Civil Engineering Reference
In-Depth Information
of a surface is discussed in more detail in
Section 3.4.3.
•
Large aperture can result from shear
displacement of discontinuities having appre-
ciable roughness and waviness, from tensile
opening, from outwash of infillings, and from
solution. Steeply dipping discontinuities that
have opened in tension as a result of valley
erosion or glacial retreat may have very large
apertures.
Descriptive terms that can be used to define
roughness are a combination of small-scale fea-
tures (several centimeters dimensions):
rough,
smooth, slickensided
, and larger-scale features
(several meters dimensions):
stepped, undulat-
ing, planar
. These terms can be combined to
describe decreasing levels of roughness shown in
Table II.5.
•
In most sub-surface rock masses, apertures
will probably be less than half a millimeter,
compared to the tens, hundreds or even thou-
sands of millimeters width of some of the
outwash or extension varieties. Unless discon-
tinuities are exceptionally smooth and planar,
it will not be of great significance to the shear
strength that a “closed” feature is 0.1 mm
wide or 1.0 mm wide. However, hydraulic
conductivity is influenced by minor changes
in aperture.
G Aperture
•
Aperture is the perpendicular distance separ-
ating the adjacent rock surfaces of an open
discontinuity, in which the intervening space
is air or water filled. Aperture is thereby
distinguished from the width of a filled discon-
tinuity. Discontinuities that have been filled
(e.g. with clay) also come under this category
if the filling material has been washed out
locally.
•
Unfortunately, visual observation of small
apertures is inherently unreliable since, with
the possible exceptions of drilled holes and
bored tunnels, visible apertures are bound to
be disturbed by blasting or surface weathering
effects. Aperture can be measured indirectly
by hydraulic conductivity testing.
Table II.5
Descriptive terms for roughness
•
Apertures are recorded from the point of view
of both their loosening and conducting capa-
city. Joint water pressure, inflow of water and
outflow of storage products (both liquid and
gas) will all be affected by aperture.
I
Rough, stepped
II
Smooth, stepped
III
Slickensided, stepped
IV
Rough, undulating
V
Smooth, undulating
VI
Slickensided, undulating
VII
Rough, planar
VIII
Smooth, planar
Apertures can be described by the terms listed in
Table II.6.
IX
Slickensided, planar
Table II.6
Aperture dimensions
Aperture
Description
<
0.1 mm
Very tight
0.1-0.25 mm
Tight
“Closed” features
0.25-0.5 mm
Partly open
0.5-2.5 mm
Open
2.5-10 mm
Moderately wide
“Gapped” features
>
10 mm
Wide
1-10 cm
Very wide
10-100 cm
Extremely wide
“Open” features
>
1 m
Cavernous
