Published in June 2007 Australian Longwall Magazine

The Hilti “OneStep” Self Drilling Bolt was first introduced into the Deutsche Steinkohle (DSK) coal mines in Germany in 2004-05. The aim was to achieve quicker, stable and consistent bolt cycle times. The results were increased development rates with areas achieving 50–100% improvement, the quality of bolting standards improving and overall bolting safety standards increasing dramatically.

Hilti is currently installing the OneStep Bolts in Australian coal mines, with large-scale test sites at BMA’s Broadmeadow mine in Queensland, Solid Energy’s Spring Creek mine in New Zealand, Centennial Coal’s Mannering bord and pillar operation and specialised projects at Centennial’s Angus Place in New South Wales.

The Hilti OneStep Bolt combines the drill bit head, drill steel, bolt and adhesive resin in a single unit. The two-component adhesive resin system is contained in a tubular cartridge. This resin cartridge is located in a steel inner tube enclosed in a second 4mm gauge steel tube. The design of the bolt ensures the resin cartridge is completely protected and will not damage or leak.

To use the bolt, a dispenser is fitted to the drill motor on the bolting rig and the OneStep Bolt is inserted and the drilling operation commences. Drill fragments/fines are flushed out by the flow of water through the annular gap between the bolt and the drill hole. The flushing water is fed to the bolt through the gap between the inner and outer tubes.

After reaching the intended drilling depth, dispensing is begun encapsulating the bolt with mixed resin.

According to Hilti mining sales and marketing manager Andrew Worth, the productivity gains using the OneStep can be measured in two ways. Firstly, faster bolt installation and improved bolt installation processes lead to a reduction in bolt cycle time.

“Special test results obtained from mines using the OneStep suggest that there is a high probability that primary ground support densities can be reduced as well as secondary support requirements. Improved strata reinforcement capability derived from the quality of resin mixing and encapsulation results in the total utilisation of the bolt length installed,” he said.

“By completely bonding the roof bolt the load generation in the bolt will be higher and will be applied to a greater proportion of the roof strata.”

Eliminating many of the installation processes and improving face management through the ability to quickly and safely place bolts where roof instability is occurring are also major benefits for the bolt.

Angus Place trials

“During the introduction phase of the Hilti system into Australia and New Zealand we realised that there were specialised applications where the OneStep Bolt could be used,” Worth said. Following a presentation of the bolt at Centennial’s Angus Place, personnel at the mine were keen to investigate several applications where the OneStep Bolt could prove very successful. “They realised the inherent benefit of the One Step Bolt being able to be installed remotely from the operator, thus removing personnel from potentially hazardous strata conditions,” he said.

During development drivage at Angus Place in geologically disturbed areas of the mine, roof guttering and cavities formed at the face and then tended to increase in height and width until they were unable to be supported by the bolting rigs on the continuous miner. In the past, hand-held roof bolters operated out of a QDS work platform were used to support the roof in the cavity areas.

In June 2006 Angus Place approached Hilti to investigate the viability of a self-drilling bolt solution for a spiling application being worked on by the mine and southern NSW roof bolter manufacturer Rambor.

“Their idea was to drill and install 2.5m Hilti OneStep Bolts at an approximate angle of five degrees to the horizontal into the face of the development roadway at roof level in the area of the cavity. This would then break the cycle of guttering and stabilise the roof in the spiled area. The continuous miner would then be able to continue the development cycle with minimum disruption to the drivage schedule,” Worth said.

With some minor modification to the system and operator familiarisation, the spiling rig successfully installed the full round in an eight-hour shift with only two operators (this included the mounting and dismounting of the rig from the continuous miner).

The use of the Hilti self-drilling bolt not only enhanced the operational efficiency with a significant reduction in spiling cycle time, but also enhanced the safety of the spiling process. This process allowed operations to be conducted at a safe distance from the face and under supported roof.

The next challenging application where the Hilti OneStep Bolt was used was on the longwall face bolt up. Due to the weak geological nature of the coal face and high longwall abutment stress, previous attempts at supporting it with conventional resin anchored bolts failed due to hole closure. Sprayed products had also been trialled but were unsuccessful due to ongoing convergence causing spalling. Friction bolts had been used but involved hand-held bolting equipment, and Angus Place wanted to reduce operator exposure during face bolting operations.

It was decided to mount two Rambor longwall face drilling rigs with a custom designed slide assembly to the ranging arm of the shearer and install Hilti OneStep Bolts into the longwall face. Two teams installed the bolts up to a rate of 100 bolts per shift.

“The OneStep eliminated the collapsing drillhole issue as it always remained in the hole during the installation process and if coal broke away the operator just kept drilling thus maximising the effectiveness of the bolt length. As with the spiling application all operations were conducted at a safe distance away from the coal face,” Worth said.

“Both the spiling and longwall take-off were very successful at Angus Place and Hilti are proud of the performance and safety delivered by the Hilti self-drilling bolt during these projects.”

The next challenge for the OneStep at Angus Place is for standard rib bolting. Hilti and Angus Place are currently conducting time studies and process mapping to evaluate the suitability. Hilti is also looking at ways of maximising the confinement of the rib to prevent ongoing spalling.

Finally the target market for the OneStep is in roof and rib bolting. Hilti is currently in the latter stages of a major roof bolt trial at Broadmeadow where over 5000 OneStep Bolts have been successfully installed in the roof. The focus of the trial is to deliver productivity gains and improve safety standards by installing an efficient and effective system of ground support.

Results of the large-scale trial will be published in the near future.