Engineering Rock Mechanics

2 12 Rock mass classification

value of RQD threshold is

2 2 3.861

3.861 - 1.143 In (1.143) = 0.90m.

t* =

hmax - hmin

The new values of RQD and Q -called XQD* and Q* to distinguish them

from the customary formulations - are as shown in the following table.

Borehole 123456789101112

Spacing (m) 0.259 0.304 0.875 0.292 0.467 0.412 0.350 0.368 0.438 0.389 0.280 0.318

Frequency (m-l) 3.861 3.289 1.143 3.425 2.141 2.427 2.857 2.717 2.283 2.571 3.571 3.145

RQDf (“30) 14.0 20.7 72.7 18.9 42.9 36.1 27.5 30.1 39.4 33.0 17.1 22.8

O* 2.9 4.3 15.1 3.9 8.9 7.5 5.7 6.3 8.2 6.9 3.6 4.8

These results for e*, together with the results from the earlier calcula-

tion, are plotted in the figure below. We can see from this that Q* has a

greater discrimination than Q with regard to the inhomogeneity, at the

expense of much reduced values.

RQD threshold=O.l m ROD threshold=0.90 m frequency

20.0 4.00

b

U I

c E

15.0 3.00

0 a

c^0 C

m a, (^3) U
I 10.0 2.00 .g
9
5.0 1 .oo
0.0 0.00
123456789101112
borehole number
Although this technique has improved the sensitivity of the classific-
ation values, the reduced values of Q* in comparison to Q show that
we have effectively developed a completely new classification scheme,
and as such we cannot use any of the customary correlations between
Q and engineering behaviour. Thus, this technique must only be used to
improve the sensitivity of a classification scheme when the scheme itself
is being used to delineate different zones of rock properties, and not yet,
for example, for selection of support and reinforcement.
12.3 Additional points
A substantial extension to the RMR system has been made by Romana
(1993) with his SMR system for assessing slope stability. The SMR value
Romana M. R. (1993) A geomechanical classification for slopes: slope mass rating,
in Comprehensive Rock Engineering, Vol. 3, Ch. 23 (J. A. Hudson, ed.), Pergamon Press,
Oxford, pp. 575-599.