Geology Reference
In-Depth Information
SAND
-10
SANDSTONE
-12
VOLCANIC
-14
LIMESTONE
DOLOMITE
SILTSTONE
META MORPHIC
-16
GRANITE
CLAY
SHALE
-18
WELDED
TUFF
-20
DIABASE
GNEISS
-22
-24
Fig. 3.4 Typical values of permeability k in laboratory specimens of various rocks. The data in
this figure are derived from Brace (
1980
)
where R is now generalized to be the ''hydraulic radius'' of the channel (i.e., the
ratio of the pore volume to the solid-fluid interfacial area, which serves to a first
approximation as a measure of the equivalent channel cross-sectional dimension; it
is one quarter of the diameter for a uniform circular cross-section, one-half of the
narrow dimension for a slot-shaped channel, and /d
=
61
ð Þ
for an assemblage
of spheres of diameter d having porosity /); C is a dimensionless ''shape factor''
which is somewhat less than the value 1/2 that applies for a circular cross-section
in the Poiseuille formula and which in effect allows for the error involved in the
use of the hydraulic radius as the equivalent channel cross-sectional dimension in
the Poiseuille analogy; the porosity / enters to allow for the fact that the equiv-
alent channel does not occupy the whole of the cross-sectional area of porous solid
