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Soil–water characteristic curve of lime treated gypseous soil
Abdulrahman Aldaood a,b, Marwen Bouasker a, Muzahim Al-Mukhtar a,⁎a Centre de Recherche sur la Matière Divisée CRMD-CNRS and Laboratoire PRISME, Université d ’ Orléans, Polytech’ Orléans,Orléans, Franceb Mosul University, College of Engineering, Civil Engineering Department, Al-Majmooah street, Mosul, Iraq
a b s t r a c ta r t i c l e i n f o
Article history:
Received 4 April 2014
Received in revised form 14 September 2014
Accepted 17 September 2014Available online xxxx
Keywords:
Gypseous soil
Lime stabilization
Curing conditions
SWCC
Micro structure
The determination of water holding capacity variations with environmental conditions, in particular relative
humidity (suction), is essential in the assessment of the behaviour of gypseous soil. The relationship between
suction and moisture content is expressed by the soil-water retention curve (SWRC) or soil-water characteristic
curve (SWCC).This relationship wasdetermined forthersttime forlime treated gypseous soil, using tensiomet-
ric plate, osmotic membraneand vapour equilibrium techniques, in the suction pressure range of (10–1,000,000
kPa). Soil samples containing (0, 5, 15 and 25%) gypsum were treated with 3% lime and cured for 28, 90 and
180 days at 20 °C and 40 °C. Results showed that the water holding capacity of the soil samples increased with
increasing gypsum content, curing period and curing temperature. The effect of gypsum content on SWCC was
greater than the effect of curing conditions, although microstructural properties of the treated soil samples
showed that curing conditions also had a signicant effect on the SWCC. All the experimental data tted well
to the Fredlund and Xing (1994) and Van Genuchten (1980) models for SWCC.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
In most cases, in situ compacted soils are unsaturated and are char-acterized by soil suction, which plays a signicant role in determining
the performance of soil as foundation materials in terms of permeabili-
ty, strength and volume change(Linand Cerato, 2012). Further, manyof
the geotechnical engineering problems, especially in arid or semiarid
climatic areas, are associated with unsaturated soils (Fredlund and
Rahardjo, 1993). Soil suction (total suction) has two components:
matric and osmotic suction (Fredlund and Rahardjo, 1993). Total suc-
tion is dened as the total free energy of thesoil water per unit volume.
Matric suction refers to a measure of the energy required to remove a
water molecule from the soil matrix without the water changing state.
It represents the difference between the pore air pressure and the
pore water pressure. Osmotic suction arises from differences between
the salt concentration of the pore water and that of pure water. The
total soil suction is given by the sum of matric and osmotic suction.
For low suction values, only a small inuence of osmotic suction is
observed; for higher suction values, above 1500 kPa, the contribution
of osmotic suction is negligible (Burckhard et al., 2000; Çokça, 2002).
Unlike tests in traditional soil mechanics, tests that directly measure
unsaturated soil properties are not as easily accessible and are often
extremely labor intensive. One tool that has made the analysis of unsat-
urated soil data simpler and morepractical is the soil-water characteris-
tic curve (SWCC) (Fredlund and Rahardjo, 1993; Zhai and Rahardjo,2012; Satyanaga et al., 2013; Li et al., 2014). SWCC is dened as the
relationship between gravimetric water content, volumetric water
content, degree of saturation and soil suction (or equivalent relative
humidity).The keys of the SWCC are air entry value AEV(Ψa), saturated
water content (θs), residual water content (θr) and water entry value
(Ψr) (Fredlund and Xing, 1994; Vanapalli et al., 1999). SWCC indirectly
allows for the determination of the geotechnical properties of unsatu-
rated soilthat can be used to determinethe shear strength, permeability
and volume change of soils. Further, the water retention ability of a soil
is also usually characterized by a SWCC. Therefore, in recent years, ana-
lyzing suction in the context of the aforementioned geotechnical prop-
erties has become the subject of much research in the rapidly growing
eld of unsaturated 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).
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 unsatu-
rated soils near the surface. Gypseous soils can be improved by various
methods. Chemical stabilization of gypseous soils is very important for
many geotechnical engineering applications such as pavement struc-
tures, roadways and infrastructures, to avoid damage due to gypsum
Applied Clay Science xxx (2014) xxx–xxx
⁎ Corresponding author. Tel.: +33 2 38 25 78 81 (ou), +33 2 38 49 49 92, + 33 2
38255379; fax: +33 2 38255376 (Secr.).
