soil–water characteristic curve of gypseous soil
TRANSCRIPT
ORIGINAL PAPER
Soil–Water Characteristic Curve of Gypseous Soil
Abdulrahman Aldaood • Marwen Bouasker •
Muzahim Al-Mukhtar
Received: 22 June 2014 / Accepted: 14 October 2014
� Springer International Publishing Switzerland 2014
Abstract The soil–water characteristic curve
(SWCC), also known as soil–water retention curve,
is a key tool in assessing the behavior and properties of
unsaturated soil. The SWCC of gypseous soil with 0,
5, 15 and 25 % gypsum content was determined, using
tensiometric plate, osmotic membrane and vapour
equilibrium techniques, with suction pressures rang-
ing between 10 and 1,000,000 kPa. The effect of two
compaction efforts, standard and modified, was
examined on the SWCC of soil samples. The water-
holding capacity of soil samples increased with
increasing gypsum content and applied compaction
effort. Mercury porosimetry tests and scanning elec-
tron microscope images revealed that compaction and
the presence of gypsum increased the number of
capillary pores. These changes in the pore size
distribution of soil samples induced modifications in
the volumetric water content at air-entry value of the
tested samples. All experimental SWCC data were
fitted using the Fredlund and Xing (Can Geotech J
31(4):521–532, 1994) and Van Genuchten (Soil Sci
Soc Am J 44:892–898, 1980) models. Results showed
that a high coefficient of determination (R2) can be
achieved by using both models to fit the experimental
results of gypseous soil SWCC.
Keywords Gypseous soil � Soil–water characteristic
curve (SWCC) � Micro-structure � Compactive effort
1 Introduction
Gypseous soils are commonly found in many arid and
semiarid zones in the world. These soils typically
exhibit low strength, and high collapse and settlement
characteristics upon wetting. However, the problems
caused by gypseous soils are usually associated with
climate because in arid and semiarid zones climatic
conditions change over time, and these climate
changes cause moisture changes within unsaturated
soils near the surface.
Determining the properties of unsaturated soil in a
wide range of gypsum content is a fundamental issue
in many geotechnical and geoenvironmental applica-
tions such as pavement layers, embankments, engi-
neered liners and covers (Gourley and Schreiner 1995;
Meerdink et al. 1996; Vanapalli et al. 1996; Rao and
Revanasiddappa 2000). The engineering behavior of
A. Aldaood � M. Bouasker � M. Al-Mukhtar (&)
Centre de Recherche sur la Matiere Divisee, CRMD -
CNRS and Laboratoire PRISME UPRES n�4229, 8 Rue
Leonard de Vinci, 45072 Orleans Cedex 2, France
e-mail: [email protected]; muzahim.al-
A. Aldaood
e-mail: [email protected]
M. Bouasker
e-mail: [email protected]
A. Aldaood
Civil Engineering Department, College of Engineering,
Mosul University, Mosul, Iraq
123
Geotech Geol Eng
DOI 10.1007/s10706-014-9829-5
soils that are typically in an unsaturated state can be
better interpreted if the influence of suction is taken
into account (Fredlund 2000). The important role of
suction in the shear strength, permeability, deforma-
tion and volume change properties of unsaturated soils
has long been recognized. Moreover, in recent years,
analyzing suction in the context of the aforementioned
geotechnical properties has become the subject of
much research in the rapidly growing field of unsat-
urated soil mechanics (Delage et al. 1998; Al-Mukhtar
et al. 1999; Melinda et al. 2004; Guan et al. 2010;
Thyagaraj and Rao 2010; Sheng et al. 2011).
