Site investigation - Practical Engineering Geology - page 149

Box 4-7 Collection of discontinuity data in exposures (modi ed fromHencher &

Knipe, 2007)

1. First take an overview of the exposure. Examine it from different directions.
2. Develop a preliminary geological model and split it into structural and weathering zones units. Sketch
the model.
3. Broadly identify those joint sets that are present, where they occur, how they relate to geological
variation and what their main characteristics are, including spacing, openness as mechanical
fractures (or otherwise), roughness, in ll and cross cutting or terminations in intact rock or against

other discontinuities. Surface roughness characteristics such as hackle marks should be noted as these
are indicative of origin and help differentiate between sets.
4. Measure suf

cient data to characterise each set geologically and geotechnically. Record locations on
plans and on photographs. This might be done using line and window surveys but quite often these are
time consuming and not very productive. It is generally best to decide what to measure and then measure
it, rather than hope that the answer will be revealed from a statistical sample.
5. Plot data and look at geometrical relationships. Consider how the various sets relate to one another
and to geological history as evidenced from faults, folds and intrusions ( Chapter 3).
6. Search for missing sets that might have been expected given the geological setting.
7. Analyse and reassess whether additional data are required to characterise those joints that are most
signi cant to the engineering problem.

Where the data collection point is distant from the project location, consider whether the collected data
might be unrepresentative.

Remote measurement of fracture networks is becoming more reliable
using photogrammetry (Haneberg, 2008) or ground-based radar
( Figure 4.19) and research is progressing into the automatic interpretation
of laser-scanned data into rock sets (orientation and spacing) (Slob, 2010).
Currently, this approach, however, lacks any link to an interpretation of
origin of the discontinuities and their geological inter-relationships
( Chapter 3), which would make it much more valuable. In the author

s
opinion, probably the best use for laser scanning at the moment is as an aid
to the

'

field team, in particular for measuring data in areas of an exposure
with dif cult access, but they cannot replace mapping and characterisa-

tion by experienced persons at the current stage of development.
Rock joint data are generally represented on stereographic projections,
as illustrated in Figure 4.20. The technique allows sophisticated analysis of
geological discontinuity data (Phillips, 1973), but its most common use in
engineering geology is for determining the potential for speci

c rock
discontinuities to cause a failure in a cut slope or in an underground
opening (Hoek & Bray, 1974 and Chapter 6). Plotting of data, statis-
tical grouping and comparison to slope geometry is now easily done
using software such as Dips (Rocscience), but care should be taken in
interpretation and especially against masking important but relatively
rare data (Hencher, 1985). Bridges (1990) demonstrates the importance
of differentiating sets on the basis of geological characteristics rather
than just geometry.