Civil Engineering Reference
In-Depth Information
3.3.1 Line and window mapping
three spatial points defines a triangle. From these
co-ordinates, the orientation of the triangles can
be defined in terms of the dip and dip direction,
together with the co-ordinates of the centroid.
The software allows a mouse to be used to outline
the surface to be analyzed, and the calculated dip
and dip direction can then be imported directly to
a stereonet.
Methods of structural mapping that will system-
atically examine all significant geological features
are “line” and “window” mapping (see also
Appendix II).
Line mapping comprises stretching a tape along
the face and mapping every discontinuity that
intersects the line; line lengths are normally
between 50 and 100 m. If the ends of the line
are surveyed, then the location of all the discon-
tinuities can be determined. Window mapping
comprises mapping all discontinuities within a
representative segment or “window” of fixed size,
spaced at regular intervals along the exposure.
The intervening areas are examined for similarity
of structure. The dimensions of a window would
normally be about 10 m. Either of these mapping
techniques may be used in both the reconnaissance
and final design stages of a project, depending on
the extent of the face available for mapping. If the
initial investigations identify a particular feature
that is likely to have a significant effect on stability,
then more detailed mapping, such as roughness
and persistence measurements, could be carried
out on these structures.
3.3.3 Types of discontinuity
Geological investigations usually categorize dis-
continuities according to the manner in which
they were formed. This is useful for geotech-
nical engineering because discontinuities within
each category usually have similar properties as
regards both dimensions and shear strength prop-
erties that can be used in the initial review of
stability conditions of a site. The following are
standard definitions of the most common types
of discontinuities:
(a)
Fault
—Discontinuity along which there has
been an observable amount of displacement.
Faults are rarely single planar units; nor-
mally they occur as parallel or sub-parallel
sets of discontinuities along which move-
ment has taken place to a greater or less
extent.
(b)
Bedding
—Surface parallel to the surface of
deposition, which may or may not have a
physical expression. Note that the original
attitude of the bedding plane should not be
assumed to be horizontal.
(c)
Foliation
—Parallel orientation of platy min-
erals, or mineral banding in metamorphic
rocks.
3.3.2 Stereogrammetric mapping of
discontinuities
There are circumstances where it is not possible to
directly access a rock face for mapping because,
for example, there is a hazard to the geolo-
gists of rock falls or the face is overhanging in
places. Under these conditions, there are available
indirect methods of geological mapping using ter-
restrial photography. The basic principle involves
obtaining the co-ordinates of at least three points
on each surface from which its orientation can be
calculated.
One system that provides this facility is SIRO-
JOINT (CSIRO, 2001). A digital image of the
slope face taken by a camera at a known loc-
ation, and the image is converted into three-
dimensional spatial data defining the surface of
the rock face. Each spatial point has a position
(
x
,
y
,
z
co-ordinates) in space, and each local set of
(d)
Joint
—Discontinuity in which there has
been no observable relative movement. In
general, joints intersect primary surfaces
such as bedding, cleavage and schistosity. A
series of parallel joints is called a
joint set
;
two or more intersecting sets produce a
joint
system
; two sets of joints approximately at
right angles to one another are said to be
orthogonal
.
