Civil Engineering Reference
In-Depth Information
either by direct measurement of the surface,
or by comparing the surface with standard
profiles of irregular joint surfaces. These tech-
niques are described in Section 3.4.3. Usual
practice would be to use the standard rough-
ness profiles during the preliminary mapping.
If there is a critical discontinuity that will
control stability, then the values estimated
from the profiles could be calibrated with a
limited number of detailed measurements of
roughness.
G
Wall strength
—The strength of the rock form-
ing the walls of discontinuities will influ-
ence the shear strength of rough surfaces.
Where high stresses, compared to the wall
strength, are generated at local contact points
during shearing, the asperities will be sheared
off resulting in a reduction of the rough-
ness component of the friction angle. In the
initial stages of weathering, there is often
a reduction in rock strength on the discon-
tinuity surfaces that may result in a dimin-
ished roughness value. It is usually adequate
to estimate the compressive strength from
the simple field tests as shown in Table 3.1
(ISRM, 1981b), or if core or lump samples
are available, by carrying out point load tests.
The Schmidt hammer test is also a method of
estimating the compressive strength of rock at
discontinuity surfaces.
Table 3.1
Classification of rock and soil strengths (ISRM, 1981b)
Grade
Description
Field identification
Approx. range
of uniaxial
compressive
strength (MPa)
R6
Extremely strong rock
Specimen can only be chipped with geological
hammer.
>
250
R5
Very strong rock
Specimen requires many blows of geological
hammer to fracture it.
100-250
R4
Strong rock
Specimen requires more than one blow of
geological hammer to fracture it.
50-100
R3
Medium strong rock
Cannot be scraped or peeled with a pocket
knife, specimen can be fractured with single
firm blow of geological hammer.
25-50
R2
Weak rock
Can be peeled by a pocket knife with difficulty,
shallow indentations made by firm blow with
point of geological hammer.
5.0-25
R1
Very weak rock
Crumbles under firm blows with point of
geological hammer and can be peeled by
a pocket knife.
1.0-5.0
R0
Extremely weak rock
Indented by thumbnail.
0.25-1.0
S6
Hard clay
Indented with difficulty by thumbnail.
>
0.5
S5
Very stiff clay
Readily indented by thumbnail.
0.25-0.5
S4
Stiff clay
Readily indented by thumb but penetrated
only with great difficulty.
0.1-0.25
S3
Firm clay
Can be penetrated several inches by thumb
with moderate effort.
0.05-0.1
S2
Soft clay
Easily penetrated several inches by thumb.
0.025-0.05
S1
Very soft clay
Easily penetrated several inches by fist.
<
0.025
Notes
Discontinuity wall strength will generally be characterized by grades R0-R6 (rock). Some rounding of strength values has been
made when converting to SI units (ISRM, 1981b).
