Environmental Engineering Reference
In-Depth Information
TABLE 5.24
Engineering Classification for
In Situ
Rock Quality
a
RQD (%)
Velocity Index
Rock-Mass Quality
90-100
0.80-1.00
Excellent
75-90
0.60-0.80
Good
50-75
0.40-0.60
Fair
50-25
0.20-0.40
Poor
25-0
0-0.20
Very poor
a
After Coon, J.H. and Merritt, A.H., American Society for Testing and Materials, Philadelphia, PA. Copyright
ASTM. Reprinted with permission.
Complex Classification Systems
Systems have been developed to provide detailed information on rock quality that
includes joint factors such as orientation, opening width, irregularity, water conditions,
and filling materials, as well as other factors. They are most applicable to tunnel engi-
neering. Rock-mass classification systems are described in detail in Bieniawski (1989) and
ASTM SPT 984 (1988).
The rock mass rating system (RMR) for jointed rock masses
, proposed by Bieniawski (1974,
1976, 1989), is given in
Table 5.25.
It is based on grading six parameters: uniaxial com-
pressive strength (from the point-load test), RQD, joint spacing, joint conditions, joint ori-
entation, and groundwater conditions. Each parameter is given a rating, the ratings are
totaled, and the rock is classified from ”very good” to ”very poor.” In their applications to
tunnel engineering, the classes are related to stand-up time and unsupported tunnel span,
and to ranges in the rock-strength parameters of friction and cohesion. The system defines
”poor rock” as having an RQD between 25 and 50% and
U
c
between 250 and 500 tsf, which
places poor rock in the range of “moderately sound, somewhat weathered” of Jaeger
(1972)
(
Figure 5.7)
and in the ”hard” range given in
Table 5.20
.
Bieniawski (1978) and Serafim and Periera (1983) propose the following equations for
estimating the rock modulus of deformation (
E
m
) based on the RMR:
E
m
(2RMR)-100(GPa)
for RMR
50,
(5.2)
E
m
10
(RMR
10)/40
(GPa)
for RMR
50,
(5.3)
Note
: 1 tsf
10,440 tsf.
Engineering classification of rock masses for tunnel support design
(
Q
-system), proposed by
Barton (1974, 1977), is given in
Table 5.26
.
It is based on a very detailed grading of six
basic parameters including RQD, description of joint sets, joint roughness, joint alter-
ation, joint water conditions, and a stress reduction factor which provides for rating
major zones of weakness in the mass, residual stresses, squeezing rock, and swelling
rock. The rock mass quality
Q
is calculated from Equation 5.5 given in Table 5.26(2). “
Q
”
is related to other factors to obtain estimates of tunnel support requirements. Barton
et al. (1974) present a relationship for estimating RQD from rock outcrops or other expo-
sures.
95.76 kPa; 1 GPa
RQD
115-3.3
J
v
(5.4)
where
J
v
is the number of joints per meter. Sample lengths of 5 or 10 m are usually used.
RQD
100 for
J
v
4.5.