Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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as 
porewater (in the macroscopic sense). However, for geoenvironmental engi-
neering purposes, where water uptake, unsaturated flow, solute transport, 
heat flow, and so forth are important considerations, there is a need to distin-
guish between the mechanisms by which water is held in the soil. Many of 
these issues will be discussed in the later sections of this chapter and in the 
other chapters at the appropriate juncture.
3.4.1.1 Components of Soil\u2013water Potential
Figure  3.1 in Section 3.2 showed the simple capillary experiment using a 
glass column containing clean sand, demonstrating that water is held in the 
sand by capillary action and that the height h of the water drawn up into 
the sample is h = 2Tcos\u3b1/r\u3b3wg. This glass column is shown schematically in 
the left-hand illustration in Figure  3.13, where the height of capillary rise 
in the sand column is designated as hs. The sketch on the right-hand side of 
the figure shows a similar glass column containing a dry compact inorganic 
clay also placed in a shallow pan of water. In this column, we will see that 
the height of water rise (water uptake) H in the column, which is the sum of 
hc and \u394hc, far exceeds hs of the sand column in the equilibrium state. The 
height of the capillary rise in the sand column hs in Figure 3.13 is determined 
by the factors described in Figure 3.1 in the derivation of the relationship 
for h. In the case of the clay column shown on the right side of the figure, 
the height of capillary rise hc is a computed value based on the average pore 
radius of the clay sample. In reality, this is not distinguishable from the total 
height of rise in the clay column, as will be discussed later when we address 
the mechanisms involved in water uptake and movement in soils.
In geotechnical and soil engineering, the height of capillary rise hs in the 
sand column is generally described as being due to capillary suction, mean-
ing that capillary forces are responsible for suction of the water in the sand 
column. From a thermodynamic point of view, one could define a capillary 
potential \u3c8c that represents a measure of the energy by which water is held 
by the sand particles by capillary forces. Buckingham (1907) defined it as the 
potential due to capillary forces at the air\u2013water interfaces in the sand pores 
holding water in the sand.
The requirements of analyses of the soil\u2013water system as a whole are bet-
ter satisfied if the height of water uptake in a soil column is determined in 
104 Environmental Soil Properties and Behaviour
terms of the work required to move water up to its equilibrium height H. The 
total work required to move water into (and out of) a soil, which is defined 
as the soil\u2013water potential \u3c8, describes the water-holding capability of soil; 
that is, it describes the energy by which water is held to (or attracted to) the 
various soil solids comprising the soil under consideration. In soil\u2013water sys-
tems where the soil solids possess reactive surfaces such as the clay column 
shown in the right-hand sketch in Figure  3.13, the interaction of the reac-
tive surfaces of the soil solids (e.g., clay particles) with water in combination 
with the microstructure of the clay are responsible for the height H of water 
uptake in the clay column. It is particularly useful in providing a simple 
picture of the kinds of internal forces that will contribute to water movement 
and retention in soils.
In general, the work associated with actions on soil water can be quanti-
tatively represented by Gibbs free energy if one uses thermodynamic con-
cepts. Specifically, one can consider the energy with which water is held 
in a soil mass as the energy of the water in a soil. Gibbs free energy per 
unit mass of soil water is termed the chemical potential. Analyses of water 
retention and movement in a soil\u2013water system are better satisfied using 
chemical potentials. Since the energy state of soil water (porewater) in partly 
\u394hc
hc
Inorganic clay
in column
Pan filled with water
Clean quartz sand
in column 
Glass column
hs = Height of
capillary
rise in sand
column 
H = Total height 
of water
uptake in
clay column
hc = Computed height 
of capillary rise in 
clay column based
on average pore 
radius
\u394hc = Additional height
of water over and
above computed
height of
capillary rise
FIguRE 3.13
Capillary rise in quartz sand column (left) and water uptake in an inorganic clay (right). Note 
that fractioning of H into hc and \u394hc is performed theoretically; that is, the capillary rise in the 
clay column is a calculated value based on the average pore radius in the sample. In reality, it 
is not possible to partition H.
105Soil\u2013Water Systems
saturated soils is determined by such factors and forces such as (a) capil-
lary action, (b) solute concentration, (c) interactions with the reactive particle 
surfaces, and (d) external forces such as gravity, overburden load, surcharge, 
and static water pressure, it follows that the chemical potential of soil water 
is the summation of the free energies associated with all these factors and 
forces. It is less than that of free water under one atmospheric pressure at the 
same elevation and temperature of the test sample. This chemical potential, 
which is defined as the total soil\u2013water potential \u3c8 (Iwata et al., 1995), is a 
negative quantity.
The components of the total soil\u2013water potential \u3c8 are described as follows:
\u2022	 Total soil\u2013water potential \u3c8 is defined as the chemical potential derived 
from the sum of the various forces, and is equivalent to the work 
required to move a unit quantity of water from the reference pool 
to the point under consideration in the soil. It is a negative quantity.
\u2022	 Matric potential \u3c8m is a reflection of the property of the soil matrix. 
This is specified as \u2212(2\u3c3/r)\u3bdw , where \u3c3 is the surface tension of water, 
r is the pore radius, and \u3bdw is the specific volume of water (= 1 cm3/g). 
This is equivalent to the Buckingham capillary potential, where the 
sorption forces in the sand column experiment are due to capillary 
phenomena. In the case of soils containing particles with reactive 
surfaces, there are other types of forces that need to be taken into 
account. A good example of this is the case of clays, and especially 
the case of swelling clays. Complications surrounding analysis of 
interlayer swelling and microstructural effects and influences do 
not permit easy resolution in terms of capillary \u201cforces.\u201d According 
to Sposito (1981), the matric potential \u3c8m includes the effects of dis-
solved components of the soil\u2013water system on the chemical poten-
tial \u3bcw.
\u2022	 Gravitational potential \u3c8g = \u2212gh; where h is the height of the clay above 
the free water surface, and g is the gravitational constant. If the point 
in the clay under consideration is below the surface, h is a negative 
quantity, and hence the relationship becomes positive.
\u2022	 Osmotic potential \u3c8\u3c0 is specified as 
\u2212 =\u2211RT ni i w1000 pi \u3bd\u3a0 for nonideal 
solutions, where \u3c0i is the osmotic coefficient (1 for ideal solutions), ni 
is the molality of solute i, and \u3a0 is the osmotic pressure and is equiv-
alent to the work required to transfer water from a reference pool 
of pure water to a pool of soil solution at the same elevation, tem-
perature, and so forth. The term osmotic potential is generally used in 
conjunction with swelling clays. For nonswelling soils, the general 
term solute potential \u3c8s is more appropriate. To avoid confusion, the 
more general term solute potential is favoured. It is not unusual for 
the literature to report on the use of \u3c8s as the osmotic potential \u3c8\u3c0, 
106 Environmental Soil Properties and Behaviour
that is, \u3c8\u3c0 = \u3c8s = nRTc, where n is the number of molecules per mole of 
salt, c is the concentration of solutes, R is the universal gas constant,