Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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lateral support, a generally accepted pro-
cedure is to contain individual soil samples in lucite collars as shown in the 
figure. Soil samples are usually 1 to 2 cm high and 5 to 7 cm in diameter. 
Under an applied pressure, medium- to coarse-grained soils usually reach 
equilibrium in one or two days, whereas clays may take a week or more 
to reach equilibrium. Vapour losses can occur with long periods required 
for equilibration. Under such circumstances, the controlled vapour pressure 
technique (discussed in the next subsection) for determination of the soil\u2013
water potential might be more appropriate.
110 Environmental Soil Properties and Behaviour
For each applied pressure to the pressure membrane, when equilibrium 
is reached, the water remaining in the sample is considered to be held 
within the soil by internal forces (i.e., forces originating within the soil) 
that are greater than those applied by the air pressure introduced into 
the chamber. Figure  3.12 shows some typical soil\u2013water potential rela-
tionships between three kinds of soils, generally obtained with pressure 
membrane techniques.
A thermodynamic analysis of the processes associated with the pressure 
membrane procedure shows that if a sample in the chamber is fully saturated 
at the onset of a test where the initial pressure Pi in the pressure membrane 
apparatus is zero, that is, Pi = 0, then the applied pressure Pw is a direct mea-
sure of the matric and solute potentials, \u3c8m and \u3c8s, respectively; that is, Pw 
provides a measurement of \u3c8 = \u3c8m + \u3c8s.. Similar to the case of measurements 
with the tensiometer, the effect of dissolved solutes is included in the mea-
surements obtained with the pressure membrane technique. Accordingly, it 
is necessary to keep this in mind and to be sure to distinguish between mea-
surements that do or do not include the effect of dissolved solutes. There are 
at least two different schools of thought concerning the matric potential \u3c8m. 
These revolve around whether the matric potential does or does not include 
the effects of solutes. For nonswelling soils, this issue is not particularly sig-
nificant. However, for swelling soils, if one maintains that the solute poten-
tial is the osmotic potential (i.e., \u3c8s = \u3c8\u3c0), it becomes necessary to account for 
this potential. The following relationship can be used: \u3c8s = \u2212RTCs, where 
R is the universal gas constant, T is the absolute temperature, and Cs is the 
Test sample in rigid lucite
collar resting on top of
cellulose membrane
Pressure chamber with
cellulose membrane on
top of ceramic porous
stone underlying test
samples
Air/gas pressure
Open to atmosphere
Gauge
FIguRE 3.16
Pressure membrane apparatus used for determination of soil\u2013water potential for silts and 
clays.
111Soil\u2013Water Systems
concentration of solutes. So long as one is careful in differentiating between 
the various effects, either concept is acceptable.
3.4.2.4 Vapour Pressure Technique
Extended periods required for equilibration of samples under pressure 
may result in vapour losses in the samples. A way in which this can be 
avoided is to use the controlled vapour pressure method for determination 
of soil\u2013water potential. In this method, a controlled vapour pressure of 
water is introduced to the chamber. The soil samples in the chamber will 
either gain or lose water until the potential of the soil water is the same 
as that of the surrounding air. At this juncture, the sample water content 
can be determined (Figure 3.17). A simple method consists of placing soil 
samples in a desiccator containing sulphuric acid. Since sulphuric acid 
maintains a definite vapour pressure which can be regulated by regulat-
ing the concentration, this permits one to use the hypsometric relationship 
which relates the vapour pressure to soil\u2013water potential to determine the 
soil\u2013water potential of the samples in the desiccator. Strictly speaking, it 
should be noted that the sulphuric acid is sorbed in the soil sample.
3.4.2.5 Thermocouple Psychrometer Measurements
The thermocouple psychrometer, which can be used in laboratory studies 
or as a field instrument, measures vapour pressure. This is a well-defined 
0
20
40
60
80
Soil-water Potential, kPa
Re
lat
ive
 V
ap
ou
r P
re
ss
ur
e P
/P
o, 
× 
10
0
100
\u20131.0 \u201310 \u2013103\u2013102 \u2013104 \u2013105 \u2013106
FIguRE 3.17
Soil\u2013water potential at different relative vapour pressures with which soil is in equilibrium.
112 Environmental Soil Properties and Behaviour
thermodynamic quantity which depends on both the matric \u3c8m and solute 
\u3c8s (osmotic) components of the soil\u2013water potential \u3c8w. The psychrometer 
probe essentially consists of a small ceramic bulb within which the ther-
mocouple end or juncture is embedded (Figure  3.18). Cooling of the junc-
ture is obtained by passing an electrical current through it (Peltier effect). 
Cooling of the juncture below the dew point will result in condensation at 
the juncture. The condensed water will evaporate when the electrical current 
is removed or discontinued. There is an inverse relationship between the 
rate of evaporation of the condensed water and the vapour pressure in the 
psychrometer bulb. Evaporation of the condensed water at the juncture will 
result in a drop in the temperature, the magnitude of which will depend on 
the relative humidity and temperature of the immediate volume surround-
ing the psychrometer. The drier the surroundings, the faster the evaporation 
rate, and hence the greater the temperature drop. The drop in temperature 
is measured as the voltage output of the thermocouple. The relationship 
between \u3c8w and the relative humidity is given as
 
\u3c8w
m
o
RT
V
p
p
= ln (3.22)
where R is the universal gas constant, T represents the absolute temperature, 
Vm is the molal volume of water, p is the vapour pressure of the air in the soil 
\u2022
Copper (+) wire
Copper (\u2013) wire
Constantan wire
Porous ceramic bulb
Chromel-constantan
thermocouple
Welded junction
Teflon insert
Connecting pin
Epoxy resin
FIguRE 3.18
Schematic illustration of thermocouple psychrometer cross-section showing principles of 
Peltier cooling technique for determination of soil\u2013water potential.
113Soil\u2013Water Systems
voids, po is the vapour pressure of saturated air at the same temperature, and 
the ratio of p/po is the relative humidity.
3.4.3 Hysteresis of Soil\u2013water Potential Relationships
As illustrated in the left-hand sketches in Figure 3.2, soil pores include differ-
ent masses of water in the drying process and in the wetting process at the 
same capillary potential. This is because the demonstrated capillary force 
is a function of only the structure or geometry of the pores, as witness the 
left-hand illustration of the pore radii involved in the wetting and drying 
processes. It follows that the matric potential \u3c8m relationship for volumet-
ric water content \u3b8, which is defined as the water characteristic curve or water 
retention curve, varies between the drying process and the wetting process. 
This phenomenon, which is defined as hysteresis of the water characteristic 
curve, is more predominant in sands as opposed to clays.
When a soil begins the drying process in the fully saturated water content 
state, and continues onward until the water content reaches the fully dried 
state, the resultant water characteristic curve is defined as the primary drying 
curve (Figure 3.19). Conversely, when a fully dried soil begins its wetting pro-
cess and continues until it reaches the apparent saturated state, the resultant 
water characteristic curve is defined as the primary wetting curve. The final 
First primary drying curve
Second primary drying curve
Secondary drying curve
Secondary wetting curve
Primary wetting curve
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