E-mail addresses: [email protected], [email protected]
(M. Al-Mukhtar).
CLAY-03172; No of Pages 11
http://dx.doi.org/10.1016/j.clay.2014.09.024
0169-1317/© 2014 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Applied Clay Science
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c l a y
Please cite this article as: Aldaood,A., et al., Soil–water characteristic curve of lime treated gypseous soil, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.024
http://dx.doi.org/10.1016/j.clay.2014.09.024http://dx.doi.org/10.1016/j.clay.2014.09.024http://dx.doi.org/10.1016/j.clay.2014.09.024mailto:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.clay.2014.09.024http://www.sciencedirect.com/science/journal/01691317http://www.elsevier.com/locate/clayhttp://dx.doi.org/10.1016/j.clay.2014.09.024http://dx.doi.org/10.1016/j.clay.2014.09.024http://dx.doi.org/10.1016/j.clay.2014.09.024http://dx.doi.org/10.1016/j.clay.2014.09.024http://www.elsevier.com/locate/clayhttp://www.sciencedirect.com/science/journal/01691317http://dx.doi.org/10.1016/j.clay.2014.09.024mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.clay.2014.09.024
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dissolution. Lime stabilization is often performed in order to overcome
such problems. The improvementin the geotechnical properties of gyp-
seous soil and the chemical stabilization process using lime, take place
through two basic chemical reactions: short and long term reactions.
The short-term reactions include cation exchange, occulation and
agglomeration; these processes are primarily responsible for modifying
engineering properties such as workability and plasticity reduction
(Little, 1995; Bell, 1996; Al-Mukhtar et al., 2010a). The long term
reactions, called pozzolanic reactions, lead to the creation of new calci-um hydrates which contribute to occulation by bonding adjacent soil
particles together and as curing occurs they strengthen the soil (Ingles
and Metcalf, 1972). Pozzolanic reactions are time and temperature
dependent and thus strength develops gradually over a long period
(Al-Mukhtar et al., 2010a,b, 2012).
Many collapsible soils, such as loess, loosely compacted lls or
gypseous soils can undergo substantial settlement as the materials are
wetted at relatively large overburden pressures, bringing about damage
to the overlying structures. Future climate changes (especially relative
humidity), which could potentially cause signicant changes in the
soil moisture regime for many areas of the world, as well as rapid
developments in many arid areas and the tropics, will be factors induc-
ing further problems associated with unsaturated soils. The behaviour
of unsaturated lime treated gypseous soils in general appears to be
complex due to the large number of physical and chemical phenomena
involved, in particular gypsum dissolution and ettringite formation. A
sound understanding of the unsaturated behaviour (especially the
soil-water characteristic curve) of lime treated gypseous soil is thus
required, in order to nd safe and cost-effective solutions to the
engineering problems that can occur with this typeof soil. In thepresent
study, the SWCC of lime treated gypseous soil (containing different
amounts of gypsum) under different curing conditions (curing temper-
ature and curing periods) were measured. The SWCC of soil samples
were studied in the suction range of (10–1,000,000 kPa) using three
different techniques: tensiometric plates, osmotic membrane and
vapour equilibrium. The experimental test results were tted using
the Fredlund and Xing (1994) and Van Genuchten (1980) equations.
2. Materials and experimental methods
2.1. Materials
Thesoil samples were a naturalne-grainedsoil, obtained froma bor-
row pit near Jossigny in the eastern part of Paris-France. The soil sampleswere collected at a depth between (1.5–2.0 m) below the surface. After
sampling the soil was homogenized and kept in plastic bags then
transported to the laboratory for testing. The natural water content in
situ was found to beabout 18.5%. The soilhad a liquid limit of29%, a plas-
tic limit of 21%, and a plasticity index of 8%. The percentages of clay, silt
and sandwere 19,64 and17%respectively. The chemicalanalysis showed
the presence of clay minerals (SiO2 = 68.8% and Al2O3 = 8.4%) and of
calcite (CaO = 5.9%). The high amount of silica reected the presence
of quartz. The results of the chemical analysis correlated well with the
results of the X-ray diffraction(Fig. 7): silica reected the presence of
quartz, alumina indicated the presence of clay mineral (kaolinite and
illite) and calcium oxide indicated the presence of calcite mineral. The
specic gravity of the soil was 2.66. The soil can be classied as sandy
lean clay (CL) accordingto theUnied Soil Classication System(USCS).
