Geoscience Reference
In-Depth Information
8
WATER BENEATH THE GROUND:
FLUID MECHANICS OF FLOW
IN POROUS MATERIALS
8.1
POROUS MATERIALS
The great majority of all near-surface geologic formations, in which water is stored and
transported, are unconsolidated porous rocks made up of particles of different sizes. This
type of formation is usually referred to as a
soil
close to the surface, and as an
aquifer
at
greater depths. However, the terms soil and aquifer material are often used interchange-
ably. Many of these formations consist of alluvial and colluvial deposits, which as ripar-
ian aquifers are major contributors to streamflow. Although in some regions underlain
by limestone or karst formations, large quantities of water can be transported through
solution channels and caves, globally they are of much less importance. Formations
consisting of volcanic rock, shale and clay layers, which are porous but which transmit
water relatively slowly, are often considered impermeable for hydrologic purposes; as
such they are referred to as
aquicludes
.
The voids or open spaces between the particles of soils and other granular materials
are referred to as pores. An important property of such water-bearing formations is their
porosity. This can be defined as follows
volume of voids in
∀
n
0
=
lim
∀→
(8.1)
∀
0
where
is a small volume of porous material. This definition is subject to the continuum
paradox: on the one hand, the limit is necessary to allow the description of phenomena
at a point by means of infinitesimal calculus; on the other hand, the volume
∀
must
be kept large enough so that
n
0
represents a meaningful ensemble average over pores
of many different sizes. The porosity of a soil depends primarily on its particle size
distribution and on its structure. Some of these features are illustrated in Figures 8.1 and
8.2. A soil with a wide distribution of particle sizes tends to have a smaller porosity than
a soil consisting of particles or grains of a more uniform size. The structure of a granular
porous material refers to the arrangement of the particles among one another and to their
aggregation into larger structures. Thus the porosity can be increased by agricultural
operations, such as ploughing or raking, or by frost; these processes “open up” the soil
simply by rearranging the relative positions of the particles. Similarly, the porosity of the
soil can be decreased by compaction. In principle, in the case of soils consisting of inert
material, their texture, that is the size of the particles, should not affect the porosity, as
long as their structure, particle size distributions and chemical composition are similar.
However, actual soils are not inert, but the surfaces of their particles carry electrical
∀
