Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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Matric Suction, (cm)
Vo
lu
m
et
ric
 W
at
er
 C
on
te
nt
, \u3b8
FIguRE 3.19
Hysteretic phenomena observed for water characteristic curves for sand. Arrows indicate wet-
ting and drying processes.
114 Environmental Soil Properties and Behaviour
saturated water content obtained in this wetting process is less than that 
of the fully saturated water content since the soil will entrap air bubbles in 
the wetting process. When drying of the apparently saturated soil (which 
includes the entrapped air bubbles) occurs, the measured water characteris-
tic curve will differ from the primary boundary curves. When the wetting or 
drying occurs in the middle of the drying or wetting process, the developed 
characteristic curves, which are defined as the secondary wetting or secondary 
drying curves, will be different from the primary curves. The water charac-
teristic curves are important pieces of information that reflect the changing 
water content processes, and are useful in analyses of water uptake, drain-
age, and water flow in engineering projects.
The hysteretic phenomena of water characteristic curves can be analyzed 
and simulated quantitatively using two kinds of pore models since it is a 
function of the characteristics of the soil pore structure:
\u2022	 Blown tube model: The pore geometry is simplified by assuming a 
shape similar to the pore structure model illustrated by the left-hand 
sketches in Figure 3.2 in Section 3.2.1 (Nakano, 1980).
\u2022	 Sphere packing model: The pore geometry is assumed to be the space 
formed in an ideal packing of spheres of similar radii (Zou, 2007).
Hysteretic performance in clays is not easy to explain with a simple model. 
This is because of the ink-bottle effect (as shown in Figure  3.2) and also 
because of changes in the nature and sizes of the various microstructural 
units. Movement of particles in these microstructural units occurs in the wet-
ting and drying processes, resulting in fabric distortion and \u201cplastic readjust-
ment.\u201d When volume change occurs, the fabric changes accompanying volume 
changes will produce corresponding changes in pore size distributions and 
clay\u2013water interactions arising from physicochemical forces. Interparticle con-
tacts and forces at the points of contacts differ on wetting and drying.
3.5 Water Uptake and Transfer
The discussion in this section directs its attention to the principles govern-
ing movement of moisture into partly saturated soils. The discussion on water 
movement in fully saturated soils will be found in the chapters and sections 
dealing with the hydraulic properties and performance of soils (Chapter 7). The 
term water is generally considered to mean liquid water, whereas the term mois-
ture is used as a more inclusive term, that is, liquid water and vapour. Where 
it is necessary to avoid confusion, the term liquid water will be used in place of 
water. The term partly saturated soil is used in preference to unsaturated soil.
115Soil\u2013Water Systems
The water content of a soil is a significant property of a soil because of its 
role in establishing the properties of soils. It is not a static quantity. Additions 
of water come naturally from rainfall, snow-melt, subsurface flow, and con-
densation, and natural depletion of water content occurs as water losses due 
to evaporation, transpiration, and drainage. Additions and depletions of 
water content also occur due to anthropogenic activities such as irrigation in 
agricultural practices, alteration of surface hydrology features, land reclama-
tion, and so forth. The terms water uptake and migration refer specifically to 
water entering a dry or relatively dry soil (water uptake), and water transfer in 
partly saturated soils after moisture uptake (water migration).
3.5.1 Moisture Transfer
In natural circumstances, most of the water movement in soils is due to gra-
dients of matric potential \u3c8m or capillary potential \u3c8c that arise from differ-
ences in water content. Concentration gradients (differences in concentration 
of solutes in porewater in different parts of the soil) will also provoke water 
transfer. The osmometer experiment shown as a schematic in Figure  3.20 
illustrates this phenomenon. In this example, the soil sample in the left-hand 
cell is fully saturated with ionic solutes in its porewater. The total potential 
in the soil is \u3c8 = \u3c8s. For a partly saturated, the potential will be, \u3c8 = \u3c8m + \u3c8s .
De-ionized water
Suction device
Selective membrane permitting only diffusion
of water molecules through membrane 
Saturated soil
with ionic solutes
in porewater
Soil-water potential
\u3c8 = \u3c8m + \u3c8s
Mercury manometer for
measurement of soil suction S
S = SS + SM
Osmometer system
FIguRE 3.20
Osmometer-type cell showing development of suction required to counter flow of water into 
the left-hand side chambers because of the total potential \u3c8 in the saturated soil. S, SS, and SM 
refer to the total suction, solute suction, and matric suction, respectively.
116 Environmental Soil Properties and Behaviour
The right-hand cell in the osmometer system contains deionized water 
which is separated from the left-hand cell by a fixed-position selective mem-
brane that permits only diffusion of water molecules through the mem-
brane. When the concentration of a solution differs from that at another 
point, there is a tendency for the more dilute liquid to diffuse into the region 
of higher concentration. This is the case for the left-hand and right-hand 
cells shown in Figure 3.20. The potentials in the soil in the left-hand cell pro-
duce gradients that will induce the deionized water in the right-hand cell to 
diffuse into the soil to attain a more uniform ionic concentration. To restrain 
diffusion of water in the right-hand cell through the selective membrane to 
the left-hand cell, one needs to apply suction to the water in the right-hand 
cell. The suction required to restrain diffusion, S, can be considered to con-
sist of SS and SM , where SS and SM are the solute suction and matric suction, 
respectively.
3.5.1.1 Water Transfer and Wetting Front
Water movement in soil above the water table occurs when both water and 
air are present in the voids. This is the partly saturated soil zone of interest 
in many soil and geoenvironmental engineering projects. Water transfer in 
partly saturated soils is of considerable interest also to the agro industry. 
Commonly accepted terminology that describes water transfer in partly sat-
urated soils as unsaturated flow will be used in this book. The characteristics 
of unsaturated flow in soils are demonstrated in the portrayal of wetting-front 
advance results obtained from a horizontal soil column permeation experi-
ment (Figure 3.21). Beginning with a dry soil, a constant head source of water 
is supplied by the double Mariotte flask. The ability to locate the Mariotte 
flask air entry position (up or down) allows one to conduct the permeation 
experiment with various values of negative or positive heads.
The profile depicted in the diagram, which is called a wetting front pro-
file, consists of a wetting zone ahead of a transmission zone and behind 
the wetting front. The wetting zone and wetting front combine to form 
characteristic shapes that can inform one on the nature of water diffusion 
into the soil.
In general, moisture transfer without convective flow is expressed using a 
Darcy-type equation as follows:
 
v k
x
= \u2212
\u2202
\u2202( )\u3b8
\u3c8
 (3.23)
where v is the flux, k(\u3b8) is the Darcy coefficient, \u3b8 is the volumetric water con-
tent, x is the spatial distance from the source of water, and \u3c8 is the soil\u2013water 
potential\u2014that is, the sum of the various potentials associated with the vari-
ous forces operating in the soil\u2013water system (see Section 3.4.1).