Civil Engineering Reference
In-Depth Information
diameters attached to the lid. If the diameter of
the larger plates is about the same dimensions as
the wavelength of the roughness, then the meas-
ured orientation will be approximately equal to
the average orientation of the surface. However,
the smaller diameter plates will show a scatter
in the orientation measurements as the plates lie
on irregularities with shorter wavelengths. If the
orientation measurements are plotted on a ste-
reonet, the degree of scatter in the poles about the
mean orientation is a measure of the roughness.
Quantitative methods of profile measurement
have been established by Tse and Cruden (1979)
using a mechanical profilometer, and Mearz
et al
.
(1990) have developed a shadow profilometer
that records the shape of the surface with a video
camera and image analyzer.
The method developed by Tse and Cruden is
illustrated in Figure 3.10(a). The profile is defined
by measuring the distance
(y
i
)
of the surface from
a fixed reference line at specified equal intervals
(x)
over a length of
M
intervals. From these
measurements the coefficient
Z
2 is defined as
(a)
∆
x
i
=
M
y
i
y
i
+1
(b)
(c)
01 23cm
1
M(x)
2
y
i
)
2
1
/
2
M
Z
2
=
(y
i
+
1
+
(3.10)
Figure 3.10
Measurement of joint roughness:
(a) procedure for measuring roughness with
mechanical profilometer (modified from Tse and
Cruden (1979); reprinted with kind permission from
Elsevier Science Ltd. The Boulevard, Langford Lane,
Kidlington, UK); (b) photograph of profilometer;
(c) profile of joint with JRC of 11.4.
i
=
1
A study has also been carried out to assess the
effect of the size of the sampling interval
(x)
along the profile on the calculated value of JRC
(Yu and Vayssade, 1991). This study found that
the calculated value of JRC was dependent on
the size of
x
, and that the most accurate res-
ults were obtained with small sampling intervals.
The value of JRC can be calculated from the coef-
ficient
Z
2 using one of the following equations for
the appropriate sampling interval:
One means of making profile measurements is to
use a carpenter's comb that consists of a series of
metal rods, positioned in a frame, such that they
can slide relative to each other (Figure 3.10(b)). If
the comb is pressed against a rock face, the rods
will slide to conform to the shape of the surface.
This profile can then be traced on to a piece of
paper and the shape of the rock surface quanti-
fied by measuring the distance of each rod from a
reference line.
Figure 3.10(c) shows the results of a profile
measurement with a carpenter's comb of a rough,
planar joint in granite. The profile of the surface
JRC
=
60.32
(Z
2
)
−
4.51
for
x
=
0.25 mm
(3.11a)
JRC
=
61.79
(Z
2
)
−
3.47
for
x
=
0.5 mm
(3.11b)
JRC
=
64.22
(Z
2
)
−
2.31
for
x
=
1mm
(3.11c)
