9 Permeability
9.1 Permeability of intact rock and rock masses
Permeability is a measure of the ease with which a fluid will flow
through rock. Fluids do not flow easily through solid rock, so intact
rock has a low permeability. However, rock masses are fractured and,
because fractures conduct fluids more readily, it is easier for fluids to
flow through them (Fig. 9.1).
Often, the permeability of a rock mass is considered as a scalar
quantity, i.e. it can be characterized by one value which is therefore inde-
pendent of the flow direction. However, in the general case, there will be
normal flows (analogous to normal stresses) and there will be cross-flows
(analogous to shear stresses). Thus, an improved approach is to consider
the permeability of a continuum as a second-order tensor, analogous
to the quantities stress and strain discussed in Chapters 3 and 5. This
means that there will be principal permeability directions and mag-
nitudes (similar to principal stresses) acting on planes along which there
is no cross-flow (similar to zero shear stress on principal stress planes).
The rock mass is not a continuum, but the tensor assumption, with its
six independent components, is a significant improvement on a scalar
characterization, especially when we know that rock mass properties are
directionally dependent because of the fractures.
We noted in Chapter 3 that, to eliminate internal moments in the rock,
it is assumed that not all the shear stresses are independent, and this
reduces the number of independent stress components from nine to six
Fluid + in Fluid .- out -
Fluid
in Fluid outIntact rock: small scale Rock mass: large scale
Figure 9.1 Fluid flow through intact rock and fractured rock.