring at the mine. The work is now planned to commence during February 2002.

The risk assessment procedure previously developed for the Parkgate seam at Thoresby Colliery has been expanded to include risks from rib instability as well as roof instability. The revised risk assessment comprises a written description of the procedures involved, a purpose designed booking sheet, for recording the results of visual examination of roadway condition, and a series of spreadsheets for inputting the survey results and other relevant information such as monitoring data and remedial support data. The spreadsheets automatically produce tabular outputs for roof and ribs which identify a level of residual risk for each 20m zone of the roadway.

Incorporation of rib stability into the assessment has involved identification of probable rib failure mechanisms for the local conditions and consideration of the likely effects on rib stability of the main recordable parameters.

The revised assessment procedure also provides a guide as to possible appropriate remedial support for any given set of circumstances, though it emphasises that the appropriateness of remedial support for a particular risk should be ascertained by the Colliery Rockbolting Engineer through detailed on-site examination of the conditions and available monitoring data.  A similar risk assessment procedure is currently being developed for the Stanley Main seam at Riccall colliery following the roof fall there and investigations are underway to identify the fall mechanism.

NATURAL FLUIDS’ INFLUENCE ON EXCAVATION STABILITY AND SUPPORT

This project is focused on the effect of strata water and crude oil on the stability of rock bolted roadways.  ECSC funding is supporting laboratory testing at the University of Nottingham and fieldwork at UK Coal’s Thoresby Colliery.  The work has been in progress since September 1998 and is due to be completed in September 2003.  It has broadly identified the minimal short term effects on support system integrity but there is a time based deterioration dependant upon strata types, chemical composition of the water, effect of this on support materials and the importance of the geotechnical environment.  Research is currently in progress on long term corrosive effects using a nine year old wet roof bolted roadway in the recently reopened Ollerton Colliery as a field site.  A number of recent advances in the field of non-destructive testing of in situ rock anchors are to be applied.   

Numerical Modelling For Mine Design

Work has continued on developing improved means of modelling essentially 3 dimensional non-elastic mine design problems using the tools currently available. True non-elastic 3 dimensional modelling is now practical for room and pillar design where symmetry can be applied in 2 dimensions and work has continued in developing and improving these techniques. It has also been applied experimentally to consideration of roofbolt row spacing. For longwall layout and interaction problems a combination of elastic 3 dimensional and non-elastic 2 dimensional modelling remains the only practical method. This is currently being applied to modelling workings above, beneath and alongside existing goaf.

In-Situ Stress Characterisation Through Acoustic Emission (Kaiser Effect.)

The objective of this work is to assess the feasibility of using Acoustic Emission (AE) testing for rock stress determination. This is based on directly comparing AE results with those from stress measurements at the same site.  The initial results, reported previously, from both coal measures and gold mine strata, indicated that the Kaiser Effect can indicate the stress to which the samples have been subjected.  However it was not observed in all samples.

Further testing has been undertaken on quartzite strata from a second gold mine in South Africa where an in-situ stress measurement was undertaken.  The acoustic emission results from the maximum horizontal stress direction correlated very well with those obtained from the stress measurement.  However a Kaiser Effect was not identifiable from the samples tested from the vertical stress direction.

Numerous tests have been carried out on coal measures strata from three sites at a shallow coal mine in South Africa.  Three different types of strata, a dark siltstone, a fine sandstone and a coarse sandstone, have been tested. 

The results indicated that:

  1. The Kaiser Effect was not detectable for the vertical and minimum horizontal stress directions.  This was because both these stresses were less than 5 MPa and therefore were within the area where AE can occur associated with ‘bedding in’ of the sample relative to the loading platen.
  2. Identification of the Kaiser Effect may be rock type sensitive. The coarse grained sandstone samples were able to indicate a clear Kaiser Effect in some cases whereas the results from the siltstone samples were more difficult to interpret
  3. Even for relatively low stress, 15 MPa, the stress indication from the Kaiser Effect correlated well with that measured.
Updated 2013-12-23