Permeability and Seepage Flow - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 140

Permeability
To describe the permeability of jointed rock with the aid of a homogeneous model we
assume impermeable intact rock and laminar fl ow in the discontinuities. In the same
way as for soil and for porous rock, a seepage velocity v s is introduced, being defi ned
as the quotient of the discharge Q and the cross-sectional area A perpendicular to the
direction of fl ow (Fig. 6.10, left). The cross-sectional area A incorporates the cross-sec-
tion of both the discontinuities and the intact rock. Fig. 6.10 (right) illustrates Darcy's
law for a rock mass containing one discontinuity set D with persistent, open joints,
which are assumed to be oriented parallel to the hydraulic gradient I. The aperture
2a i and the spacing s of the discontinuities are assumed to be constant. According to
Wittke (1990), for this case the seepage velocity is given by (Fig. 6.10)
(6.26)
where
(6.27)
is the permeability coeffi cient parallel to D, which can also be described by means of a
cubic law with regard to the aperture of the discontinuities.
Figure 6.10 Darcy's law for a rock mass with one set of persistent, open discontinuities (Wittke 1990)