Geoscience Reference
In-Depth Information
of high-quality waters for Dijon. Putting all his knowledge into the service of water
supply for the town Dijon, he succeeded and the town opened one of the most
advanced water supplies at that time with many public fountains in 1844, a couple
of years after completion of the project. In 1856, shortly before his death, he wrote
the topic
Public Fountains of the City of Dijon
(
Les Fontaines Publiques de la Ville
Dijon
) to guide other engineers in constructing similar edifi ces of public impor-
tance. It was well before water supply systems were common in European or US
cities. The equation and the basic law about fl ow of water in porous materials are
named in honor of his legacy.
As mentioned earlier, hydraulic conductivity has the dimension of velocity
(length/time or L/T). Unfortunately, we fi nd less exact terms are sometimes intro-
duced in nonscientifi c literature, especially, e.g., permeability and fi ltration coeffi -
cient. However, the scientifi c word permeability has an exactly defi ned meaning
with the dimension of
L
2
used for description of “fl ow” of air and generally of gas
through porous materials, like soils and various fi lters. And the scientifi c term fi ltra-
tion coeffi cient is reserved for catching soil or solid particles bigger than is the size
of pores.
Soil hydraulic conductivity depends upon several soil properties. Soil porosity
and pore-size distribution are those most frequently cited for their dominating infl u-
ence and control of its value. The saturated hydraulic conductivity of sands and
sandy soils is very high because water fl ows primarily through their big pores with
minimal viscous resistance because the water comes in contact with only a small
solid area. The conductivity values of sands and sandy soils are usually about
100 cm/day. Loamy soils have lower values, usually several tens of centimeters per
day, since they contain a combination of big and small pores. And, if small pores
dominate their porosity, the conductivity sinks to about a centimeter per day or even
less. The value of conductivity depends strongly upon the degree of soil aggrega-
tion. Loamy soils with well-developed, stabile structure may have a conductivity up
to 50-100 cm/day, but when the structure is totally deteriorated, the conductivity
decreases by as much as 50 times. Clays manifest the lowest values of hydraulic
conductivity. With their specifi c surface being very large (50-200 m
2
/g), water
moves with great diffi culty owing to the very high friction losses at the boundary
between solid clay particle surfaces and water. Conductivity of clay soils may fall
well below 1 cm/day with their specifi c values strongly dependent upon the compo-
sition of the clay minerals (Fig.
9.3
).
If a clay soil composed mainly of smectites is dry and cut by vertical cracks due
to shrinkage, its hydraulic conductivity would be high during the fi rst 10-60 min of
infi ltration. Subsequently, as a result of wetting and swelling, its conductivity would
be almost nil. On the other hand, when a clay soil remains moist and never dry, it is
considered as an impervious material frequently used for the internal sealing core of
inundation dams that protect the plains from fl oods during high discharge in the
river. Clay is used in a similar way in dams of reservoirs. In the middle of the dam
constructed of compacted loamy soil, there is a trapezoidal cross section of a com-
pacted clay core. Without this type of sealing core, water from the reservoir soaks
through the dam and escapes to the downhill side of the dam. This leaking water
