Because of the presence of discontinuities in a rock
mass, we have the concepts of primary permeability and
secondary permeability. Primary permeability refers to the
rock matrix permeability, whereas, the secondary
permeability refers to the rock mass permeability.
In some circumstances, e.g. petroleum engmeering,
we will be specifically interested in the primary permeability,
but in most rock engineering it is the secondary permeability
which dominates the design and construction procedures.