Rock Mechanics and Rock Engineering: From the Past to the Future – Ulusay et al. (Eds) © 2016Taylor & Francis Group, London, ISBN 978-1-138-03265-1 DeterminingtheGeologicalStrengthIndex(GSI)usingdifferentmethods B.Vásárhelyi Department of Engineering Geology & Geotechnology, Budapest University of Technology and Economics, Budapest, Hungary G.Somodi,Á.Krupa&L.Kovács RockStudy Ltd, Pécs, Hungary ABSTRACT: During the design process in rock engineering Hoek-Brown failure envelope is used for the determinationofrockmassfailureenvelopemainlyinbrittlerocks.AnimportantinputparameteroftheHoek- BrownfailureenvelopeistheGeologicalStrengthIndex(GSI ),whichvariesbetween0and100,andconcentrates onthedescriptionofrockstructureandblocksurfaceconditions.Thereareseveralmethodswhichdefine GSI but a general international standard has not been specified yet. Our aim is to analyze different methods of GSI determination on the basis of observations during the construction phase of the Bátaapáti radioactive waste repository. Examinations of the values determined on-site gave significantly different results. Different correlationsweredeterminedbetweenthecalculated GSI values. 1 INTRODUCTION TheGeologicalStrengthIndex(GSI )representstoday the most widely used engineering index for the cat- egorization of rock mass quality for obtaining input dataintothecontinuumnumericalanalysiscodesand closedformsolutionsbasedontheHoek–Brownfail- ure criterion (e.g. Marinos & Hoek 2000, Marinos et al. 2007). The exact determination of this value is veryimportantfortheexactcalculationofthefailure envelopeorthedeformationmodulioftherockmass. Ván & Vásárhelyi (2014) determined the sensitiv- ity of the GSI based on mechanical properties, such as the Hoek-Brown equation (failure envelope of the rockmass)andtheHoek-Diederichsequation(defor- mation moduli of the rock mass). It was shown that sophisticatedempiricalequationscanbehighlysensi- tivetotheuncertaintiesinthe GSI values–evenifthe errorofthe GSI isonly5%,therelativesensitivitycan reach100%! Recently, Morelli (2015) analyzed the different calculationmethodsof GSI.UsingMonte-Carlosimu- lations,hissimulationresultsindicatethatthediverse relationshipsmaypredictdissimilarvaluesofthe GSI for the same rock mass. He obtained the highest GSI valuefromtheequationswhichapplytheconventional RMR 1989 values,andthelowestresultswereobtained byusingthe RMi methodfor GSI calculation. Similar results were found and published by Sari (2015).Usingprobabilitybasedanalysisisperformed inhisstudytoaccountfortheuncertaintyand/orvari- ability reflected by most rock masses encountered in largeconstructionprojects.Heanalyzedthreedifferent empiricalmethods.Hefoundthatdifferentsuggested equations generated completely different values of GSI. It was also found that the most influential parameters depended on the equations used in the estimationofrockmassstrength. Bertuzzi et al. (2016) determined the GSI values of four different rock masses (namely: Hawkesbury Sandstone and Ashfield Shale of Sydney; the Green- land Group greywacke and argillite of South Island, New Zealand; and the Otago Schist of South Island, NewZealand).Theircalculationsbasedonthesugges- tionofHoeketal.(2013):quantifyingtheGeological StrengthIndex(GSI )withtheRockQualityDesigna- tion(RQD)andwiththejointconditionrating(J Cond89 ) oftheRockMassRatingsystem(RMR).Accordingto theirresults,thetwomethodsproducedatapointsthat aregenerallywithin ±10ofeachother.Theexceptions arerockmassesthatmaynotbecapturedwellby RQD. Theyrecommendedthatthequantified GSI approach beusedtosupplementandcheckthevisuallyassessed chart GSI. They also suggested different GSI values that may be needed to cater for different numerical methods. Thegoalofthispaperistocomparethedifferent GSI calculationsystemsbyusingthedatameasuredinthe radioactivewasterepository,constructedinBátaapáti (Hungary). Up to now, more than 6km long tunnel system was constructed and all the tunnel faces were documented (Deák et al. 2014, Kovács et al. 2015). Thegeographicalpositionoftheresearchareaisshown in Figures 1 and 2 present the schematic view of the tunnelsystem. 2 GEOLOGICALANDGEOTECHNICAL CONDITIONS The repository was constructed in an intruded and displaced Paleozoic granite batholith body. The 1049