Rock mass Classification Systems

Rock mass Classification Systems
The rock mass classification schemes are very useful during feasibility design stages of a Project.
The first attempt in the definition of a rock mass classification system was made by Terzaghi in 1946.
DESIGN OF TUNNEL SUPORT

BASED ON A DESCRIPTIVE CLASSIFICATION

THERE ARE DEFINED DIFFERENT DESCRIPTIONS FOR THE ROCK MASS TYPES

a) Intact rock: may present problems of spalling and popping during construction.
b)Stratified Rock: Spalling problems
c)Moderatly jointed rock: sometimes is not required lateral support. Feasible problems, spalling and popping.
d)Blocky and seamy rock: consists in rock fragments imperfectly interlocked. Vertical walls require lateral support.
e)Crushed rock: there are fine grains and no recementation.
f)squeezing rock mass: there is a high perecentage of microscopic particles of clay with low swelling capacity.
g)swelling: the rock advances into the tunnel with expansion phenomenons.


In 1967, is developed another definition to determine or to analize the properties of the rock mass: the Rock Quality Designation by Deere.

The RQD, can be defined as a percentage of intacto core pieces longer than 10 cm in the total length of the core:


Here is an example:




In the RQD determination, there may be several problems with core breakage. The method ignores joint orientation (is a directionally dependent parameter, and the result may present variatons): but is a simple, inexpensive and reproducible method.

After the first definition of the RQD, there have been developed different correlations, for example Tunneling(its original use), modulus of elasticity, joint volume, is a parameter in the Rock Mass Rating and a parameter in the Q index.


ROCK STRUCTURE RATING


Developed by Wickham in 1972, is the first approximation to obtain a quantitative method to describe the quality of a rock mass.


The RSR is formed by the combination of three components:

Observation: the RSR does not give any indication of the compresive strength and modulus of the rock mass. It does not include any laboratory or experimental results.

The next tables show the values for the parameters according to the rock properties ( A, B and C components).

Parameter A






Parameter B






Parameter C





Bieniawski - Geomechanics classification

This classification system is based in six parameters which define the Rock Mass Rating (RMR):




In the use of this system is necessary to divide the rock mass into different structural regions, each one with support needs acording to it's properties.


The total RMR is the result of the sum of the ratings, and the next step is the selection of the support according to the guidelines proposed by the author of the method.


Reference: E.Hoek "Practical rock engineering" - Rock mass classification

January 03 2011