links between the classification systems and rock properties 201In the same vein, extensions to the RMR and Q-classification systems
have been made to estimate rippability, dredgeability, excavatability, cutta-
bility and cavability (Bieniawski, 1989).
12.4 Links between the classification systems
and rock properties
The rock mass classification systems have some common parameters, so we
might expect a link to exist between the RMR and Q-systems. The most
well-known correlation is given by the equation
RMR = 9log,Q + 44.
Naturally, this has to be an approximation, because in situ stress and rock
strength are not common to the two systems.
Since the rock mass properties, e.g. deformability and strength, are also
functions of the intact rock properties and the discontinuity properties, it
follows that we may be able use the classification scheme values to estimate
the modulus and strength of rock masses. Empirical relations are available
for both of the systems described.
Bieniawski (1989) has suggested that
EmSS = 2 x RMR - 100 GPa (for RMR > 50).
Another relation has been proposed by Serafim and Pereira (1983), which
covers the entire RMR range, and is
EMASS = GPa.
For the Q-system, Barton et al., (1985) proposed that
10 logloQ < EMASS < 40 loglo Q, with EM,,, = 25 loglo Q.Other expressions have been developed (Hoek and Brown, 1988). For the
Hoek-Brown strength parameters rn and s (see Section 6.5.3) as:
undisturbed rock : rn = mi exp (yyoo] =exp['""R9-'"']disturbed rock : rn = rn, exp [ (R,:,,,,,)) = exp[RMR6-~ooj
where RMR is the unadjusted rating from the system.
12.5 Discussion
It is important to place the value of rock mass classification schemes and
the estimations described above within the context of practical rock
engineering. It is easy to point to the value of the classifications when, often
inexperienced, personnel have to make assessments of rock mass quality
and support requirements, especially when faced with no other clear alter-
native. Similarly, it is easy to say that none of the techniques has any solid
scientific foundation and can quite clearly be dangerously misleading if the