Soil suction (total suction) is defined as the total
free energy of the soil water per unit volume. It
comprises two types of suction: matric and osmotic
suction (Fredlund and Rahardjo 1993). Matric suction
refers to a measure of the energy required to remove a
water molecule from the soil matrix without the water
changing state. In another words, matric suction is the
energy required to move a water molecule within the
soil matrix. It represents the difference between the
pore air pressure and the pore water pressure. Osmotic
suction arises from differences between the salt
concentration within the pore water and that of pure
water. It is generally the result of the chemical content
(mineral salt). The total soil suction is given by the
sum of matric and osmotic suction. For low suction
values, only a small influence of osmotic suction is
observed; for higher suction values, above 1,500 kPa,
the contribution of osmotic suction is absolutely
negligible (Burckhard et al. 2000; Cokca 2002).
The soil–water characteristic curve (SWCC), also
called the soil–water retention curve (SWRC), is
defined as the relationship between suction and the
corresponding state of wetness of the soil. The state of
wetness can be expressed in various ways, namely
volumetric water content (h), gravimetric water con-
tent (w/c) or degree of saturation (Sr) (Fredlund and
Rahardjo 1993; Fredlund and Xing 1994). The soil–
water characteristic curve is a measure of the water-
holding capacity (i.e. storage capacity) of the soil as
the water content changes when subjected to various
suction values. The soil–water characteristic curve is a
conceptual and interpretative tool through which the
behavior of unsaturated soils can be understood. As
the soil moves from the saturated state to a drier state
(unsaturated state), the distribution of the soil water
and air phases changes, as the stress state changes. The
relationships between these phases take on different
forms and influence the engineering properties of
unsaturated soils (Vanapalli et al. 1999a).
Several factors influence the soil–water character-
istic curve such as soil type: mineralogy, texture,
structure and plasticity (Likos et al. 2003; Khattab and
Al-Taie 2006; Nam et al. 2009); compaction param-
eters: compaction water content, dry unit weight and
compaction effort (Vanapalli et al. 1999b; Romero
et al. 1999; Miller et al. 2002; Yang et al. 2004; Thakur
et al. 2005; Osinubi and Bello 2011); void ratio, stress
history, heat, suction measurement methodologies and
other factors (Vanapalli et al. 1999a; Romero et al.
2001; Lee et al. 2005; Tang and Cui 2006; Salager
et al. 2011). Soil samples of a particular soil, that have
the same texture and mineralogy, can have different
soil–water characteristic curves. As a result, the
engineering behavior of each of the samples will also
differ.
Along with the development of experimental
methods to determine the SWCC, numerous models
have been proposed for fitting analytical functions
through experimental results. Many of these models
are derived from the pore-size distribution through
micromechanical relationships between effective pore
size and soil suction (Sillers et al. 2001).
In Fredlund and Xing (1994) proposed a model
using a three-parametric continuous function as shown
below:
h ¼ hs 1�ln 1þ W
Wr
� �
ln 1þ 1;000;000Wr
� �24
35 1
ln eþ Wa
� �n� � !m
ð1Þ
where h = the volumetric water content at desired
suction, hs = the saturated volumetric water content,
W = the soil suction (kPa), Wr = the soil suction
(kPa) corresponding to the residual water content, hr,
a = a soil parameter that is related to the air entry
value of the soil (kPa), n = a soil parameter that
controls the slope at the inflection point in the soil–
water characteristic curve, m = a soil parameter that
is related to the residual water content of the soil; and
e = the natural number, 2.71818….
These parameters are characterized by a clear
physical meaning: parameter (a) is influenced mainly
by grain-size distribution, therefore a fine-grained soil
has a higher air entry value than a coarse-grained soil;
(n) is linked to pore size distribution and depends on
Geotech Geol Eng
123
soil density, and m depends on the asymmetry of the
model.
Van Genuchten (1980) proposed a closed-form
equation for the entire range of suction, given by:
h ¼ hr þhs � hrð Þ
1þ awð Þn½ �m ð2Þ
where the parameters h, hs and W are as in Fredlund
and Xing’s equation, hr = the residual volumetric
water content, a = the parameter related to the air
entry value, n = the parameter related to the pore size
distribution of soil, while m is related to the asymme-
try of the model curve and is equal to 1-n-1.
