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.

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

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:
1. decide on the objective of the rock mass classification scheme;
2. decide on the parameters to be used, their ranges and ratings;
3. decide on the algebra to be used for the rock index (e.g. do we simply select values from a table, do we add rating values together, do we multiply ratings together, or something else?); and
4. calibrate the rock index value against the objective.

The advantage of using a rock mass classification scheme is that it is a simple and effective way of representing rock mass quality and of encapsulating precedent practice. The disadvantage is that one cannot use it for a different objective or in significantly new circumstances.

The rock mass classifications that have been developed to date follow this basic approach, but include more parameters and use a greater number of classes than the simple 'good', 'fair', 'poor' example we gave above.

For example, by adding a third parameter to the classification given in Table 12.1, 'thickness of the layers', and using more rating values (Vervoort and de Wit, 1997'), a useful rock index for rock dredging has been developed. By judicious choice of the relevant parameters, such rock mass classification schemes can be a powerful tool for rock engineering.

The two main classification systems, Rock Mass Rating and Tunnelling Quality Index (XMR and Q), have both been widely applied and there is now a large database of projects where they have been used as the main indicator of rock stabilization requirements in rock tunnelling.

The systems provide a coherent method of using precedent practice experience and can now be linked to numerical analysis approaches.

With all schemes, the key issues are the objective of the classification system, choice of the optimal parameters, assigning numerical ratings to parameter values, the algebraic manipulation of the parameter ratings, and drawing conclusions from the mean and variation of the overall rock quality index values.

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