Comparison of quantified and chart GSI for four rock masses

• Data from four different rock masses are provided to extend the GSI case histories.
• The two methods produce data points that are generally within ± 10 of each other.
• The exceptions are rock masses whose blockiness may not be captured well by RQD.
• The quantified GSI should be used to supplement the visually assessed chart GSI.
• Different GSI values may be needed to cater for different numerical methods.

This paper discusses the Geological Strength Index (GSI). It provides data from four different rock masses to extend the case history offered by Hoek et al. (2013) in their paper that proposes to quantify GSI with RQD and the joint condition rating of RMR.The correlation between the GSI assessed from its chart and the quantified GSI was found to be fair for the datasets from the four rock masses; the Hawkesbury Sandstone and Ashfield Shale of Sydney; the Greenland Group greywacke and argillite of South Island, New Zealand; and the Otago Schist of South Island, New Zealand. The two methods produce data points that are generally within ± 10 of each other. The exceptions are rock masses whose blockiness may not be captured well by RQD.It is recommended that the quantified GSI approach be used to supplement and check the visually assessed chart GSI.The authors also suggest different GSI values that may be needed to cater for different numerical methods.