The quick lime used in this study, supplied by the French company
LHOIST, is a very ne lime and passes through an 80 μ m sieve opening.
The activity of the lime used was 94%.
The gypsum (CaSO4.2H2O) used in this study, supplied by the Merck
KGaA company, Germany, is a veryne gypsum and passes through an
80 μ m sieve opening, and with a purity of more than 99%.
2.2. Sample preparation
The soil samples were treated by 3% lime, which represents the
“optimum lime percent” based on the Eades and Grim method (1966).
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
28 days at 20°C
90 days at 20°C
180 days at 20°C
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
28 days at 20°C
90 days at 20°C
180 days at 20°C
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
28 days at 20°C90 days at 20°C
180 days at 20°C
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
28 days at 20°C90 days at 20°C
180 days at 20°C
15% G
0% G 5% G
25% G
Fig. 1. Experimental soil-water characteristics curve of soil samples cured at 20 °C.
2 A. Aldaood et al. / Applied Clay Science xxx (2014) xxx– xxx
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An experimental program was performed on soil samples with varying
percentages of gypsum (0, 5, 15 and 25%) of the dry weight of soil. A
standard Proctor compaction effort (ASTM D-698) was adopted in the
preparation of soilsamples. To ensurethe uniformity of thesoil samples,
only soil passing through a 4 mm sieve opening was used. The soil was
initially 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 standard Proctoroptimum moisture content of the
natural soil (i.e. 11%). During mixing, proper care was taken to prepare
homogeneous mixtures.
The soil mixtures were then stored in plastic bags for a period of
24 hours before compaction for moisture equalization. For lime treated
gypseous soil samples, the mixtures were prepared rst by thorough
mixing of dry predetermined quantities of soil, gypsum and lime to
obtain a uniform color. Then the required amount of water (11%) was
added and again mixed to obtain a uniform moisture distribution. The
mixture was then placed in plastic bags and left for 1 hour mellowing
time. After that, the soil samples were statically compacted to the
maximum dry unit weight of the natural soil (17.7 kN/m
3
). The soilsamples were 50 mm in diameter and 10 mm in height. After compac-
tion, the samples were immediately wrapped in cling lm and coated
with paraf n wax to reduce moisture loss. In order to study the effect
of curing periods on the SWCC, the compacted soil samples were
cured at 20 °C and 40 °C for 28, 90 and 180 days.
2.3. Suction measurement
Suction measurements ranging between (10–1,000,000 kPa) were
carried out using three complementary techniques: tensiometric plates,
osmotic membrane and vapour equilibrium techniques. The SWCC of
lime treated soil samples were determined after 28, 90 and 180 days
of curing. The SWCC in the suction range of 10–20 kPa was measured
using tensiometric plates. A period of 21 days was required for soil
samples to reach equilibrium. The SWCC in the suction range of 100–
20°C
5%G
Ettringite
Ettringite
Ettringite
Ettringite
Ettringite
Ettringite
20°C
15%G
20°C
25%G
40°C
25%G
40°C
15%G
40°C
5%G
Fig. 2. Microstructure changes and ettringite minerals formation during 180 days of curing at 20 °C and 40 °C.
Table 1
Volumetricwater content withs uction of soil samples at differentcuring temperatureand
time.
Suction, kPa Soil with 5% gypsum Soil with 25% gypsum
28 days
of curing
180 days
of curing
28 days
of curing
180 days
of curing
20 °C 40 °C 20 °C 40 °C 20 °C 40 °C 20 °C 40 °C
10 38 38.9 39.5 40.9 41.4 43.1 43 45.3
100 35.4 36.9 37.3 38.9 39.7 40.9 41.5 42.81000 28.8 30.2 30.6 31.9 32.5 33.9 33.7 36.1
10,000 13.6 16.2 16.4 16.9 16.5 19.8 17.7 19.4
150,000 3.8 4.2 4 4.3 4.1 5.1 4.3 5.5
3 A. Aldaood et al. / Applied Clay Science xxx (2014) xxx– xxx
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1500 kPa was determined using the osmotic membrane technique. The
soil samples were placed inside a semi-permeable membrane, then the
soil sample and membrane were submerged in a polyethylene glycol
(PEG) solution with different concentrations to impose various suction
values (i.e. 100–1500 kPa). A period of 28 days was required for thesoil
samples to reach equilibrium. The SWCC in high suction ranges (over
1500 kPa) was determined using the vapour equilibrium technique.
