The Geological Strength Index (GSI) has been developed over a number of years to provide a means of estimating inputs for the Hoek-Brown failure criterion for derivation of rock mass shear strength. The GSI should ideally be assigned from field observation of the rock mass by suitably experienced engineering geological personnel. At the time of slope design, exposure of critical rock mass units is often limited. Information may be limited to data collected from drill core, from which GSI cannot be directly assigned. Attempts to quantify the GSI directly from rock mass parameters have been made by numerous practitioners, and in 2013 Hoek et al. provided their method in the paper entitled Quantification of the geological strength index chart. The method utilizes two well-established geotechnical parameters – joint condition and Rock Quality Designation. These parameters are both routinely assessed from drill core, providing a methodology for assigning GSI from drill core data, superseding the unreliable relationships between Bieniawski's Rock Mass Ratings and GSI. This paper presents a comparative study between values of GSI derived using the Hoek et al. relationship and those derived using the methods that it supersedes, and the effect on slope design. Introduction The Geological Strength Index (GSI) has been developed over a number of years to provide a means of estimating inputs for the Hoek-Brown failure criterion for derivation of rock mass shear strength for intact rock or heavily jointed rock masses considered to be homogeneous and isotropic. The GSI should ideally be assigned from the field observation of the rock mass by suitably experienced engineering geological personnel (Marinos et al., 2005). It relies on qualitative assessment of the structure and surface conditions of the rock mass, and assigns a numerical value based on this assessment. At the time of open pit slope design, exposure of critical rock mass units is often limited, or unavailable. Information on rock mass conditions may be obtainable only from drill core, from which GSI cannot be directly assigned. In order to allow the derivation of the GSI from drill core data, empirical relationships were developed between GSI and a number of rock mass classification schemes. Hoek et al. (1995) note correlations with Bieniawski's Rock Mass Rating (RMR 76 , 1976) system, the updated version, known as RMR 89 (Bieniawski, 1989), and Q (Barton et al., 1974). Although these relationships have since been considered unreliable (Hoek, 2007), they are still widely used in open pit slope design. A number of methods have been proposed for the direct quantification of the GSI. These include those of Somnez and Ulusay (1999), Cai et al. (2004), and Russo (2007, 2009), although none of these have been readily adopted for use in open pit slope design. More recently, Hoek et al. (2012) proposed their own methodology, providing the open pit geotechnical engineer with an updated methodology with which to estimate GSI. The various methods that can be used to estimate GSI are discussed below, and the implications of the use of the updated method on historic slope designs are explored using a number of case studies.