Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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for interactions of particles with face-to-face, face-to-edge, 
and edge-to-edge configurations, it is difficult to assign the proportions of any 
of these configurations to the overall soil fabric and soil structure.
Common methods for obtaining parallel particle orientation for swelling 
pressure tests include controlled slow evaporation of shallow pans of very 
dilute montmorillonite suspensions (Yong and Warkentin, 1959) and high-
pressure consolidation (Alammawi, 1988). Figure 3.10 shows the results from 
the high-pressure consolidation tests on a Na-montmorillonite saturated 
with a 10\u22123 MNaCl reported by Alammawi, together with calculations from 
DLVO theory and from a modified Gouy Chapman DDL model (G-C DDL 
model). The modifications for the G-C DDL model reported by Yong and 
Mohamed (1992) were required to account for the energies in the Stern layer 
(i.e., energies in the inner Helmholtz and outer Helmholtz planes). In the 
unmodified G-C model (without inclusion of the Stern energies), the charge 
on the surface of the particle \u3c3s must be balanced by the total space charge 
\u3c3\u3c1 in the soil solution to preserve electroneutrality. To account for the inter-
actions in the Stern layer, the surface charge at the surface is balanced by 
the additional contributions from the charges due to the ions in the inner 
Helmholtz and outer Helmholtz planes, \u3c3ihp and \u3c3ohp, respectively.
100 Environmental Soil Properties and Behaviour
The results shown as \u201cBolt\u201d in Figure 3.10 are from the experimental results 
reported by Bolt (1956) also for Na-montmorillonite at the same salt concen-
tration. The \u201cvan Olphen\u201d results added to the previously reported results 
of Yong and Mohamed (1992) are interpreted from calculations reported by 
van Olphen (1963) for pressures required to remove the first four water layers 
next to a typical montmorillonite unit. The comparison of calculated values 
and experimental results shown in the graph indicate acceptable agreement 
between calculated and measured swelling pressure results, especially at 
the higher particle separation distances (2x). We will discuss these results in 
greater detail when we address the topic of water uptake and movement in 
partly and fully saturated clays.
3.4 Soil\u2013Water Energy Characteristics
3.4.1 Concept of Soil\u2013Water Potential
Of all the physical properties of soil, the energy relationship of soil for water 
is perhaps the most important. It describes the energy with which water is 
held in a soil mass in relation to its water content. Many different terms have 
been used to describe the energy with which water is held in soils. This is 
1 10 102 103 104 105 106
Pressure, kPa
0
2
4
6
8
10
12
14
Se
pa
ra
tio
n 
Di
sta
nc
e 2
x, 
nm
DLVO
Modified Gouy Chapman
Bolt
Experimental
van Olphen
FIguRE 3.10
Comparison of experimental swelling pressure results with calculated results from theoretical 
models. Experimental results are from tests reported by Alammawi (1988).
101Soil\u2013Water Systems
because terms were required, both in research where thermodynamics ter-
minology was usually used, and in practical soil\u2013water work and engineer-
ing, where descriptive terms were used. These descriptive terms included 
such terms as soil\u2013water tension and soil suction, indicating that the water in 
soil is in equilibrium with a pressure less than atmosphere (soil\u2013water ten-
sion), and that the soil exerts a force to take in water (soil suction).
The energy with which water is held to a soil at any water content has 
been specified as the soil\u2013water potential. For example, Buckingham (1907) 
defined the capillary potential of a soil as the work required per unit weight 
of water to pull water away from the mass of soil, having in mind that this 
potential is due to the capillary forces holding water in soil. This potential 
decreases as the water content increases, and vice versa. To demonstrate the 
macroscopic relationship between energy by which water is held in soil and 
its water content, we consider a simple laboratory desorption test using a 
Buchner-type apparatus as shown in the top left-hand sketch in Figure 3.11.
This type of laboratory test measures the amount of energy required to 
push water out from the soil sample contained in the apparatus, using air 
pressure acting on the top of the soil sample to drive water out from the 
sample. In this demonstration exercise, one applies air pressure to the soil 
sample contained in the apparatus as illustrated schematically in the figure. 
Initial state, \u3b8initial P1
\u3b81
Vx = V1 P2
\u3b82
Vy = V1 + V2
P3
\u3b83
Vz = V1 + V2 + V3
P = Applied air pressure
P1 < P2 < P3
\u3b8 = Volumetric water content
V = Volume of extruded water
P1 P2 P3
Vz
Vy
Vx
Ex
tru
de
d 
W
at
er
 V
ol
um
e
Applied Air Pressure
P
FIguRE 3.11
Schematic illustration of desorption-type experiment using the Buchner-type pressure appa-
ratus. The right-hand sketches illustrate the steps in the test procedure, and the bottom left-
hand graph shows the results obtained.
102 Environmental Soil Properties and Behaviour
After equilibrium is reached at each pressure increment, the amount of water 
extruded or discharged is determined.
A relationship can be developed between water content in the soil and 
pressure applied, taking note of the initial water content (before pressure 
application), the amount of water discharged under the pressures applied, 
and making the necessary calculations to track the equilibrium water content 
remaining in the soil. This is the water-holding capacity of the soil at that par-
ticular applied pressure. This sequence of pressure application is shown in the 
right-hand sketches in Figure 3.11 and the results plotted in the graph shown 
at the bottom left-hand portion of the figure. The graphical results portrayed 
in Figure 3.11 are presented in terms of water content and applied pressure. 
Water retention or water-holding capacity is viewed from the soil\u2013particle 
frame of reference in terms of suction. This is the opposite of the frame of ref-
erence which considers water-holding capacity in terms of the work required 
to move water into or out of the sample: the soil\u2013water potential. In that sense, 
whilst both measurements should give equal results in terms of magnitude 
of effort required, suction measurements are expressed positively, whereas 
potential measurements have the opposite (negative) sign. Figure 3.12 shows 
the results for three typical kinds of soils using the pressure membrane as 
the desorption-provoking device. In this case, the soil\u2013water potential is used 
in place of the applied pressure as the abscissa. The discussion relating to 
Soil-water Potential, kPa
\u20130.1 \u20131 \u201310 \u2013102 \u2013103 \u2013104 \u2013105 \u2013106
0
40
10
20
30
50
60
Vo
lu
m
et
ric
 W
at
er
 C
on
te
nt
 \u3b8
, P
er
ce
nt
Medium-type clay
Silty-clay loam 
Sand
FIguRE 3.12
Typical soil\u2013water potential relationships for sand, silty-clay loam, and a medium-type clay. 
The results are obtained as desorption curves using the pressure membrane apparatus.
103Soil\u2013Water Systems
the pressure membrane technique is given in Section 3.4.2 when methods for 
determining soil\u2013water potential are addressed.
As we have learnt from the discussions in the previous section, different 
types of forces are involved in respect to how water is held to soil parti-
cles, between soil particles, and within the soil mass itself. This means that 
water in soils is held within the soil with different mechanisms depending 
on whether one is concerned with water surrounding individual soil par-
ticles, interlayer water, interparticle water, or \u201cbulk\u201d water in soils. For soil 
engineering purposes, all of these kinds of \u201cwaters\u201d are lumped together