Numerical Modelling of the dynamic response of threadbar under laboratory-scale conditions

Abstract

New technologies related to the mining industry have been developed in the last 50 years as the mining industry has expanded in depth. These technological developments have responded not only to production and planning issues but also to problems related to physical phenomena that take place in the field and hinder the mining progress. Prompting research into rockburst events in high stress underground mining has increased internationally in the last few years. Efforts to understand, quantify damage, and mitigate the effects of these events have been the object of many studies carried out by recognized institutes within the mining industry and based on the work done by the Canadian RockBurst Research Program (Cai and Kaiser, 2018; Kaiser et al., 1996). One study trend has been on how underground excavations are supported and how support systems absorb dynamic impacts. Several researchers have dedicated their efforts to study reinforcement and retainment elements that are part of the support system in an attempt to improve the standard designs widely used and initially conceived for static load resistance. Institutions such as the CanMet - Mining & Mineral Sciences Laboratories (CanMet-MMSL) of Canada, the Western Australia School of Mines (WASM) and recently the new Dynamic Impact Tester (DIT) of New Concept Mining, have been studying the behaviour of reinforcement and retainment elements under dynamic loads. Their studies have evolved from simply comparing loads to analysing the capacity of support system elements to absorb energy from impacts and deform during the process. Through laboratory-scale testing representative of in-situ conditions, the aforementioned institutions have worked to quantify the deformation and absorption of energy of these elements, resulting in comparative parameters and an adaptable design under dynamic loads. However, laboratory-scale testing involves a high cost in preparation time and validation; hence a limited number of these tests are carried out. Numerical modelling is an alternative that, in addition to complement the results from laboratory testing, should be useful to explain the deformation and energy absorption process. Yi and Kaiser (1994a), Tannant et al. (1995), Ansell (1999; 2005), Thompson et al. (2004), St-Pierre (2007) and Marambio et al. (2018) have modelled the dynamic behaviour of reinforcement elements in laboratory-scale testing centred around load-displacement relationships. The role of grout, however, has not been fully incorporated into those models even though the grout/rockbolt interface has been shown to be the place where the failure occurs to most of the reinforcement elements in situ. In Chile, the process of developing a new laboratory-scale dynamic testing facility supported by the University of Chile, the Geomechanics Research Center MIRARCO and Compañia de Aceros del Pacifico (CAP), using a mechanism similar to the CanMet-MMSL, has conducted several studies (appended) in which numerical modelling is taken into account as an important part of the process. In this study, a methodology is proposed to numerically model the process of laboratory-scale dynamic testing of reinforcement elements, i.e. rockbolts plus grouting. Based on the finite difference method, a model was developed, calibrated and verified with a specific focus on the threadbar (also known as rebar or gewibar), which is widely used as rock reinforcement in Chilean underground mining and globally.