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Early strength development of shotcrete for rapid mine development and behaviour under dynamic loads

Ever deeper mine developments combined with wider spans and layouts with close infrastructure alignments represent an increasing challenge for installed underground support. Destressing and sudden seismic events frequently create severe damage to the vital underground support within the haulage, ore handling and extraction infrastructure. In-cycle shotcrete has proven to be the primary support of choice in most large-scale mine developments, including the recent super-cave developments, such as Oyu Tolgoi, Chuquicamata, El Teniente and Grasberg. As a result, increased efforts have been undertaken over the past years to understand the behaviour of fibre-reinforced sprayed concrete under sudden dynamic loads, with evaluations of the latest developments in fibre technologies and shotcrete mix designs. Together with Luossavaara-Kiirunavaara Aktiebolag (LKAB) and the operating team at the Kiirunavaara and Malmberget operations in Sweden, among the world’s most stringent operations when it comes to early strength requirements and energy absorption of the shotcrete liner, Sika undertook extensive testing of different shotcrete mix designs. Simulated seismic shocks were used in order to evaluate primary shotcrete liners by observing their behaviour under such rapid deformation events (Swedberg 2013; Swedberg et al. 2014). The major findings and test results form part of this paper. Particular focus was also given to the early strength development of the sprayed shotcrete because destressing events usually occur soon after a face has been blasted (Amberg 2006), and thus rapid strength development is of particular interest. For reliable in situ onsite strength development measurement, Sika developed an ultrasonic-based tool to measure the strength of the sprayed concrete at the sprayed location and will allow development teams to re-enter secured perimeters at the right time. This is seen to have a major positive impact on safety and productivity of any rapid mine development project.