Geology Reference
In-Depth Information
C.6 Rock mass classi
cations
This review is not comprehensive but considers the most commonly
used rock mass classi
cation systems. A particularly useful review,
mostly with respect to tunnelling and underground structures, is
given by Professor Hoek at:
http://www.rocscience.com/hoek/corner/
3_Rock_mass_classi
cation.pdf
Rock Quality Designation (RQD) was proposed by Deere (1968) as
an index of rock quality by which a modi
ed core recovery percentage
is measured by counting only pieces of
core 4 inches
(100mm) or greater in length as a percentage of core run. This
measure is usually recorded on core logs and has stood the test of
time, partly because it is simple (like the SPT test for soils and weak
rock). Many authors have pointed out inconsistencies and suggested
modi
'
sound and hard
'
cations, for example, over core lengths to be considered and
have questioned the de
nition of sound and hard, but most of these
have not been widely adopted. Basically, one adds the lengths of intact
core greater than 100mm in length along the centre line of core and
expresses this as a percentage of core run. If there is 100% recovery
and all sticks between natural discontinuities are >100mm in length,
the rock has an RQD of 100%. If all sticks of core were 95mm, the
RQD would be 0%. It is a crude measure but used in practice and is a
key parameter in other rock mass classi
cations, RMR and Q, as
discussed below. Details of measurement are given in BS 5930:1999
and most other standards, including Geoguide 3 (GCO, 1988).
Various authors have used RQD directly to correlate with rock mass
parameters such as Young
s modulus or to estimate bearing capacity
for foundations (Peck
et al
., 1974). It should be borne in mind that
rock with 100mm spacing of open joints (100% RQD) is in fact
severely fractured rock that would be described as closely spaced (60
mm to 200mm). According to BS 8004 (BSI, 1986), any rock with joint
spacing less than 100mm would require
'
'
tests on rock
'
, as discussed in
Chapter 6,
to assess allowable bearing pressure.
Palmström (1982) suggests that RQD can be estimated from the
number of discontinuities per unit volume based on visible disconti-
nuity traces surface exposures, using the following relationship:
RQD ¼ 115
−
3
:
3J
v
where J
v
is the sum of the number of joints per unit length for all
discontinuity sets and known as the volumetric joint count. There is
a practical dif
culty here in that traces of discontinuities do not