This technique is based on the observation that the relative humidity
in the airspace above a salt solution is unique to the concentrationand chemical composition of that solution. The soil samples inside the
desiccators will absorb or desorb the moisture until suction equilibrium
is reached (this takes more than 4 weeks). All three techniques were
generated under null stress and at room temperature (20 °C).
2.4. Mineralogical and microstructural tests
Mineralogical andmicrostructuraltests were conducted at theend of
28 and 180 days of curing at 20 °C and 40 °C for all soil samples with
various amounts of gypsum. Microscopic observations were performed
to explain soil behaviour along with SWCC and to evaluate the presence
of pozzolanic compounds and ettringite minerals in the samples.
The high resolution scanning electron microscope (SEM) equipment
PHILIPS XL 40 ESEM, was used. The fractions of soil samples were
injected with epoxy x resin, gold coated and then scanned. Several
digital images at different magnications were recorded in order to
examine the cementitious compounds and the formation of ettringite.
A pore sizedistribution assessment was carried out to determine the
fabric of the soil samples by using a Pore Size Porosimeter (9320), in
which the mercury pressure was raised continuously to reach more
than 210 MPa, and to measure the apparent pore diameter in the
range 3.6 nm to 350 μ m. Soil samples were lyophilized using ALPHA
1–2 Ld Plus – GmbH apparatus before applying mercury tests to mini-
mize micro-cracks due to thermal drying. Only soil samples cured for
28 days at 20 °C and those cured at the higher temperature (40°) for
180 days were tested.
For the X-Ray diffraction test (XRD), fractured samples produced on
completion of the desired curing periods for all soil mixes were
powdered and sieved through a 400 μ m sieve to serve as samples forthe test. Before testing, the samples were dried for 24 hours at 40 °C.
A PHILIPS PW3020 diffractometer was used for XRD analysis. The
diffraction patterns were determined using Cu-Kα radiation with a
Bragg angle (2θ) range of 4°-60° running at a speed of 0.025/6 sec.
3. Results and discussion
3.1. Effect of curing periods on SWCC
The SWCC of lime treated soil samples with different gypsum
contents are presented in Fig. (1). These curves were determined after
28, 90 and 180 days of curing at 20 °C. During curing periods, soil
samples experience continuous changes in micro structure, which
should induce considerable variations in SWCC. This means that the
experimental results composing the SWCC of samples that undergo
variable curing periods cannot be determined in the same conditions.
Curing periods have an insignicant effect on the shape of SWCC of
soil samples for all gypsum contents (i.e. all curves have an S-shaped
curve). For the same gypsum content, it can be seen that despite the
slight difference between the SWCC obtained, the overall trend of the
SWCC is similar.
In general, the soil samples cured for 180 days have a higher water
holding capacity than samples cured for 28 and 90 days. The effect of
curing time is more visible at 180 days than at 90 days in comparison
with water content at 28 days. The kinetics of lime–clay reactions is
low as the tested soil contains kaolinite and illite and these reactions
depend on the mineralogy of clayey soils (Al-Mukhtar et al., 2014).
Table 1 presents the values of volumetric water content with suction
of soil samples at different curing temperatures (20 °C and 40 °C)
and curing times (28 days and 180 days). The effects of curing periods
on SWCC are greater at low suction pressure than at high suction
pressure (N10,000 kPa). The difference in the SWCC of soil samples
with curing period is attributed to the formation of cementitious
materials. During lime treatment many clay particles are chemically
bound together and form coarser aggregates, resulting in an increasedpore size (occulation). As the curing periods increase, the pore space
decreases due to the increase in hydration products and the formation
of more cementitious materials. At the same time, the presence of
gypsum leads to the formation of ettringite minerals, as shown in
Fig. (2).
Cementitious materials and ettringite minerals cause changes in the
pore space of the soil samples. Fig. (3) and Table (2) show the pore size
distribution of soil samples cured for 28days at 20 °C. It canbe seen that
increasing the curing period resulted in more macro pores centered on
6 μ m and reduced the number of pores centered on 2 μ m, while there
was a slight and insignicant variation in the number of pores centered
Table 2
Pore size distribution of soil samples with curing conditions.
