Page 75 - Rock Mechanics For Underground Mining

COLLECTING STRUCTURAL DATA

less than half a millimetre. It will be appreciated, of course, that unlike the examples
given in Figure 3.12, the apertures of real discontinuities are likely to vary widely
over the extent of the discontinuity. This variation will be difficult, if not impossible,
to measure.
Clearly, aperture and its areal variation will have an influence on the shear strength
of the discontinuity. Perhaps more important, however, is the influence of aperture on
the permeability or hydraulic conductivity of the discontinuity and of the rock mass.
For laminar flow, the hydraulic conductivity of a single discontinuity with plane,
parallel sides is given by

ge 3
k = (3.5)
12v
−1
where k = hydraulic conductivity (m s ), g = acceleration due to gravity
−2
(m s ), e = discontinuity aperture (m) and v = kinematic viscosity of the fluid
2 −1
2 −1
◦
(m s )(= 1.01 × 10 −6 m s for water at 20 C).
If e = 0.05 mm, for example, k = 1.01 × 10 −7 ms −1 for water at 20 C, but if e is
◦
−1
increased to 0.5 mm, k is increased by a factor of 1000 to 1.01 × 10 −4 ms .
Filling is the term used to describe material separating the adjacent rock walls
of discontinuities. Such materials may be calcite, chlorite, clay, silt, fault gouge,
breccia, quartz or pyrite, for example. Filling materials will have a major influence on
the shear strengths of discontinuities. With the exception of those filled with strong
vein materials (calcite, quartz, pyrite), filled discontinuities will generally have lower
shear strengths than comparable clean, closed discontinuities. The behaviour of filled
discontinuities will depend on a wide range of properties of the filling materials. The
following are probably the most important and should be recorded where possible:
(a) mineralogy of the filling material taking care to identify low-friction materials
such as chlorite
(b) grading or particle size
(c) water content and permeability
(d) previous shear displacement
(e) wall roughness
(f) width of filling
(g) fracturing, crushing or chemical alteration of wall rock.

3.4 Collecting structural data

The task of collecting the data referred to in section 3.3 is usually the responsibility
of the mining or engineering geologist, although rock mechanics engineers or mining
engineers may sometimes be called on to undertake the necessary fieldwork. In either
case, it is essential for the rock mechanics or mining engineer (who will generally
initiate a request for the data, and who will use it in mine planning studies) to be
familiar with techniques used in collecting the data and with the potential difficulties
involved.
The starting point for the development of an engineering understanding of the rock
mass structure is a study of the general regional and mine geology as determined
during exploration. This will provide some knowledge of the lithologies and of the
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