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https://www.onepetro.org/conference-paper/ARMA-2016-384
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4. Barton N, Bieniawski ZT (2008) RMR and Q—‘setting records straight’. Tunn Tunn Int 2008:26–29
5. Barton NR, Grimstad E (2014) An illustrated guide to the Q-system following 40 years use in tunnelling. In-housepublisher, Oslo. Retrieved 9 Aug, 2016 from
www.nickbarton.com