Permeability and Seepage Flow - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 139

- due to entry into discontinuities,
- due to changes of aperture,
- at bifurcations,
- at bends and
- at intersections.
However, as discussed in detail in Wittke (1990), all these energy losses, apart from ex-
ceptional cases, are negligible.
The application of discrete models of seepage flow in jointed rock with several sets of
discontinuities to rock engineering problems is limited in practice because of the lack
of detailed information on the geometric data of discontinuities. In particular, the lo-
cation and extent of water-bearing discontinuities cannot practically be determined by
exploration and testing. Also, apertures cannot be reliably determined (Section 13.9).
Normally it is only possible to evaluate mean values of the permeability coefficients in
different directions of homogeneous areas by permeability tests, discharge measure-
ments and the results of back analyses (Section 15.8 and Chapter 24).
Thus, discrete models are practically limited to flow through fault zones and master
joints, the location and permeability of which cannot always be evaluated.
6.4.2 Homogeneous Model
General
Similar to the stress-strain behavior of jointed rock (Section 3.4.2) the permeability of
jointed rock can be described by means of a homogeneous model. For this purpose, a
representative elementary volume (REV) must be defined in which the rock mass can
be considered as statistically homogeneous in the sense that an increase of this vol-
ume does not change the mean permeability of the rock mass which is referred to as
“equivalent permeability” (Bear 1972, Long et al. 1982, Khaleel 1989, Zimmermann &
Bodvarsson 1996, Min et al. 2004a, Baghbanan & Jing 2007).
To evaluate the dimension of the REV discrete models can be helpful. For this purpose,
seepage flow through networks of discontinuities so-called “discrete fracture networks”
(DFN) has been simulated (Long et al. 1982, Khaleel 1989, Zimmermann & Bodvars-
son 1996, Min et al. 2004a, Baghbanan & Jing 2007).
In Baghbanan & Jing (2007) a crystalline rock in the Sellafield area (England) with four
discontinuity sets was examined by simulating flow through a large number of two-di-
mensional DFNs using the distinct element model (DEM). In this study DFNs with
constant and statistically distributed apertures and trace lengths of discontinuities were
investigated. Accordingly, the size of the REV was found to be dependent not only on
the mean values of apertures and trace lengths but also on their statistical distribution
and degree of correlation. It was shown that homogeneous models are applicable if the
statistical distributions of the discontinuity properties exhibit sufficiently low standard
deviations. This requirement is normally fulfilled in regularly jointed rock.