Temperature Curing
period
Gypsum Small
pores
b0.1 μ m
Medium
pores
0.1–10 μ m
Large
pores
N10 μ m
Porosity
(°C) Day % % % % %
20 28 0 22 76 2 26
5 38 59 3 26
25 30 66 4 32
40 180 0 39 59 2 28
5 28 69 3 28
25 21 73 6 34
0
0.005
0.01
0.015
0.02
0.025
0.001 0.01 0.1 1 10 100
I n c r i m e n t a l I n t r u s i o n ( m L / g )
I n c r i m e n t a l I n t r u s i o n ( m L / g )
Entrance Diameter (µm)
0% G
5% G
25% G
0
0.005
0.01
0.015
0.02
0.025
0.001 0.01 0.1 1 10 100
Entrance Diameter (µm)
0% G
5% G
25% G
28 days
20°C
180 days
40°C
Fig. 3. Pore size distribution of soil samples cured for 28 days at 20 °C and for 180 days at 40 °C.
4 A. Aldaood et al. / Applied Clay Science xxx (2014) xxx– xxx
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on 0.06 μ m. The increase in macro pores with curing period is attributed
to the development of ettringite minerals. Lastly, the inuence of the
curing period may vary depending on the gypsum content because of
the variations in time-dependent pore redistribution.
3.2. Effect of curing temperatures on SWCC
The SWCC of lime treated soil samples cured for 28 and 180 days at
two curing temperatures of 20 °C and 40 °C (Fig.4) shows that thewater
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( %
)
Suction Pressure (kPa)
28 days at 20°C
28 days at 40°C
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( %
)
Suction Pressure (kPa)
180 days at 20°C
180 days at 40°C
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
28 days at 20°C
28 days at 40°C
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
180 days at 20°C
180 days at 40°C
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
28 days at 20°C
28 days at 40°C
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
180 days at 20°C
180 days at 40°C
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
28 days at 20°C
28 days at 40°C
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
180 days at 20°C
180 days at 40°C
25% G 25% G
15% G 15% G
5% G 5% G
0% G0% G
A B
Fig. 4. Experimental SWCC of soil cured at different curing temperature for (A) 28 days and (B) 180 days.
5 A. Aldaood et al. / Applied Clay Science xxx (2014) xxx– xxx
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holding capacity of all soil samples, with or without gypsum, increased
with increased curing temperatures. The results reported in Table 1
show that for all suctions, the water content at a xed curing time
(28 days or 180 days) is higher for soil samples cured at 40 °C than for
samples cured at 20 °C. The difference in water content increased
when suction decreased in the samples. This behaviour is attributed to
theacceleration of chemical reactions in thesoil samples. In fact, a higher
temperature promotes the pozzolanic reaction within the mixture and
the formation of calcium silicate hydrate (CSH) and calcium aluminate
hydrate (CAH) which act as cementitious materials, so that they in
turn contribute to thechange in thepore size distributionof soil samples.
The continuous reaction between soil, lime and gypsum with in-
creased temperature, as well as the formation of CSH, CAH and ettringite
minerals,caused the soil samples cured at 40 °C to have a ner pore size
distribution than samples cured at 20 °C, as shown in Fig. (3) and
Table (2). In soil samples without gypsum, long term lime treatment
and a higher temperature increased the proportion of small pores (by
22% to 39%) reected in thereduction of medium-sizedpores. No chang-
es were observed in large pores. In gypseous soil samples and for the
same curing conditions, lime treatment reduced the number of small
pores andincreased themedium pores.Againno changes were observed
in large pores. The changes in thepore space of soil samples with curing
temperature aredue to the pozzolanic reaction products. The pozzolanic
products (CSH and CAH) not only enhanced the inter-cluster bonding
strength but also lled the pore space. As a result, the water holding
capacity of the soil samples signicantly increased with an increasing
curing temperature. Further, the ettringite mineral lls the pores within
the soil matrix, thus leading to a decrease in the void ratio of the gypse-
ous soil samples. This assumption is in agreement with the results of the
SEM analysis (see Fig. 2). Ettringite was observed to have formed and
precipitated in the pores of the soil matrix, especially in samples with
a higher amount of gypsum. Finally, the inuence of curing temperature
was found to be more signicant at low suction pressure (below 1500
kPa). Thepresenceof ettringite may also inuencetheSWCC ofsoilsam-
ples. Depending on the curing conditions, the time-dependent changes
in the properties of the soil samples, such as gypsum dissolution or
lime hydration can considerably inuence the SWCC.