Studies by Leong and Rahardjo (1997) and Sillers
et al. (2001) have shown that the experimental data for
various soils over a wide suction range can be well
fitted using the above equations.
The aim of this paper is twofold:
• Firstly, to study experimentally the soil–water
characteristic curves (SWCC) of gypseous soil.
The effect of the following parameters on SWCC
was studied: gypsum content and compactive
effort.
• Secondly, to determine the theoretical model
which gives the best fit of the experimental data.
Two well-known models were studied, Fredlund
and Xing (1994) and Van Genuchten (1980). A
simple program was developed to assess the
determination of the parameters of these models.
2 Materials and Experimental Methods
2.1 Materials
The soil used in this research work is a fine-grained
soil, obtained from a borrow pit near Jossigny in the
eastern part of Paris–France. The soil samples were
collected at a depth between (1.5 and 2.0 m) below the
ground surface. After sampling the soil was homog-
enized and kept in plastic bags then transported to the
laboratory for testing.
The soil has a liquid limit of 29 %, a plastic limit of
21 %, and a plasticity index of 8 %. The percentages
of clay, silt and sand are 19, 64 and 17 % respectively.
The soil can be classified as sandy clean clay (CL)
according to the unified soil classification system
(USCS). The compaction properties of the soil as
obtained from the standard and modified Procter tests
are presented in Fig. 1.
The gypsum (CaSO4�2H2O) used in this study,
supplied by the Merck KGaA company, Germany, is a
very fine gypsum and passes through an 80 lm sieve
opening, with a purity more than 99 %.
2.2 Sample Preparation and Compaction
In order to conduct a precise parametric study
(focusing on the influence of gypsum content on
SWCC), all the samples were prepared in the labora-
tory. An experimental program was performed on soil
samples with varying percentages of gypsum (5, 15
and 25 % of the dry weight of soil). To ensure the
uniformity of the soil samples, only soil passing
through a 4 mm sieve opening was used. SWCCs of
soil samples were developed on statically compacted
soil samples prepared at two different initial water
contents and two densities. The initial water contents
selected in this study represent the optimum moisture
contents (OMC) corresponding to the maximum dry
unit weights from the standard (ASTM D-698) and
modified (ASTM D-1557) compaction tests, respec-
tively, as shown in Fig. 1.
To prepare the soil samples, the soil was first oven-
dried for 2 days at 60 �C. The required amount of soil
was mixed with gypsum under dry conditions. Water
was added to the soil samples to reach the desired
OMC values of both standard and modified tests, as
mentioned above. During mixing, proper care was
taken to prepare homogeneous mixtures. The prepared
soil samples were then stored in plastic bags for a
14
15
16
17
18
19
20
21
0 5 10 15 20 25 30
Dry
Uni
t Wei
ght (
kN/m
3 )
Water Content (%)
Standard
Modified
Sr = 100%
Fig. 1 Compaction characteristics of soil samples
Geotech Geol Eng
123
period of 24 h before compaction for moisture equal-
ization. After a mellowing time (i.e. 24 h) the soil
samples were statically compacted to the maximum
standard and modified dry unit weight of the natural
soil, respectively. The soil samples were 50 mm in
diameter and 10 mm in height. After that, the samples
were immediately tested to find the SWCC. In order to
study the SWCC of uncompacted (powder) soil
samples, the soil was mixed with a predetermined
percentage of gypsum (i.e. 5, 15 and 25 %) in dry
state, as mentioned previously. Then the powder
sample (20 g in weight) was placed in a 75 mm
diameter pan.
2.3 Determination of Soil–Water Characteristic
Curve (SWCC)
The SWCC is determined using complementary direct
or indirect techniques that induce soil suction. The direct
method measures the negative pore water pressure due
to suction directly, whereas the indirect method requires
the measurement of other parameters such as relative
humidity (RH) or water content, and then relates the
results to suction through calibration. In this study, three
techniques were used to control the suction in the range
of 10–1,000,000 kPa: tensiometric plates, osmotic
membrane and vapour equilibrium techniques.
