On the rock mass classifications with indices in tunnelling

Classifications based on indices such as Q or RMR, which result from the multiplication or addition of various physical parameters or ratings, should not be used to estimate the rock mass behaviour or the necessary tunnel support. A classification system should take account only of the characteristics and properties that are significant when addressing specific planning issues. The mentioned indices do not fulfil this condition. The proposed relationships between the so-called Geological Strength Index (GSI) and the rock mass parameters under the empirical Hoek-Brown failure condition are also scientifically untenable. This failure condition is neither more accurate nor easier to handle than the familiar linear Mohr-Coulomb condition. The proposed relationships defy scientific testing because their empirical basis has never been published and, in addition, they contain non-measurable entities.