Engineering Rock Mechanics

Permeability of intad rock and rock masses 143

Figure 9.3 Example of water flow through a fractured rock mass. To the right side of

the picture, the rock mass is relatively dry. However, in the central portion the rock is

wet and there are several jets of water issuing from the rock mass, e.g. at the position

indicated by the white arrow.

imal fracture frequencies are generally not orthogonal. The ramifications
this has for the concept of principal values and directions for secondary
permeability are not clear, but the difficulties of modelling fluid flow in
fractured rock masses are evident from the photograph in Fig. 9.3.
Currently, the best way to estimate fracture flow is to use numerical
programs that can simulate the fractures directly. From the results,
an equivalent tensor representation can be deduced. Initially, these
numerical programs were developed for two dimensions and solved for
the fracture flow and nodal heads at the fracture intersection points by
setting up a series of equations based on Darcy's Law and the fact that
the flows into and out of a node are equal (i.e. the continuity equation or
Kirchoff's Law). Later, it became possible to solve the problem in three
dimensions but, because pressure distributions have to be incorporated
along the lines of intersection of the fractures, the extension to 3-D is not
simply achieved by 'adding a z component' to the 2-D x-y analysis. It is
now also possible to include the primary permeability of the rock blocks
between the fractures. In many cases, the contribution of the intact rock
will be small; in other cases, such as the sandstone forming petroleum
reservoirs, the intact rock contribution is significant.
The quantity of water flowing through a planar rock fracture is
theoretically proportional to the cube of the fracture aperture. The con-
sequence of this for modelling and for rock engineering is that the
localized nature of water flow through the rock fractures is further con-
centrated by the effect of the wider fractures (Fig. 9.3). For example,
in tunnels in fractured rock, water inflow generally occurs through the
wider fractures which are well connected with the surrounding frac-