For the compacted soil samples, the SWCC in the
suction range of 10–20 kPa was measured using
tensiometric plates. The required suction value was
imposed on the soil sample by adjusting the height of a
column of water in equilibrium with a high air entry
ceramic disk. The suction in the soil sample is
determined directly as a function of the height of
water (where a 1 m water column corresponds to a
suction value of 10 kPa). A time of about 3 weeks is
required for soil samples to reach equilibrium. The
SWCC in the suction range of 100–1,500 kPa was
determined using the osmotic membrane technique.
The soil samples are placed inside a semi-permeable
membrane, then the soil sample and membrane are
submerged in a polyethylene glycol (PEG) solution
with different concentrations to impose various values
of suction (i.e. 100–1,500 kPa). However, good con-
tact is required between the soil sample and the
membrane, and the fragility of the membrane is also a
consideration in this technique. A period of 28 days is
required for the soil samples to reach equilibrium.
The SWCC in high suction ranges was determined
using the vapour equilibrium technique. This tech-
nique is based on the fact that the relative humidity in
the airspace above a salt solution is unique to the
concentration and chemical composition of that solu-
tion. Therefore, by choosing a chemical solution with
the correct target relative humidity, a soil sample
placed in a closed system (desiccator) with this
solution will absorb or yield water vapour to the
airspace until it comes into equilibrium with that
solution. Given the equilibrium relative humidity of
the airspace, it is possible to calculate the total suction
using Kelvin’s equation:
w ðkPaÞ ¼ �RT
Vln
P
P0
� ð3Þ
where R = universal gas constant (8.31432 J/mol K),
T = absolute temperature, V = molar mass of water
vapour, P/P0 = the relative humidity of air in equi-
librium with the pore water, P = partial pressure of
water vapour, P0 = saturated water vapour pressure in
equilibrium with pure water with a flat surface at the
same temperature.
SWCCs of uncompacted soil samples and pure
gypsum were also measured using powder samples.
Only suction with vapor equilibrium and osmotic
membrane techniques were used for these samples as
the suction pressures ranged between (30 and
1,000,000 kPa). The powder samples with pans were
placed in the desiccators to find the SWCC with a
suction pressure of (2,700–1,000,000 kPa), while
samples without pans were placed directly in the
osmotic membrane to complete the SWCC with
suction ranging between (1,500 and 30 kPa). The
tensiometric plate technique (suction pressure less
than 30 kPa) was not used to determine the SWCC of
powder samples, since placing the powder (soil–
gypsum mixture or pure gypsum) on the ceramic disk
will modify the porosity of the disk: gypsum particles
and/or clay particles penetrate the pore spaces of the
disk, due mainly to the dissolution of gypsum with
increasing water content. Thus, gypsum dissolution
causes errors in the results of the SWCC obtained.
The soil samples inside the desiccators absorb or
desorb the moisture until suction equilibrium is
reached (this takes more than 4 weeks). All the
previous techniques were generated under null stress
and at room temperature (20 �C).
Geotech Geol Eng
123
2.4 Micro-structural Measurements
The micro-structural aspect of the soil samples was
studied using scanning electron microscope (SEM)
and mercury intrusion porosity tests. The SEM test
was performed in this study according to the procedure
reported by (Tessier and Berrier 1978) to minimize
micro-fabric changes. The fractions of the soil samples
were injected by epoxy fix resin, gold-coated and then
scanned by a high resolution scanning electron
microscope (Hitachi TM 3000). Several digital images
at different magnifications were recorded in order to
examine the soil structure. All the soil samples were
prepared in the same manner.
