The Q-method is a numerical description of the rock mass quality with respect to rock stability, and is also used for estimating required permanent rock support in tunnels and caverns.
By means of six different parameters a Q-value can be calculated, based on the expression above. The six parameter values can be determined from mapping in tunnels or in the field and by logging of drill cores. A high Q-value means good stability, whereas low values indicate poor rock stability. Required support is dependent on the Q-values together with the span width or height of walls and the safety requirements (ESR, Excavation Support Ratio).
The Q-system was developed at NGI and introduced in 1974. Since then it has been updated several times to include the most modern types of tunnel support, and this empirical method is now based on data from more than 1250 examples from existing tunnels around the world.
In 1999 the Q-method was individually adjusted for TBM-bored i tunnels (QTBM -N. Barton, 1999). This is a method to evaluate the rate of penetration (PR) and propagation (AR) for TBM-drilling, based on an expanded Q-system and an average cutting force relative to anticipated rock mass strength. In 2002 sprayed reinforced concrete arches and energy absorption were included, in addition to adapting the method to present levels of required support.