Application of 50 to 60 Tonne Cable Pre-Loads to Roof Control in Difficult Ground Conditions

This project has successfully met all aspects of the following original objective: "to establish a package of existing technology items that allows the installation and pre-loading of cable tendons in the range of 50 to 60 tonnes and to conduct installation trials and basic field evaluations in the underground environment".

An initial survey and discussions with suppliers lead to the development of tensioning equipment that not only proved to be robust and mineworthy, but markedly lighter, such that early safety concerns related to ergonomics were quickly addressed. Risk-based operating procedures were developed that facilitated the application of up to 50t of cable pre-load without significant incident.

Project trials have produced good results in some of the weakest, most difficult roof conditions in the industry. The sites covered a number of situations, namely:

• a maingate travel road on development (Crinum Mine),
• two longwall installation roads during widening (Oaky North Mine) and
• a maingate belt heading during longwall retreat (Wyee Colliery).

Monitoring indicated the following benefits of increased levels of pre-load:

• Improved roof stability during roadway development, including better control of long-term creep and reduced guttering.
• Improved roof stability in wide excavations.
• Improved roadway serviceability under longwall abutment loading, including delayed roof displacement and again reduced guttering.

The results are consistent with the results of earlier research, that demonstrated the benefits of pre-tensioned over passive cables.

The level of roof skin and mesh deterioration associated with tensioning to levels of up to 50t was at worst minor. Furthermore, improved longer-term roof behaviour associated with higher levels of pre-load was associated with reduced skin deterioration both over time and under longwall abutment loading.

Optimisation of reinforcing cable systems requires the intelligent use of quantified experience (i.e. the application of databases of monitored roof behaviour as part of a strata management plan). This project has shown that both local and industry databases can be very powerful (and cost-effective) tools.

Finally, the Crinum trial demonstrated that improved roof behaviour can be achieved at a reduced cable density and therefore a lower overall cost at higher levels of cable pre-load.