The Bieniawski geomechanical classification or RMR (acronym for the English rock mass rating) is a geomechanical classification system presented by Eng. Bieniawski in 1973 and modified successively in 1976, 1979, 1984 and 1989.[1]​[2]​.

It allows a classification of a rock mass 'in situ'. It is usually used in the construction of tunnels, slopes and foundations. It also consists of an RMR (Rock Mass Rating) quality index, independent of the structure, and a correction factor.[3][4]​.

Contenido

El RMR se obtiene estimando cinco parámetros:[5]​.

Al resultado de cada uno de los parámetros se le asigna, según las tablas, un valor y se suman todos ellos para obtener el índice de calidad RMR sin correcciones. A este valor se le debe restar un factor de ajuste en función de la orientación de las discontinuidades.[6]​.

Leaderboard

The value is classified based on the following table:[7]​.

The correction factor, defined qualitatively, depends on the orientation of the discontinuities and has different values ​​depending on whether it is applied to tunnels, foundations or slopes.[8]​.

However, the Rock Mass Rating presents some drawbacks when applied to rock slopes, given that the parameter that takes into account the influence of the orientation of the discontinuities was defined in detail for dam foundations and tunnels, but not for slopes.[9]​ To solve this difficulty, Romana[10]​ defined the Slope Mass Rating[10]​ that adopts Bieniawski's original discontinuity orientation correction values and defines them rigorously, decomposing them into four different factors which he called F1, F2, F3 and F4. The first three factors depend on the geometric relationships between the slope and the discontinuities, while the fourth factor depends on the excavation method.

The RMR index is obtained by subtracting the adjustment factor from the values ​​obtained. This index can vary between 0 and 100 and defines five classes of rock designated with Roman numerals that correspond to five qualities of the rock mass: very good, good, average, bad and very bad.

The main advantage of this classification method is its simplicity and economy.

Correlation

The RMR is empirically correlated with the Young's modulus of the rock mass (Em), not the intact rock (Er):.

for RMR > 50.

for RMR =< 50.

Although these correlations exist, it must be clarified that they are only approximations to the real values ​​exhibited by the rock mass.