Rock support testing in Luossavaara-Kiirunavaara AB's underground mines, Sweden

The mining company Luossavaara-Kiirunavaara AB (LKAB) operates two underground iron ore mines in Sweden. Both mines use large scale sublevel caving to extract the ore at a depth of nearly one kilometre and are, since 2008, regarded as seismically active. Rock support is therefore essential in order to maintain safe and accessible production areas. The support system used by LKAB primarily consists of steel fibre reinforced shotcrete together with external steel mesh and rockbolts. Accurate methods for design and testing of the support elements are important parts of ensuring the stability of the system. This paper explores three aspects of the design and testing of rock support, namely: Pull-out tests on rockbolts and stud penetration tests in a shotcrete liner were conducted in the field, while round determinate panel (RDP) tests were performed in a laboratory environment. RDP tests were performed on fibre reinforced shotcrete panels with and without external steel mesh, as well as shotcrete panels with and without steel fibres reinforced with embedded steel mesh. The pull-out tests show that it takes about 19 h for a grouted rebar bolt to achieve its failure load capacity, while it takes 15 h from application for the shotcrete liner to obtain about 70% of the required energy absorption. Results from the RDP tests indicate that the bearing capacity of a rock support system is highly influenced by the shotcrete thickness. Doubling the shotcrete thickness results in a threefold increase of the residual load capacity, suggesting that the residual strength increases with shotcrete thickness by a polynomial function. Shotcrete panels with embedded steel mesh are shown to be able to sustain higher peak loads compared to panels with external steel mesh in these tests. However, at large deflections the reverse occurs, i.e., the latter panel type has a higher load bearing capacity.