A pore size distribution assessment was carried out
by using a Pore Seizer Porosimeter, in which the
mercury pressure was raised continuously to reach
more than 210 MPa and measure the apparent pore
diameter in the range 3.6 nm–350 lm. In a mercury
intrusion porosimetry test, the mercury is forced into
the soil samples; the applied mercury pressure and the
intruded volume of mercury are monitored during the
test. Soil samples were lyophilized using ALPHA 1-2
Ld Plus—GmbH apparatus before applying mercury
test, in order to minimize micro-fabric changes (Al-
Mukhtar et al. 1996).
3 Results and Discussion
3.1 SWCCs of Uncompacted Soil Samples
As the porosity of the soil samples tested in uncom-
pacted conditions (powder) cannot be measured
precisely, the SWCCs are presented in terms of
gravimetric water content and suction, as illustrated
in Fig. 2. The shape of the SWCC of the powder
samples is between those of clayey soil and silty soil
(Fig. 3 according to Fredlund and Xing 1994).
In the range of applied suctions, results show that the
SWCC curve of pure gypsum appears slightly below the
curve of the natural soil (without gypsum). The amount
of clay (19 %) in the natural soil could be the reason for
this difference. Adding gypsum to the natural soil
induces an increase in the water absorption; as the
gypsum content increases, the gravimetric water content
increases for a fixed applied suction. This expected result
was confirmed by the SWCC measured for the tested
samples prepared with different amounts of gypsum.
3.2 SWCCs of Soil Samples Compacted
at Standard Proctor
SWCCs of soil samples compacted at standard comp-
active effort are presented in terms of volumetric water
0
10
20
30
10 100 1000 10000 100000 1000000Suction Pressure (kPa)
Gra
vim
etric
w/c
(%)
0% G5% G15% G25% GPure Gypsum
Fig. 2 SWCC of un-compacted soil samples and pure gypsum
Fig. 3 SWCC for a sandy soil, a silty soil and a clayey soil
(Fredlund and Xing 1994)
0
10
20
30
40
10 100 1000 10000 100000 1000000Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
0% G
5% G
15% G
25% G
Fig. 4 SWCC of soil samples compacted at standard Proctor
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content and suction, as shown in Fig. 4. The main
observations are:
• The general shape of the curves is similar to that of
a silty to clayey soil. The results obtained confirm
the SWCC results of the soil powders.
• All the curves present similar trends: decreasing
suction induces higher water content in the tested
samples.
• As gypsum content increases, the volumetric water
content increases.
However, the water-holding behavior of the tested
soil can be divided into three ranges of suction effects:
• For very high suctions (more than 56,000 kPa
corresponding to a relative humidity of less than
66 %), the water contents obtained in the different
samples are practically the same, whatever the
gypsum content.
• For suctions between 56,000 and 2,700 kPa (RH of
98 %), a slightly higher water content can be
observed in samples with a higher gypsum content.
• For suctions below 2,700 kPa, water content
variations are in the form of a curve that starts
with high variation followed by a smaller variation
which then continues towards saturation. The
differences between the water contents of samples
with different gypsum contents seem to stabilize
for suctions below 100 kPa (close to the saturated
state).
3.3 SWCCs of Samples Compacted at Modified
Proctor
The observations concerning the SWCCs of samples
compacted with modified compaction effort are sim-
ilar to those for samples compacted with standard
compaction energy (Fig. 5). However, it seems that
the three ranges of suctions are slightly different: the
first range of high suctions concerns suction higher
than 100,000 kPa (instead of 56,000 kPa) and so the
second range is increased as it goes up to 2,700 kPa.
Moreover, for suctions below 2,700 kPa, the water
content increased significantly up to 1,000 kPa and
then more slowly. Finally, for a given suction value, as
the gypsum content increases in the soil, the water
content also increases.
