Safety and health in mines research advisory board
Annual review 2006
Geotechnical classification, modelling and exploitation for safe and efficient mine layout and tunnel support design
Practical work by Rock Mechanics Technology Ltd (RMT), UK Coal Mining Ltd and the University of Nottingham on this RFCS collaborative project, RFC CR 03011, was completed in August 2006. Since then, RMT, as Project Co-ordinator, has been preparing the final report, which will include the contributions of all the other European Partners.
The Project has mainly involved development of numerical modelling tools for improved mine layout and support design, particularly 3D modelling of strain softening rock behaviour. The work included considerable rock testing, incorporation of the rock testing results into modelling techniques and improved scaling of laboratory test results to in-situ conditions. RMT’s work has concentrated on developing new three dimensional modelling techniques to solve practical problems of mine layout and support design and improved modelling of goaf and abutment stresses. UK Coal has provided access to underground sites and other practical assistance.
Particular examples of the application of these techniques during the Project by RMT have included:
- modelling of coal rib deformation and reinforcement at a mine with a highly deforming coal rib; this has provided a basis for evaluating the probable effectiveness of alternative rib reinforcement consumables for the mine and is highly relevant to the work now underway at the mine following a recent fatality from a rib fall. In particular the work indicated the order of magnitude of bond strength required from a reinforcing member to be effective in a coal rib. This provides a target for laboratory short encapsulation pull testing in coal and underground pull testing of alternative consumables,
- comparison of the effectiveness of remedial support through placing flexible tendons in the roof to act as trusses/slings or to act as rock reinforcement,
- initial examination of the effectiveness of pretensioned flexible tendons,
- examination of many mine rockbolting patterns to optimise row spacing as well as bolt spacing within each row,
- modelling of many alternative layouts where longwall panels were to be affected by vertical and horizontal interaction.
Development of more innovative support systems for gateroads under the influence of rock stress
This three year RFCS project, CT-2005-00002, has just passed its half way stage. RMT’s main activities under the Project are:
- continued development and application of coal mining related geotechnical instrumentation;
- development of appropriate laboratory tests for assessing the performance of flexible reinforcement systems, feeding into the committee revising BS7861:Part 2:1996; and
- comparison of tensioned and untensioned cable bolt behaviour, also feeding back to the BS7861:Part 2 committee.
An important issue regarding coal mine geotechnical instrumentation, which is being addressed under the Project, is the obsolescence of two of the main instruments currently in daily use for monitoring rockbolting roadways, namely the Sonic Probe Extensometer and Strain Gauged Rockbolt Readout. Both have been discontinued by the manufacturer, Soil Instruments, and all stocks have been depleted. No alternatives were available on the world market.
Progress to date is summarised below:
- A potential replacement sonic extensometer was sourced but problems have been encountered with interfacing to the new probe. This issue is under investigation, but the industry should assume, at least for the time being, that the sonic probe extensometer is obsolete and may not be replaced with a directly equivalent instrument.
- ATEX approvals were granted for a portable remote reading extensometer readout and new multi-height versions of RMT’s remote reading extensometer. These were successfully field trialled at non-UK sites (India, Spitsbergen, Czech Republic). These instruments are now commercially available in 2, 3 and 4 height versions as well as a fully groutable multiheight version. They are about to be used for the first time at a UK colliery. We believe that they are the best currently available alternative to the sonic probe extensometer.
- A new datalogging strain meter has been developed and successfully field trialled in a Norwegian mine. This will also shortly be used for the first time in a UK Coal mine under Regulation 19 (2) g procedures, as it is not ATEX approved. This is now commercially available as a replacement for the Soil Instruments strain gauge bolt readout.
- New designs of strain gauged rockbolts (forged head and smaller diameter bolts) were developed and successfully trialled at Boulby Potash.
- The EXBOLT analysis software was updated for compatibility with the new instrumentation described above.
- A bolt integrity “Optobolt” was developed but its effectiveness has still to be determined. Ten of these bolts are being field trialled at Thoresby colliery.
- Laboratory grout pumping tests for flexible cable bolt systems were devised for the revision of the British Standard and successful tests achieved with “bottom-up” systems.
- Tests with “top-down” grouted systems, such as the Megastrand, indicated significant problems with the thixotropic grout which is required for this type of cable bolt. It was found that it was problematic to mix and pump the grout at the manufacturer’s recommended water to solids ratio using the standard equipment available at the mines. The manufacturer has developed a new thixotropic product which has still to be field trialled and approved for underground use.
- A field instrumentation exercise to monitor the installation of tensioned flexible tendons was rescheduled due to operational problems at the site.
- Monitoring data from Harworth colliery, where Megastrands were used extensively before its closure, has been obtained and analysis of this data has commenced in order to improve our understanding of their effectiveness and in-situ characteristics.
- A new test rig for plastic and steel mesh was developed.
Properties of flexible strand rockbolts and long tendons
HSL’s Metallurgy & Materials section was asked by HSE to characterise the mechanical and metallurgical properties of stranded rockbolts and long tendons as a contribution to the revision of the British Standard for rock reinforcement in mines. A wide range of different stranded rockbolts and tendons are used by the deep mining industry. These can be used with polyester or cementitious grouts. The purpose of this work was to establish base line mechanical properties and testing methods for different types of rockbolt and grout for incorporation into the British Standard. The work has shown that it is difficult to standardise a method for tensile testing because the results are affected by failure at the rockbolt / grout interface. This stage of the work is complete.
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