Clarification Summary in response to a readers quarry: 'Installation of resin-grouted rock bolts in hard rock mining: Challenges and solutions for improved safety'

T.G. Maepa; T. Zvarivadza

School of Mining Engineering, University of the Witwatersrand, Johannesburg, South Africa

Annulus size

Hagan (2003) stated that the relationship between the diameter of the support hole and the diameter of the rockbolt has a large effect on the efficiency of reinforcement. The difference between the radius of the support hole and the radius of the bolt is known as the annulus size. Since the design diameter of the rockbolt is 25 mm, the design annulus size is 3.5 mm. According to the mine support standard, the required annulus size should not be greater than 5 mm. The design annulus size of 3.5 mm is within the required range as stipulated in the mine's code of practice. The design annulus size is also within the manufacturer's recommended optimal design annulus size of between 3-4 mm. From this observation, the design is anticipated to be highly efficient.

Optimizing the support design

In practice, the support holes require 8% more resin per hole. Possible solutions to this problem would be to increase the sizes of the rockbolts, increase the sizes of the resin capsules, or reduce the sizes of the support holes. Increasing the sizes of the rockbolts and resin capsules will have serious cost implications. Therefore, the size of the support holes should be reduced. Many authors have suggested different optimal ranges of annulus size. The manufacturer of the resin capsules that are used at the mine suggested an optimal design annulus size between 3 mm and 4 mm. While Hagan (2003) suggested an optimal practical annulus size ranging from 4 mm to 6 mm. The specifications from the manufacturer also state that the optimal length of the support holes for the resin bolt installation should be 50-60 mm shorter than the bolt length.

The support holes should be drilled at an optimal length of 2.34 m with a drill bit of 30.5 mm in diameter. This will result in a design annulus size of 3.15 mm [(30.5-24.2)/2], which is within the manufacturer's recommendations of between 3 mm and 4 mm. Taking into account the reaming of the support holes, the final diameter of the support holes will be 32 mm. This results in a practical annulus size of 4 mm, which is also within the optimal practical annulus size ranging from 4 mm to 6 mm. Summary of the optimal support design:

> Bolt length: 2 400mm

> Bolt diameter: 24.2mm

> Support hole length: 2 340mm

> Final support hole diameter: 32mm (30.5mm diameter drill bit plus 5% reaming)

The authors' recommended support design is presented in Table V. The design will not only enhance safety at the mine but it will also reduce the costs associated with mine support. With such an optimal support system, the mine can save costs by reducing the size of the slow setting resin capsules from 500 mm by 25 m to 400 mm by 24 mm. The system is still efficient in terms of support, with 2% excess resin resulting in a waste of R1.20 per hole. The 2% excess resin will come in handy where there are geological discontinuities and resin may seep into the discontinuities. With the decrease in drill bit diameter and the length of the support hole, the mine will also reduce costs associated with drilling.

Economic benefits of the investigation

With the standard mine support design, the Andre Se Resin Anchor calculation indicated that the mine is wasting resin and losing R12.61 per support hole. The optimal design recommended by the authors will provide the mine with an efficient support system with a loss of only R1.20 per support hole, thus saving R11.41 per hole. This is a huge reduction in costs considering that the design is till efficient and providing full security. A total of 5 021 rockbolts were installed in the last quarter of 2015 (Table I in the paper). With a cost reduction of R11.41 per support hole, the mine could have saved R57 289.61, sufficient for the installation of 47 741 more rockbolts.

Figure 8 shows the total amount that could have been saved per month based on Table I and the reduction of R11.41 per support hole, as a result of the optimal design. Furthermore, addressing the design and operational inefficiencies will also result in a decrease in number of rockbolts that are installed incorrectly at the mine. The costs associated with the replacement of incorrectly installed resin-grouted rockbolts will thus be reduced significantly.

Conclusions

Decreasing the design diameter of the support holes from 32 mm to 30.5 mm, supplemented by a decrease in design length from 2.4 m to 2.36 m, result in an optimal support design and a reduction of money wasted per support hole from R12.61 to R1.20.

> The standard mine support design should be optimized by reducing the diameter of the drill bit from 32 mm to 30.5 mm and reducing the length of the support hole from 2.4 m to 2.36 m. These changes will enable the mine to use different combinations of resin capsules to reduce the cost of installation per support hole.

The paper was first published in the April 2017 issue of the SAIMM Journal and a reader requested clarification of a particular issue.