Engineering Rock Mechanics

206 Rock mass classification

Table 12.4 Relative importance of rock engineering parameters in three engi-
eering activities

In situ stress

Discontinuity persistence

Topographic factors

PMenCc of faul~folds

Location of tunnel

Discontinuity apermrc

Rock mass geometry

Discontinuity fd

Tunnel water pressure.

Re-uisting w.tcr cnditions

Depth of cavem

Discontinuity orientation

In situ stress

of faults

-type

Discontinuity frcqucncy

Discontinuity apertwe

Recxisting water conditioms

Intact mck elastic modulus

RocknuJsclasticmodulus

In situ stress

Induced displacemenm

lhamal aspects

Discontinuity geometry

beability

Time dpdent properties

Elastic modulus

Compressive strength

Porosity

Density

12.6 were developed from literature reviews and could be different if they

were based on current practice or design-led practice.

We present Table 12.4 which lists the most important parameters for the

three cases which were derived from their frequency of occurrence in the

literature, reflecting the concentration of research effort. Forty-four papers

were studied for pressure tunnels, 70 for large underground caverns, and

208 for radioactive waste disposal. The key point is that the list of most

important parameters will not be the same for different types of

engineering project, nor indeed will it coincide with one of the current

classification schemes-as is clear from the table. Furthermore, we cannot

divorce the rock mass properties from some of the site and operational

characterisitics. The table includes not only the properties of intact rock,

discontinuities and the rock mass, but also factors relating to the prevailing

boundary conditions, site circumstances and project parameters.

12.7 Concluding remarks

In conclusion, it is clear that rock mass classification schemes have assisted

engineers in the past especially in the absence of any other approach. There

are pitfalls associated with the use of the schemes, primarily associated with

the absence of what may be critical parameters for various projects. The

classification systems can be enhanced by the use of fuzzy methods and

strategic parameter selection. In the long term, perhaps beyond the

lifetimes of the readers, and certainly the authors, rock mass classification

systems will be superseded by direct solution of the engineering problems,

although there will always be some comfort in using the classification

schemes to ensure that modelling results do not contravene hard-earned

practical experience. In the meantime, rock mass classification systems are

useful but must be used with care.