Quantitative GSI Determination of Singapore’s Sedimentary Rock Mass by Applying Four Different Approaches

In this paper, the quantitative Geological Strength Index (GSI) for Singapore’s sedimentary rock mass is determined by applying four different approaches proposed by Hoek et al. (Quantification of the geological strength index chart, 2013), Cai et al. (Int J Rock Mech Min Sci 41(1):3–49, 2004), Sonmez and Ulusay (Int J Rock Mech Min Sci 36:743–760, 1999) and Russo (Improving the reliability of GSI estimation: the integrated GSI-RMi system, 2009) respectively. The relevant rock mass properties, including blockiness and discontinuities surface conditions, of 420 excavated rock faces in a Singapore’s underground excavation project are assessed for such determination. The results obtained by these different approaches are observed falling approximately in the same range between 45 and 55. Moreover, those quantitative GSI results are in good correlation with qualitative GSI which were assessed on excavated rock faces using Hoek et al. (Support of underground excavations in hard rock, Balkema, Rotterdam, 1995) approach. This study demonstrates that the four different approaches perform well by giving similar range of results, although different input parameters are applied in the calculation. A new GSI relationship between joint condition factor Bieniawski (Engineering rock mass classification, Wiley, New York, 1989) and RQD (rock quality designation) is also proposed to better match the qualitative GSI values.