Civil Engineering Reference
In-Depth Information
still flowing may have a “crackled” structure
in which the joints are not persistent, curved
and have no preferred orientation. Also, some
volcanic rocks exhibit sets that only extend over
a few meters on the face, and then different sets
occur in an adjacent area.
In the design of rock slopes on civil projects, it is
usually advisable that the full length of each slope
be designed with a uniform slope angle; it is not
practical to excavate a slope with varying slope
angles because this will complicate surveying and
the lay out of blast holes. This will require that
in applying geological data to slope design, the
dominant geological structure, such as bedding or
orthogonal joint sets, be used for the design. An
exception to this guideline may be where there is a
significant change in rock type within the cut, and
it may be appropriate to prepare a separate design
for each. However, even in these circumstances, it
may be more economical in terms of construction
costs to cut the entire slope at the flatter of the
two slope designs, or to install support in the less
stable material.
Another issue in planning a geological mapping
program is to decide how many discontinuities
should be mapped in order to define the design
sets. It is usually possible, by inspection of a nat-
ural face or existing cut, to ascertain whether the
structure occurs in sets or is randomly oriented.
For locations where there is good rock expos-
ure and the structure is uniform, as few as 20
measurements should provide information on the
orientation of the sets, with a further 50-100
measurements required to define typical prop-
erties such as persistence, spacing and infilling.
Conditions in which a greater number of dis-
continuities should be mapped include faulted or
folded structure, or contacts between different
rock types. In these cases, several hundred fea-
tures may need to be mapped in order to define the
properties of each unit. A detailed procedure for
determining the number of joints to be mapped is
described by Stauffer (1966).
Structural geology is specifically addressed in
two chapters in the topic. Chapter 2 describes
the properties of discontinuities and how they
are used in kinematic analysis, and Chapter 3
(a)
(b)
Figure 2.1
Rock faces formed by persistent
discontinuities: (a) plane failure formed by bedding
planes parallel to face with continuous lengths over
the full height of the slope (shale on Route 19 near
Robbinsville, North Carolina); (b) wedge failure
formed by two intersecting planes dipping out of the
face (sedimentary formation on Route 60 near
Phoenix, Arizona).