3.3. Effect of gypsum content on SWCC
The results (Fig. 5) show the SWCC of soil samples cured during
180 days at 20 °C and 40 °C. For the same suction pressure, especiallylow pressure below 1500 kPa, a signicant change in volumetric water
content occurs for all gypsum-containing samples. In general, the effect
of gypsum on the SWCC becomes less noticeable for high suction
pressures (over 10,000 kPa), where all the volumetric water content
values were similar. The increase in the volumetric water content of
soil samples at a low suction pressure as the gypsum content increases
can be attributed to the fact that increases in gypsum content will
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V
o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
0% G5% G15% G25% G
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V
o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
0% G5% G15% G25% G
A B
Fig. 5. SWCC of soil samples cured for 180 days (A) at 20 °C and (B) at 40 °C.
Table 3
SWCC keys of soil samples at different curing conditions.
Temp.
(°C)
Curing
time (day)
Gypsum
content (%)
Saturation state Residual state
Ψa, AEV
(kPa)
θa(%)
Ψr(kPa)
θr(%)
20 28 0 190 33 90,000 2
5 200 35 60,000 6
15 200 38 80,000 6
25 200 39 100,000 5
90 0 210 33 120,000 2
5 160 36 120,000 3
15 210 38 170,000 325 210 40 130,000 2
180 0 230 33 150,000 2
5 210 37 190,000 4
15 170 39 180,000 4
25 190 41 150,000 5
40 28 0 200 34 100,000 3
5 180 36 110,000 5
15 200 39 110,000 6
25 200 40 110,000 7
90 0 200 34 110,000 2
5 210 39 110,000 4
15 190 40 90,000 6
25 240 40 80,000 7
180 0 180 34 190,000 2
5 190 38 165,000 4
15 200 40 150,000 5
25 190 42 120,000 6 Fig. 6. Typical SWCC showing the saturation, desaturation and residual zones (Vanapalli
et al., 1999).
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increase the osmotic suction pressure. Like other salts, gypsum causes
osmotic suction – the suction potential resulting from salts present in
the soil pore water (Fredlund and Rahardjo, 1993) – and the develop-
ment of an osmotic gradient attracts more water into the gypsum-soil
matrix; as a result, gypsum addition inuences the SWCC. Also, the
renement of the pore structures of soil samples, especially those
cured at 40 °C, as shown in Fig. (3) increases the volumetric water con-
tent due to the presence of capillary forces.
3.4. Key parameters of SWCC
In order to determine the key parameters of the SWCC obtained and
to analyze the effect of curing conditions (curing periods and curing
temperature) and gypsum content, these curves are presented in
terms of volumetric water content and suction. These key parameters
(Table 3) were determined using the classical method proposed by
Vanapalli et al. (1999), as shown in Fig. (6).
In the SWCC, access to the saturation zone is represented by the air-
entry value (AEV) and the corresponding volumetric water content.
The AEV is an important parameter for unsaturated soils since the
degreeof saturationstarts to drop rapidly when thesuction pressure ex-
ceeds the AEV. The de-saturation zone, also known as the residual zone,
is represented by the residual water content and the corresponding
residual suction pressure. In general, it can be observed that the AEV
of soil samples did not change signicantly with curing conditions (cur-
ing periods and curing temperature), while the θa increased slightly
with gypsum contentbut was not affected by curing conditions. Further,
as the curing period and temperature increased, the (Ψr) values in-
creased and also increased slightly with gypsum content. The variation
in saturated and de-saturated (residual) states with curing conditions
re
ects the mineralogical and microstructural changes in soil samples,as shown inFigs. (7 an d8). XRD patterns showed that all the intensities
of the kaolinite clay mineral peaks decreased with curing conditions for
all gypsum contents. This behaviour is attributed to the fact that kaolin-
ite is exhausted by the pozzolanic reaction, and is consistent with the
pozzolanic behaviour of kaolinite. Curingconditionshad an insignicant
effect on the mineralogical changes in soil samples. In other words, no
new reections were observed on the XRD patterns of soil samples
when the curing period increased from 28 days to 180 days. When
the curing period increased, these reections seemed to be more pro-
nounced, which means that crystallization of these new Ca-hydrates
has taken place. As mentioned by (Al-Mukhtar et al., 2010a,b, 2012),
newly formed Ca-hydrate cannot be observed by XRD because the
phases formed do not have a well-organized crystalline structure, and
0
200
400
600
800
0 10 20 30 40 50 60
I n t e n s i t y ( c o u n t s / s )
2θ (°)
0
200
400
600
800
0 10 20 30 40 50 60
I n t e n s i t y ( c o u n t s / s )
2θ (°)
Fig. 7. XRD patterns of the soil samples cured at 20 °C [G: Gypsum; L: Lime; E: Ettringite; Q: Quartz; K: Kaolinite; I: Illite. C: Calcite; F: Feldspar].
