Use and misuse of rock mass classification systems with particular reference to the Q-system

Rock mass classification systems have gained wide attention and are frequently used in rock engineering and design. However, all of these systems have limitations, but applied appropriately and with care they are valuable tools.The paper describes the history of the Q-system that was introduced in 1974, and its later development. The individual parameters are analysed, and their relevance for the natural geological features they seek to simulate, is discussed. This applies to both the original application for assessing rock mass quality to estimate the extent of rock support, and the later attempts to make the method into a kind of general rock mass classification with many applications. This also includes the recently introduced QTBM, which shall allow estimates of penetration and advance rate for TBM, and also the attempts to apply Q to express the effects of pre-grouting.It is concluded that the Q-system, used with full awareness of its limitations, may be applied for classification of the stability and support estimates of tunnels and rock caverns, preferably in jointed rocks. Applied here, it may be used for planning purposes. It is less useful for prescription of rock support during construction. It is not likely that Q is suitable to express the effects of pre-grouting. QTBM is complex and partly misleading and is not recommended for use in its present form.