Page 76 - Rock Mechanics For Underground Mining

ROCK MASS STRUCTURE AND CHARACTERISATION

major structural features (folds, faults and dykes) present in the mining area. Such
information provides essential background to rock mechanics studies, but in itself,
is inadequate for our purposes. Further studies, involving careful mapping of surface
and underground exposures and logging of boreholes drilled for this purpose, are
required to obtain the types of data discussed in section 3.3.
The present account of the methods used to collect structural and related geological
dataisfarfromexhaustive.FulleraccountsaregivenbyGoodman(1976,1993),ISRM
Commission (1978a), Hoek and Brown (1980), Priest (1993) and Brown (2003).

3.4.1 Mapping exposures
In the early stages of a mining project, it may not be possible to gain access under-
ground. In this case, surface outcrops must be utilised to obtain information on the
engineering properties and structure of the rock mass. Measurements may be made on
natural outcrops or on faces exposed by surface excavations. In some mining projects,
an existing open pit provides an invaluable source of data.
It must be recognised, however, that these surface exposures can be affected by
weathering and that the surface rock mass quality may be quite different from that at
depth. It is essential, therefore, that any preliminary data obtained from surface expo-
sures are validated by subsequently examining underground exposures. Exploratory
openings should be mapped at the earliest possible stage to provide data for the rock
mechanics input into mining feasibility studies. As the mining project reaches a more
advanced stage, development openings should be mapped to provide information on
which stope design can be based.
In all of these instances, there is a basic sampling problem to be considered.
What proportion of the rock mass should be surveyed to obtain satisfactory results?
What degree of confidence can be placed on mean values of discontinuity properties
determined using limited amounts of data? There are no complete answers to such
questions although the use of statistical techniques, such as those developed by Priest
and Hudson (1981) and discussed briefly below, does provide valuable guidance.
Even where it is possible to develop a statistical approach to discontinuity mapping,
practical considerations, such as a lack of access to the desired underground exposure,
can mean that surveyors must use their judgement in interpreting results.
The approaches used for mapping exposures may be of three main types:
spot mapping in which the observer selectively samples only those discontinuities

that are considered to be important;
lineal mapping in which all discontinuities intersecting a given sampling line are

mapped; and
areal mapping in which all discontinuities within a selected area of the face,

often called a window, are mapped. This method can reduce some of the biases in
mapping to be discussed below, but it suffers from some practical difficulties for
routine use in underground mines.
The basic technique used in mapping surface or underground exposures is the
scanline survey (Figure 3.13). A scanline is a line set on the surface of the rock
mass, and the survey consists of recording data for all discontinuities that intersect
the scanline along its length. An alternative approach is to measure all discontinuities
within a defined area on the rock face, but this is more difficult to control and do
systematically than are scanline surveys. In underground development excavations
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