Rock mass
72 classification
12.1 Rock parameters and classification schemes
The purpose of rock mass classification is to establish the quality of a
particular rock mass (or part of a rock mass) by assigning rating values
to a set of rock parameters. Webster’s dictionary defines ’classification’
as “the act of classifying or forming into a class or classes, so as
to bring together those beings or things which most resemble each
other, and to separate those that differ”. This definition immediately
highlights two main issues in rock mass classification: the purpose of the
classification has to be established and the method of classification has
to be commensurate with the purpose.
For example, if we only used the uniaxial compressive strength of
the intact rock and the fracture frequency of the rock mass, we could
generate a rock mass classification scheme for characterizing sections of
rock in a tunnel as shown in Table 12.1.
On the basis of this scheme, all rock masses must then be one of the
categories, AI, A2, B1, B2. We could call this a Rock Index and assign the
words ’Good’ to AI, ’Fair’ to A2 and BI, and ’Poor’ to B2. But what is the
purpose of this classification? Perhaps, the Rock Index would indicate
the excavatability and stability of the rock masses in each category. If so,
is the classification the best one for that purpose?
There are four main steps in the development of any rock mass
classification scheme:
decide on the objective of the rock mass classification scheme;
decide on the parameters to be used, their ranges and ratings;
decide on the algebra to be used for the rock index (e.g. do we simply
Table 12.1 Illustrative simple rock mass classification scheme
Parameter Ratings, R
Uniaxial compressive strength, a,
Fracture frequency, h Ifh54/m, R= 1 If h 4/m, R =2
If a, 3 100 MPa, R = A If a, < 100 Ma, R = B