7 A. Aldaood et al. / Applied Clay Science xxx (2014) xxx– xxx
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therefore X-ray reections are greatly weakened. Second, it is possible
that reections from these phases overlap with both those of primary
minerals of natural soil and/or with the reections formed during
28 days. These observations conrmed SWCC key parameters, as
shown in Table (3).
3.5. Modeling of SWCC
In this study two model equations (Van Genuchten, 1980; Fredlund
and Xing, 1994) were used to t the experimental results of SWCC. In
1994 Fredlund and Xing proposed a model using a three-parametric
continuous function as shown below:
θ ¼ θs 1−
ln 1 þ Ψ
Ψ r
ln 1 þ1000000
Ψ r
2664
3775 1ln e þ Ψ a n !m
ð1Þ
where:
θ volumetric water content at desired suction.
θs saturated volumetric water content.
Ψ soil suction (kPa).Ψr soil suction (kPa) corresponding to the residual water
content, θr.
a soil parameter related to the air entry value of the soil (kPa).
n soil parameter controlling the slope at the inection point in
the soil-water characteristic curve.
m soil parameter related to the residual water content of the
soil; ande natural number, 2.71818……….
Van Genuchten (1980) proposed a closed-form equation for the
entire range of suction, given by:
θ ¼ θr þ θs−θr ð Þ
1 þ αψð Þn½ m ð2Þ
Where the parameters θ, θs and Ψ are as in the Fredlund and Xing
equation,
θr residual volumetric water content,
α parameter related to the air entry value.
0
200
400
600
800
0 10 20 30 40 50 60
I n t e n s i t y ( c
o u n t s / s )
2θ (°)
0
200
400
600
800
0 10 20 30 40 50 60
I n t e n s i t y ( c o u n t s / s )
2θ (°)
Fig. 8. XRD patterns of the soil samples cured at 40 °C [G: Gypsum; L: Lime; E: Ettringite; Q: Quartz; K: Kaolinite; I: Illite. C: Calcite; F: Feldspar].
8 A. Aldaood et al. / Applied Clay Science xxx (2014) xxx– xxx
Please cite this article as: Aldaood, A., et al., Soil–water characteristic curve of lime treated gypseous soil, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.024
http://dx.doi.org/10.1016/j.clay.2014.09.024http://dx.doi.org/10.1016/j.clay.2014.09.024http://dx.doi.org/10.1016/j.clay.2014.09.024http://dx.doi.org/10.1016/j.clay.2014.09.024
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0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V
o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
0% G5% G15% G25% G
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V
o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
0% G5% G15% G25% G
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o
l u m e t r i c w / c ( % )
Suction Pressure (kPa)
0% G5% G15% G25% G
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o
l u m e t r i c w / c ( % )
Suction Pressure (kPa)
0% G5% G15% G25% G
28 days
Van Genuchten
Van Genuchten
180 days
Fredlund and Xing
28 days
180 days
Fredlund and Xing
Fig. 9. Experimental and Modeling SWCC with Fredlund and Xing equation and Van Genuchten equation of soil samples cured at 20 °C.
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
0% G
5% G
15% G
25% G
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
0% G
5% G
15% G
25% G
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
0% G5% G
15% G
25% G
0
10
20
30
40
50
10 100 1000 10000 100000 1000000
V o l u m e t r i c w / c ( % )
Suction Pressure (kPa)
0% G5% G
15% G
25% G
28 days
Fredlund and Xing
Fredlund and Xing
180 days
28 days
Van Genuchten
180 days
Van Genuchten
Fig. 10. Experimental and Modeling SWCC with Fredlund and Xing equation and Van Genuchten equation of soil samples cured at 40 °C.
9 A. Aldaood et al. / Applied Clay Science xxx (2014) xxx– xxx
Please cite this article as: Aldaood,A., et al., Soil–water characteristic curve of lime treated gypseous soil, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.024
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n parameter related to the pore size distribution of soil
m parameter related to the asymmetry of the model curve
(m = 1-n−1.)
