Engineering Rock Mechanics

198 Rock mass classification

This shows that the rating value for the bedding is much lower than that
of the jointing, and so it is likely that the bedding will be the most critical
fracture set for determining tunnel stability. As the dip of the bedding is
no more than 20°, this is considered to result in 'fair' conditions, and so
the rating adjustment for orientation will be -5. Note that if we consider
the joints, then when the tunnel is heading south these features will give
rise to 'very unfavourable' conditions, with a rating adjustment of -12.
Combining the fracture assessment and the rating adjustment for the
bedding gives a value of 15 - 5 = IO, and for the joints gives a value of
24 - 12 = 12. This shows that the bedding is, as we surmised above, the
most critical feature, but there is little difference between the joints and
the bedding.
Finally, we know the strength of the shale is 53 MPa, and this gives
a rating value of about 5. The basic XMR value for the shale is then as
follows:

Strength Groundwater XQD Spacing Fracture Condition Total

5 7 10 8 15 45

Taking the rating adjustment for orientation into account reduces this

to 40.

Basalt dykes

We are given no definite geomechanical data regarding the basalt dykes,

other than that they are sub-vertical features. We need to turn to our

geological knowledge in order to make an assessment of this rock type.

Firstly, we can assume that these dykes will be of limited thickness, say,

no more than 5 m. As the cooling joints in a dyke run across the plane of

the dyke - rather than parallel to it - this will set a maximum fracture

persistence of 5 m, giving a rating value of 2. The aperture of these

cooling joints could be as large as 1 mm, giving a rating value of about

3. The joints are liable to be 'slightly rough', with no infilling and only
   slight weathering. The rating values for these attributes are 3, 6 and 5,
   respectively. The overall assessment for the joints in the dykes is then 2 +
   3 + 3 + 6+ 5 = 19. The rating adjustment for the orientation of these joints
   is difficult to assess but, given the limited extent of the joints, we can
   perhaps assign an effect of 'fair' and a corresponding rating value of -5.
   The RQD of the dykes will probably be high, and a rating value of
   around 15 will be suitable. The mean fracture spacing may be of the
   order of 0.5 m, and the rating value for this is 10. Finally, the strength of
   the basalt is 71 MPa, and the corresponding rating value is about 7.
   The basic RMR value for the dykes is then as follows:

Strength Groundwater ROD Soacine Fracture condition Total

7 7 15 10 19 58

Taking the rating adjustment for orientation into account reduces this

to 53.

We must bear in mind that the dykes may have been acting as conduits

for groundwater flow for a long time, and may be highly weathered.