Engineering Rock Mechanics

Flow through discontinuities 153

Pressure Q = 2-D flow quantity

per unit time (m2/sec)

xpl Aperture

Arbitrary datum level

Flow is due to head:

Pressure head = Ply

Elevation head = z

Kinematic head = v2/2g Ignore

Ata: Hl=--!+z, Y

P P

At@: H2=L+z2 Y

Figure 9.3 Flow between two parallel surfaces.

The variation in K with both the aperture and the discontinuity
frequency for flow parallel to a single set of discontinuities is shown in
Fig. 9.4.
With reference to Fig. 9.4, Hoek and Bray pointed out that the perme-
ability of such a system is sensitive to the discontinuity apertures, and
hence also to the presence of any in situ stress. They extend the idea to two
orthogonal sets of discontinuities, and discuss pumping tests in a borehole
traversing such an array.

IO' -

10-

10-1 -

x

I I I I

0.05 0.1

10-8

0 001 0.005 0.01

Discontinuity aperture e - cm

Figure 9.4 Variation of discontinuity set permeability as a function of the aperture
and discontinuity frequency (after Hoek and Bray, 1977).