The Log-GSI Method to Estimate GSI from Borehole Data: Application in Snowy 2.0 Project
Alberto VANNI1, Davide VIETTI1, Antonio DEMATTEIS2, Emanuele CATALANO1, Manfred THÜRING1#
1Lombardi Engineering Ltd., Switzerland, 2Lombardi Engineering Ltd, Australia

Since its introduction in 1994, the Geological Strength Index (GSI) has become a popular tool to describe the rock mass condition, particularly due to its link to the previously introduced Hoek-Brown failure criterion which allows to estimate the major design parameters for underground excavation projects (shear strength and deformability).The GSI assessment is usually based on the visual inspection of a rock surface (outcrop, tunnel face, etc.). However, in the design process of underground works the available information at project depth is rather contained in boreholes, practically the information needs to be assembled from the recovered rock cores, their pictures and logged information.Among others, Hoek, Carter and Diederichs in their paper “Quantification of the Geological Strength Index Chart” (2013) have prepared the way for a quantitative evaluation of the GSI, in terms of RQD for the rock mass segmentation and a few alternative ways of describing the discontinuity quality, in terms of Bieniawski’s RMR (Jcond89) or Barton’s Q (Ja, Jr) system. Whereas the RQD is commonly available from borehole logging, the description of the joint condition is usually a more narrative aspect.We present the experiences during the design studies of the deep tunnels of the hydroelectric pumped-storage project Snowy 2.0 (New South Wales, Australia), in which the GSI was determined from borehole logging and the rock mass strength and deformability parameters were calculated by the Hoek-Brown criterion.The borehole logging was done according to the Australian geotechnical site investigations standard AS 1726-2017. A procedure was developed to translate the descriptions of the joint condition into Barton’s Ja and Jr. The GSI was finally determined according to Hoek et al. (2013) and was compared to and validated by directly logged Ja and Jr values and a visual inspection of the rock cores.The methodological approach, its results and a critical analysis are presented and discussed for 3 boreholes of differing lithology and rock mass quality (good, fair and poor). An outlook is given for the further usage of the outcomes in the design process of underground excavations.