Multi-approach Geological Strength Index (GSI) Determination for Stratified Sedimentary Rock Masses in Singapore

The main aim of this study is to present the results of the Geological Strength Index (GSI) predicted by three different empirical equations, currently available in literature, which used the input parameters applied in the well-established rock mass classification systems, namely the RMR₁₉₈₉ (Bieniawski in Engineering rock mass classification. Wiley, New York, 1989) and the Q system (Barton et al. in Rock Mech. 6(4):189–239, 1974). To meet the objective, the GSI of the stratified sedimentary rock masses at underground rock cavern excavation project in Singapore are calculated by the three different empirical equations. It is observed that the empirical approach by Hoek (in: 47th US rock mechanics/geomechanics symposium, American Rock Mechanics Association, 2013) is the highest among those calculated GSI values. In the next steps, the empirically calculated GSI values are compared with quantitative GSI and qualitative GSI determined at the site. It is noted that all three calculated vales are still higher than the quantitative GSI values determined by charts proposed by Hoek (in: 47th US rock mechanics/geomechanics symposium, American Rock Mechanics Association, 2013), Cai et al. (Int J Rock Mech Min Sci 41(1):3–49, 2004), Sonmez and Ulusay (Intl. J Rock Mech Min Sci 36:743–760, 1999) and Russo (Tunnel Undergr Space Technol 24:103–111, 2009) approaches (Winn and Wong in Geotech Geol Eng, 2019. https://​doi.​org/​10.​1007/​s10706-018-0748-8) and qualitative GSI values determined on excavated rock faces using charts by Hoek et al. (Support of underground excavations in hard rock. Balkema, Rotterdam, 1995) and Marinos and Hoek (in: GeoEng2000, Melbourne, Australia, 2000). To make a narrow gap, modification of those empirical equations is proposed for the sedimentary rock masses in Singapore. The calculated results by proposed equations with field-estimated qualitative GSI fall in the range of ± 10. The Excel add-in program Crystal Ball (Oracle Crop.) has been applied to perform probabilistic analyses using Monte Carlo simulations in all three modified empirical equations for determination of the GSI. It is observed that the results of the base case, mean and median GSI values are in the comparable ranges. The sensitivity analysis was performed to explore the influence of individual input parameter in the determination of GSI.