Bergman, A, Fjellner, E, Hallén, P, Penczek, R, Pérez Hidalgo, K, Scolari, F & Svanberg, M 2023, 'Adopting hollow bar: lessons learned', in J Wesseloo (ed.), Ground Support 2023: Proceedings of the 10th International Conference on Ground Support in Mining, Australian Centre for Geomechanics, Perth, pp. 301-316, https://doi.org/10.36487/ACG_repo/2325_20
(https://papers.acg.uwa.edu.au/p/2325_20_Bergman/)
Abstract: Boliden strives to increase efficiency and quality, and to enable automation during bolting operations. To this effect, a trial with hollow bar rockbolts with pumpable resin as grouting was performed at the Renström mine. As part of this trial, tests were carried out to verify the capacities of the new bolt and determine the required anchor length needed.
In the initial testing during 2020 using a hollow bar from DSI Underground (DSI) with standard steel (R25, 22 t ultimate strength, 5% deformation capacity) results showed that the ultimate strength was good, and the anchor length was slightly longer than for a rebar bolt. The deformation capacity was, however, not sufficient.
For the 2022 trials, DSI supplied a bolt with modified steel characteristics (Ductile R25, 15% deformation capacity) that, in external lab tests, showed the desired deformation capacity together with the required strength of the grouting material. The first tests of the field trial failed to repeat these results and the expected anchor length was not achieved. A program of eliminating error sources and finding alternative solutions followed.
The trials revealed that several factors played a part in achieving the desired anchor length. A replacement for the current rebar bolt needed at least 500 mm of anchor length and therefore, hollow bar bolts of 2.9 m length were ordered for future use.
This paper summarises the investigations, changes and lessons learned in these trials, with respect to anchor lengths, curing time, impact of temperature and installation practices.

Keywords: pumpable resin, hollow bar, short encapsulation, pull-out test