2023 – The Six-Ways Method – A More Robust Process of Estimating the Geological Strength Index

Nicola Burns, Deryk Forster

There are different methods to estimate the Geological Strength Index (GSI), and each of these require various inputs which are determined differently. For a given rock mass, this can produce conflicting (and sometimes unreliable) results causing uncertainty to geological assessments and subsequent design, which, in turn, affects safety, costs, and time. The consistency of inputs required for GSI estimations can vary due to the experience of field personnel and their understanding of the inputs, unfavourable conditions onsite or human error – and, simply, the difficulty of predicting the strength of a rock mass beneath the surface with relatively limited primary data.

The ‘GSI Six-Ways’ is an approach that is suggested in order to not rely on any one (or few) methods for estimating the GSI, which could be significantly biased by specific input data or practitioner experience. The method provides the user with a wholistic quantitative and visual output to provide a more thorough grasp of the GSI of the geomechanical unit in question.
As the name suggests, there are six ‘ways’ or calculations this process utilises to develop the visual output. All are highly established methods; Direct Methods: Hoek et al (2013) – JCond89, Hoek et al (2013) – JCond76, Cai et al (2007), and Indirect Methods: RMi (Palmstrom 1995), RMR (Bieniawski 1976), Q (Barton et al 1974). For each of these, each input can be determined once an appreciation of the block and defect conditions are known.

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