3.4 Analysis of the Compaction Efforts on SWCC
The comparison of SWCC curves (Fig. 6) obtained for
soil samples with different gypsum contents with the
two compaction efforts demonstrates that:
• The SWCC of soil samples compacted at a higher
compaction effort is above the SWCC of samples
compacted at a lower energy effort, for all gypsum
contents. It is well known that the energy for the
standard compaction is about 6 kg cm/cm3 while
that for the modified compaction (Proctor) test is
more than 4 times that of standard compaction, at
slightly higher than 24 kg cm/cm3. Comparing the
SWCC of soil samples with similar initial moisture
contents, but using different values of compaction
energy, it can be observed that SWCCs preserve
the slope of the transition branch.
• The effect of compaction is negligible for all high
suctions (C2,700 kPa) induced by the saturated
salt solutions (Hr B98 %), as all the SWCC values
are very close. These results corroborate those of
Vanapalli et al. (1999a) which indicated that at
high suction, the water relation is influenced less
by the structure and more by the composition and
specific surface of the soil.
• The effect of compaction is remarkable for the
other parts of the measured SWCC, in particular
for suctions B1,000 kPa: for a given suction value,
the water content is higher in samples compacted
at a higher energy. Similar behavior was noted by
(Vanapalli et al. 1999b; Khattab and Al-Taie 2006;
Osinubi and Bello 2011).
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
0% G
5% G
15% G25% G
Fig. 5 SWCC of soil samples compacted at modified Proctor
Geotech Geol Eng
123
These behaviors could explain the higher water
absorption measured for suctions with tensiometric
plates and osmotic membrane techniques in all
samples compacted at modified effort. Further, in this
zone of suction (i.e. \1,000 kPa) it is expected that
capillary forces will be present. This behavior can be
attributed to the changes in soil structure. When soil
samples are compacted with the standard compactive
effort, the pore space of the soil internal structure is
relatively larger, while the pore space of soil samples
with modified compactive effort is smaller and many
capillary pores may be present in the soil samples.
Thus, these changes in soil structure have an effect on
capillary forces. In fact, the specific gravity of the
prepared samples decreased with increasing gypsum
content. Moreover, porosities in the samples decreased
with gypsum content (Table 1). The added gypsum
acts as a filler, infiltrating (intruding) the pore spaces
of the samples. Mercury porosimetry results (Fig. 7)
show substantial changes in the pore size distribution
of the samples:
• In standard effort, the combined effect of com-
paction and gypsum addition eliminated all pore
spaces higher than 10 lm in samples with 15 and
25 % of gypsum.
• In modified compaction, the combined effect of
compaction and gypsum addition eliminated all
pore spaces higher than 10 lm in all the tested
samples.
0
10
20
30
40
50
10 100 1000 10000 100000 1000000Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
0%G Standard
0%G Modified
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
5%G Standard
5%G Modified
0
10
20
30
40
50
10 100 1000 10000 100000 1000000Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
15%G Standard
15%G Modified
0
10
20
30
40
50
10 100 1000 10000 100000 1000000Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
25%G Standard
25%G Modified
Fig. 6 SWCC of soil samples compacted at different compaction efforts
Table 1 Specific gravity and porosity values of soil samples
with gypsum content at different compactive efforts
Gypsum (%) Gs Standard effort Modified effort
Porosity (%)
SD ± 2 %
Porosity (%)
SD ± 2 %
Total Mercury Total Mercury
0 2.66 33 33 26 22
5 2.6 32 29 25 23
15 2.5 31 31 24 24
25 2.49 29 29 22 24
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Lastly, the porosities obtained by mercury intrusion
for standard compaction are similar to calculated
porosities. This demonstrates that all the pore spaces
of these soil samples ranged between 350 lm and
3.6 nm. SEM images (Fig. 8) confirmed the reduction
in the voids of soil samples as gypsum content
increases. The gypsum used here had a greater fraction
of fine particles (all particles passed through an 80 lm
sieve opening) than the soil used (coarser particles),
and the addition of gypsum fills up the voids between
the relatively coarser particles of the soil. As a result,
the void ratio decreases but with more capillary pores
in the samples.
