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Carbolt Self Monitored, Yieldable Carbon Fibre Cable Bolt for Ground Control

Underground » Strata Control and Windblasts

Published: April 21Project Number: C28019

Get ReportAuthor: Karsten Hoehn, Scott Barnes, Ian Greyvensteyn | Mining3, CSIRO

In general, there is a variety of rockbolts and roof support systems available on the market for mines to choose from. The most common type is the rock bolt, which consists of a metal rod with an anchoring component and a face plate. Some bolts are manufactured out of glass fibre reinforced polymer. They are more expensive than the traditional metal bolt but have special applications, for example on the panel sides of the longwall coal mines to protect the shearer from getting damaged as would occur if metal rockbolts were used.

In addition to these fixed-length rockbolts cable-bolts are available for longer holes and other challenging conditions.  The disadvantage of cable-bolts is that, although they have a high tensile strength, they are usually not designed to tolerate sheer stress. Hence, there is a technology gap and a need for a long or deep hole support system which is also tolerant to sheer loads that is also corrosion resistant. This project conducted a feasibility study for the next generation of advanced rock support systems in the form of a carbon fibre rope, the Carbolt, that can either be manufactured in pre-determined lengths, or in bulk and rolled onto a drum for deployment into a drill hole by a to-be designated machine before being cut to length as needed during the installation.

For single tows the full carbon fibre strength, as specified by the manufacturer, could be achieved when using a high extension (>130%) epoxy resin. When four strands of tow were combined the strength was reduced by 15-30% which is indicative of uneven tension in the tow strands, a risk when forming by hand and not seen in commercial pultrusion. The strength reduction in twisted 4 ply was of the same order (19-32%) indicating that the twisting was not impacting significantly on the overall tensile strength. The same fluctuations in the pre-tension of different strands caused by variations in the manual manufacturing process prevented the full Carbolt from achieving the desired load capacity. Nevertheless, it can be extrapolated that an industrially produced Carbolt at 510g/m (with a resin fraction of 60%) and 25mm diameter could have a strength in excess of 400kN (40 tonnes).

While there are more engineering refinements required in a potential next phase, the project was also able to demonstrate that the Carbolt can withstand significant shear forces. Due to its flexibility, it was able to deform, undergoing a displacement of 30mm before the shear test box ran out of travel. Further, the ductile behaviour, required to release the load stresses in the roof, was demonstrated with a Carbolt containing a modified core.

In a pre-commercial Carbolt development, twisting significantly longer carbon strands would allow uniform pretension in all carbon filaments during the twisting and cable forming phases to be achieved, as is routinely done in the commercial production of technical ropes and cables. This would then enable the Carbolt to achieve the tensile and shear strengths desired while retaining good ductile properties.

The Carbolt requires a novel locking mechanism that locks the carbon fibre strands of the rope without damaging them. This locking mechanism forms part of a new bearing plate design and a number of different designs have been investigated as part of this project.

Throughout the duration of the road way lifetime, wall and rock movement is evident, but often not monitored. The ability of integrating optical sensing fibres into the carbon fibre composite structure of the Carbolt has been demonstrated, enabling the monitoring of tensile stresses continuously along the bolt during future field trials.

Overall, during this project the feasibility of the Carbolt design has been demonstrated on a component level, bringing the concept from TRL 0 to TRL 2, see appendix C for details. In addition, a road map has been developed, see appendix B, outlaying the next steps to progress the Carbolt technology from the current feasibility stage to a number of possible realisations and application.

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