Environmental Engineering Reference
In-Depth Information
TABLE 5.26
(
Continued
)
σ
c
/
σ
1
σ
t
/
σ
1
SRF
Mild rock burst (massive rock)
5-2.5
0.33-0.16
5-10
(heavy rock burst massive rock)
2.5
0.16
10-20
SRF
Squeezing rock: plastic flow of incompetent
rock under the influence of high rock pressure
Mild squeezing rock pressure
5-10
Heavy squeezing rock pressure
10-20
Swelling rock: chemical swelling activity
depending on presence of water
Mild swelling rock pressure
5-10
Heavy swelling rock pressure
10-15
a
Where RQD is reported or measured as
10 (including 0), a nominal value of 10 is used to evaluate
Q
in
Equation (5.5) RQD intervals of 5, i.e., 100, 95, 90, etc., are sufficiently accurate:
Rock mass quality Q
(RDQ/
J
n
)(
J
r
/
J
a
)(
J
w
/SRF)
(5.5)
b
For intersections use 3.0
J
n
. For portals use 2.0
J
n
.
c
Descriptions refer to small-scale features and intermediate-scale features, in that order.
d
Add 1.0 if the mean spacing of the relevant joint set is greater than 3 m.
J
r
0.5 can be used for planer slick-
ensided joints having lineations, provided the lineations are orientated for minimum strength.
e
Last four factors above are crude estimates. Increase
J
w
if drainage measures are installed. Special problems
caused by ice formation are not considered.
f
Reduce these values of SRF by 25-50% if the relevant shear zones only influence but do not intersect the excavation.
g
For strongly anisotropic virgin stress field (if measured): when 5
1
/
3
10, reduce
c
and
t
to 0.8
c
and 0.8
t
. When
1
/
3
10, reduce
c
and
t
to 0.6
c
and 0.6
t
, where
c
is the unconfined compression strength,
3
are the major and minor principal stresses. Few case records
available where depth of crown below surface less than span width. SRF increase from 2.5 to 5 is suggested
for such cases (see low stress, bear surface).
Source:
From Barton, N. et al., Proceedings of the ASCE 16th Symposium on Rock Mechanics, University of
Minnisota, 163-178, 1977. With permission.
Additional Notes
: When rock-mass quality
Q
is estimated, the following guidelines should be followed, in addi-
tion to the notes listed in parts (a) to (f):
t
the tensil strength (point load), and
1
and
1.
When borecore is unavailable, RQD can be estiamated from the number of joints per unit volume, in which
the number of joints per meter for each joint set are added. A simple relation can be used to convert this num-
ber into RQD for the case of clay-free rock masses:
RQD
3.3
J
v
(approx.)
where
J
v
is the total number of joints per cubic meter (RQD
115
4.5).
2. The parameter
J
n
representing the number of joint sets will often be affected by foliation, schistocity, slatey
cleavage, or bedding, etc. If strongly developed these parallel “joints” should obviously be counted as a com-
plete joint set. However, if there are few “joints” visible, or only ocassional breaks in borecore because of these
features, then it will be more appropriate to count them as “random joints” when evaluating
J
n
in part b.
3. The parameters
J
r
and
J
a
(representing shear strength) should be relevant to the
weakest significant joint set or
clay filled discontinuity
in the given zone. However, if the joint set or discontinuity with the minimum value of
(
J
r
/
J
a
) is favorably oriented for stability, then a second, less favorably oriented joint set as discontinuity may
sometimes be of more significance, and its higher value of
J
r
/
J
a
should be used when evaluating
Q
from the
equation above.
The value of J
r
/J
a
should in fact relate to the surface most likely to allow failure to initiate
.
4. When a rock mass contains clay, the factor SRF appropriate to
loosening loads
should be evaluate (part f). In
such cases the strength of the intact rock is of little intrest. However, when jointing is manual and clay is com-
pletely absent, the strength of the intact rock may become the weakest link, and the stability will then depend
on the ratio rock-stress/rock-strength (see part f). A strongly anisotropic stress field is unfavorable for stabil-
ity and is roughly accounted for as in Note g, part f.
100 for
J
v
5.
The compressive and tensil strengths
t
of the intact rock should be evaluated in the saturated condi-
tion if this is appropriate to present or future in situ conditions. A very conservative estimate of strength
should be made for those rocks that deteriorate when exposed to moist or saturated conditions.
c
and
