Engineering Rock Mechanics

Questions and answers: rock mass classification 207

The next step is to establish whether an increase in each of these

seven parameters is good or bad in the context of a classification

index in which higher values indicate improved engineering circum-

stances.

Minimum principal stress value 03 an increase is good

Compressive strength of the 0, an increase is good

rock

Groundwater pressure at cavern Pw an increase is good

crown level (before cavern

excavation)

Stress anisotropy ratio an increase is bad

Maximal stored gas pressure Ps an increase is bad

Fracture frequency h an increase is bad

Fracture aperture e an increase is bad

(a, /aj) = aR

To develop a 'Gas Cavern Tightness Index' (GCTI), the parameters

or parameter ratings have to be algebraically arranged so that vari-

ations in the parameter values are correctly effected in the GCTI index

according to the table above. If we use the same approach as the Q

system, in which the quotients have an interpretation, a suitable GCTI

would be

GCTI= - *c x -^03 x Pw -.

*R he3 Pg

The individual quotients can then be interpreted as follows:

UC

OR

is related to the stability of the intact rock around the cavern;

is related to the resistance to gas leakage; and

is related to the effectiveness of the groundwater confinement.

Xe3
Pw
Pg
The reader can no doubt think of improvements to this initial index.
Our intention here is only to provide an example of the principles used
in developing new rock mass classification indices.

Q 12.9 The following parameter values have been determined for

three rock mass types found along the route of a major new highway

tunnel that passes at a high level through the flank of a mountain

range:

Strength RQD Mean fracture

(MPa) (%) spacing (m)

Sandstone 00 45 0.4

Mudstone 20 75 0.3

Syenite intrusions 250 10 0.2