Civil Engineering Reference
In-Depth Information
(e)
Cleavage
—Parallel discontinuities formed in
incompetent layers in a series of beds of
varying degrees of competency are known
as cleavages. In general, the term implies
that the cleavage planes are not controlled
by mineral particles in parallel orientation.
(f)
Schistosity
—Foliation in schist or other
coarse grained crystalline rock due to the
parallel arrangement of mineral grains of the
platy or prismatic type, such as mica.
The importance of defining the rock type
is that there is wide experience in the per-
formance of different rock types (e.g. granite
is usually stronger and more massive than
shale), and this information provides a useful
guideline on the likely behavior of the rock.
B
Discontinuity type
—Discontinuity types
range from clean tension joints of limited
length to faults containing several meters
thickness of clay gouge and lengths of many
kilometers; obviously the shear strength of
such discontinuities will be very different.
Section 3.3.3 provides a definition of the six
most common types of discontinuity.
C
Discontinuity orientation
—The orientation
of discontinuities is expressed as the dip and
dip direction (or strike) of the surface. The dip
of the plane is the maximum angle of the plane
to the horizontal (angle
ψ
), while the dip dir-
ection is the direction of the horizontal trace
of the line of dip, measured clockwise from
north, angle
α
(see Figure 2.4). For the plane
shown in Figure 3.5 that dips to the north-
east, the orientation of the plane can be com-
pletely defined by five digits: 30/045, where
the dip is 30
◦
and the dip direction is 45
◦
. This
method of defining discontinuity orientation
facilitates mapping because the dip and dip
direction can be read from a single compass
reading (Figure 3.6). Also, the results can be
plotted directly on a stereonet to analyze the
structural geology (see Section 2.5). In using
the compass shown in Figure 3.6, the dip is
read off a graduated scale on the lid hinge,
while the dip direction is read off the compass
scale that is graduated from 0
◦
to 360
◦
.
Some compasses allow the graduated circle
to be rotated to account for the magnetic
declination at the site so that measured dip
directions are relative to true north. If the
compass does not have this feature, then the
magnetic readings can be adjusted accord-
ingly: for a magnetic declination of
20
◦
east
,
for example,
20
◦
is added to the magnetic
readings
to obtain true north readings.
D
Spacing
—Discontinuity spacing can be
mapped in rock faces and in drill core, with
3.3.4 Definition of geological terms
The following is a summary of information that
may be collected to provide a complete descrip-
tion of the rock mass, and comments on how
these properties influence the performance of the
rock mass. This information is based primarily on
procedures developed by the International Society
of Rock Mechanics (ISRM, 1981b), with some
additional information from the Geological Soci-
ety Engineering Group (1977). More details of
the mapping data is provided in Appendix II,
which includes mapping field sheets and tables
relating descriptions of rock mass properties to
quantitative measurements.
Figure 3.5(a) illustrates the 12 essential features
of geological structure, each of which is described
in more detail in this section. The diagram and
photograph in Figure 3.5 show that sets of discon-
tinuities often occur in orthogonal sets (mutually
at right angles) in response to the stress field that
has deformed the rock; the photograph shows
three orthogonal joints in massive granite. Ortho-
gonal structure is also illustrated in the stereonet
in Figure 2.11. The value of recognizing ortho-
gonal structure on an outcrop or in a stereonet is
that these features are often the most prevalent in
a cut and are likely to control stability.
The following is a list, and a description of the
parameters that define the characteristics of the
rock mass.
A
Rock type
—The rock type is defined by the
origin of the rock (i.e. sedimentary, meta-
morphic or igneous), the mineralogy, the
color and grain size (Deere and Miller, 1966).
