Rock mass classification

Rock mass classification systems turn field observations into a single number that summarises rock mass quality. They give a common language between geologists and engineers, help choose excavation methods and tunnel support at the early stages, and feed into estimates of rock mass strength and modulus.

The Rock Mass Rating (RMR), developed by Bieniawski, adds ratings for five parameters: intact strength, RQD, joint spacing, joint condition and groundwater, then adjusts for joint orientation relative to the excavation. The total, out of 100, places the rock in one of five classes from very good to very poor and links to recommended support and stand-up time.

The Q system, developed by Barton and colleagues at the Norwegian Geotechnical Institute, multiplies three quotients: RQD over the joint set number, a measure of block size; joint roughness over joint alteration, a measure of inter-block shear strength; and a joint water factor over a stress reduction factor, a measure of active stress. Q ranges over six orders of magnitude, from exceptionally poor to exceptionally good, and tunnel support charts relate it to span and use.

Both systems are empirical and should be used with judgement: they do not capture every failure mode, such as a single adverse wedge in good rock or squeezing in weak rock under high stress. Approximate correlations between RMR and Q exist, and the Geological Strength Index derived from them is used with the Hoek-Brown criterion.