3.5 Key Parameters of SWCC of Soil Samples
In order to determine the key parameters of the
SWCCs obtained and to analyze the effect of
compaction and gypsum content, the curves are
presented in terms of volumetric water content and
suction. These key parameters (Table 2) were deter-
mined using the classical method proposed by (Fredl-
und and Xing 1994) and show that:
The air entry value (AEV) which is the suction
where air starts to enter the largest pores in the soil,
varies for the same compactive effort from about
100–220 kPa. This result means that as the porosity of
samples decreases, higher values of AEV can be
reached. These findings are similar to the results
obtained by (Tarantino 2009; Heshmati and Motahari
2012). However, it must be mentioned that it was not
easy to determine precise values of this parameter on
the SWCC. In general, however, AEV values were
slightly higher in samples containing more gypsum.
Therefore, the reduction in void ratio increases the
water-holding capacity of gypsum–containing soil
samples. Gypsum, like other salts, causes osmotic
suction; the suction potential resulting from salts
present in the soil pore water (Fredlund and Rahardjo
1993), and the development of an osmotic gradient,
attract more water into the gypsum–soil matrix.
Therefore, gypsum addition influences SWCCs and
the slope of these curves increased with increasing
gypsum content for both compactive efforts.
• Precise values of residual suction pressure (Wr),
where a large suction change is required to remove
additional water from the soil, were also difficult to
determine. However, these values were similar for
the same compactive effort (no influence of
gypsum content) and decreased with increasing
compaction.
• The behavior of residual water content (hr) is
similar to that of (Wr): no influence of gypsum
content on these values was observed, and slightly
higher values of (hr) were obtained with modified
compactive effort.
• The volumetric water content (ha) corresponding
to the AEV is the main parameter which changes.
ha increased with both compactive effort and
gypsum content, as shown in Fig. 9. This behavior
is due to the soil fabric induced in the soil samples
by compaction and gypsum addition. Moreover,
there is a linear relationship between the porosity
and the volumetric water content (Fig. 10): as the
porosity decreases, the ha also decreases. The main
effect of compaction is on the high pore space and
capillary pores.
0
0.005
0.01
0.015
0.02
0.025
0.001 0.01 0.1 1 10 100 1000
Incr
imen
tal I
ntru
sion
(mL/
g)
Entrance Diameter (µm)
0% G5% G15% G25% G
0
0.005
0.01
0.015
0.02
0.025
0.001 0.01 0.1 1 10 100 1000
Incr
imen
tal I
ntru
sion
(mL/
g)
Entrance Diameter (µm)
0% G5% G15% G25% G
(A)
(B)
Fig. 7 Pore size distribution of soil samples compacted at
a standard effort and b modified effort
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3.6 Modeling of SWCC of Soil Samples
Figure 11 illustrates the modeling of SWCCs of soil
samples at different compactive efforts using the
Fredlund and Xing (1994) and Van Genuchten (1980)
equations. The continuous lines of SWCC shown in
this figure represent the best fit SWCCs using Fredlund
and Xing or Van Genuchten equations, while the dots
25% G25% G
15% G15% G
5% G 5% G
0% G 0% G
Fig. 8 SEM images
showing void ratio reduction
of soil samples compacted at
standard effort (left) and
modified effort (right)
Geotech Geol Eng
123
represent the experimental SWCCs. Table 3 presents
both Fredlund and Xing and Van Genuchten equations
parameters, used to model the SWCC of soil samples.
Based on the summation of squared error (SSR)
values, good correlations were obtained between the
experimental data and the modeled SWCC. In general,
there was no clear relationship between the equations
parameters and both compactive effort and gypsum
content. However, the values of n in the Fredlund and
Xing equation decreased with gypsum addition. This
behaviour indicates that the gypseous soil samples
exhibited a less uniform pore size distribution than
samples without gypsum. Tinjum et al. (1997) showed
that there is no relationship between dry unit weight
and the Van Genuchten parameters (n and a) of four
types of clayey soils. The lack of relationship is due to
the fact that soil samples compacted to the same dry
unit weight at different water contents (dry and wet of
optimum content) have radically different pore size
distributions.
4 Conclusions
To the best of the authors’ knowledge, no studies on
the experimental determination and modeling of the
SWCCs of gypseous soil have so far been reported.
The water-holding capacity of gypseous soil is
affected by mineralogical composition, texture, struc-
ture and field conditions (compaction, relative humid-
ity….etc.). The main conclusions concerning the
SWCC of the fine-grained soil studies here with
different amounts of gypsum and compacted under
different compaction efforts were:
• The shape of the SWCC (S-shape) obtained on the
soil powder is very similar to that obtained under
compacted effort.
• For the same compaction effort, water retention of
soil samples increases with gypsum content.
• For the same gypsum content, water retention
increases with compaction effort applied on the
soil.
• The effect of compaction is negligible for all high
suctions (2,700 kPa equivalent to a relative
humidity of 98 %) while this effect is remarkable
Table 2 Keys parameters of SWCC for soil samples at dif-
ferent compactive efforts
Compactive
effort
Gypsum
content (%)
Saturation state Residual
state
Wa, AEV
(kPa)
ha
(%)
Wr
(kPa)
hr
(%)
Standard 0 120 28 80,000 3
5 110 31 63,000 4
15 200 32 70,000 4
25 220 34 80,000 4
Modified 0 110 38 37,000 7
5 100 42 50,000 6
15 130 43 61,000 6
25 120 45 51,000 8
20
25
30
35
40
45
50
0 10 20 30
Volu
met
ric w
ater
con
tent
at
Air
entr
y va
lue
(%)
Gypsum Content (%)
Standard
Modified
Fig. 9 Volumetric water content of soil samples with gypsum
content at different compaction efforts
y = -152.45x + 78.664R² = 0.98
20
25
30
35
40
45
50
0.20 0.25 0.30 0.35
Volu
met
ric w
ater
con
tent
at
Air
entr
y va
lue
(%)
Porosity (%)
StandardModified
Fig. 10 Relationship between the porosity and the volumetric
water content
cFig. 11 Experimental and modeling SWCCs with Fredlund
and Xing equation (left) and Van Genuchten equation (right)
Geotech Geol Eng
123
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
0%G Standard
0%G Modified
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
0%G Standard
0%G Modified
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
5%G Standard
5%G Modified
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
5%G Standard
5%G Modified
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
15%G Standard
15%G Modified
0
10
20
30
40
50
10 100 1000 10000 100000 1000000Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
15%G Standard
15%G Modified
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
25%G Standard
25%G Modified
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
Suction Pressure (kPa)
Volu
met
ric W
ater
Con
tent
(%)
25%G Standard
25%G Modified
Geotech Geol Eng
123
for other parts of the SWCC measured (relative
humidity higher than 98 %).
• Changes in the pore size distribution of soil
samples due to compaction and gypsum content
explain the modification in the water retention. In
standard compaction effort, the combined effect of
compaction and gypsum addition eliminates all
pore spaces higher than 10 lm in samples with 15
and 25 % of gypsum. In modified compaction, the
combined effect of compaction and gypsum addi-
tion eliminates all pore spaces higher than 10 lm
in all the tested samples. Mercury porosimetry
results and SEM images show more capillary pores
with compaction and gypsum addition.
• Among the key parameters of the SWCC, the
volumetric water content at the air entry value is
the one which changes the most because it depends
directly on soil texture, in particular on the
capillary pores.
• Both theoretical models, widely used for the predic-
tion of SWCC of soils, of Fredlund and Xing (1994)
and Van Genuchten (1980), successfully evaluated
the experimental SWCC of gypseous soil.
Finally, for gypseous soil, if the amount of gypsum
increases, the compaction effort should be lower in the
field (i.e. it is better to use the standard compaction
effort than the modified effort) because water retention
will be lower for a fixed suction (relative humidity) and
so the risk of gypsum dissolution could be minimized.
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