The results presented in Figs. (9 and 10) are representative of what
was obtained concerning the modeling of all the experimental SWCC
data. These gures illustrate the modeling SWCC of soil samples cured
at 20 °C and 40 °C for 180 days using the Fredlund and Xing (1994)
and Van Genuchten (1980) equations. The continuous lines of SWCCshown in this gure represent the best t SWCC using Fredlund
and Xing or Van Genuchten equations, while the points represent the
experimental SWCC.
In general, the t with experimental data provided by both models
was similar; however, the Fredlund and Xing equation gave better
summation of squared error (SSR) values than the Van Genuchten
equation. Table (4) gives both theFredlund andXing andVan Genuchten
equations parameters used to model the SWCC of soil samples. These
parameters were determined automatically by a computer program in
order to minimize the SSR values (difference between experimental
and modeling values). There is a good agreement between the tted
and experimental values, as evidenced by the coef cient of determina-
tion which was more than or equal to 0.99 for the two models. However,
more data are necessary to dene precisely the effect of gypsum content
on the parameters of these models. These models depend on the pore
size and particle size distributions, which are unlikely to capture the
complexities of pore and void distribution through the gypseous soil
samples, since the pores of the soil samples changed due to the curing
conditions and the formation of cementitious materials and ettringite
minerals.
4. Conclusions
Gypseous soils are commonly treated with lime in order to improve
their engineering behaviour against environmental conditions such as
humidity or wetness. Experimental results presented in this study
show the effect of different parameters (gypsum content and curing
conditions) that inuence the SWCC of lime treated gypsum soil.
Theoretical equations were used to evaluate their performance intting
experimental data. The main conclusions that can be drawn from this
study are:
- In the limetreated gypsum soil, thewaterholding capacity increased
with gypsum content. This behaviour is characterized in the SWCC
by increasing the volumetric water content at air entry and the
residual water content with gypsum content.- The curing period did not modify the saturation parameters
(volumetric water content and suction at air entry value) of the
SWCC of the lime treated soils. However, residual parameters
(suction and water content) increased with curing period and
temperature as the micro pore structure changes with the progress
of the pozzolanic reactions.
- Curing temperature acceleratedthe chemical reactions (i.e. pozzola-
nic reactions) and increased the water holding capacity mainly in
the low suction range (high relative humidity) of all soil samples,
with or without gypsum.
- Mineralogical and microstructural investigations reveal changes in
the micro structure of the lime treated gypsum soil samples with
curing conditions and provide explanations for the modications
in the key parameters of SWCC.
- Interesting agreements were obtained between the experimental
and modeled SWCC by using the well-known Fredlund and Xing
and Van Genuchten equations. Both are able to reproduce the global
shape of the SWCC of lime treated gypseous soil. However, an
improvement in these models is certainly necessary to take into
account the specicity of the type of soil and the progress of the
reaction between lime and the clay during curing.
Finally, as this study is the rst to address the SWCC of lime treated
gypseous soils, more tests are needed to determine the general features
of the SWCC corresponding to eld conditions of these problematic
soils. Future studies should also address the relationship between the
SWCC, which plays an important role in unsaturated soil mechanics,
and constitutive models to determine changes in geotechnical proper-
ties such as shear strength, volume change and permeability.
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Table 4
Equations parameters of modeling SWCCs of soil samples.
Curing
condition
Gypsum
content (%)
Fredlund equation Van genuchten
equation
n m SSR ⁎ α n SSR
28 days at 20 °C 0 1.5 0.9 20 0.016 1.46 22
5 1.7 0.78 18 0.018 1.376 22
15 1.45 0.76 18 0.016 1.374 27
25 1.28 0.8 23 0.016 1.36 40
90 days at 20 °C 0 1.35 0.93 20 0.015 1.43 27
5 1.37 0.85 21 0.018 1.373 32
15 1.3 0.84 32 0.016 1.376 47
25 1.8 0.74 35 0.014 1.41 47180 days at 20 °C 0 1.8 0.75 45 0.017 1.396 53
5 1.9 0.64 67 0.018 1.34 87
15 2 0.63 85 0.02 1.33 99
25 3.3 0.48 62 0.016 1.35 104
28 days at 40 °C 0 1.8 0.76 32 0.015 1.421 37
5 1.2 0.83 30 0.018 1.346 44
15 1.2 0.76 34 0.015 1.34 55
25 1.1 0.8 42 0.017 1.33 66
90 days at 40 °C 0 1.1 1.07 29 0.018 1.42 38
5 1.1 0.91 44 0.016 1.37 63
15 1.1 0.82 28 0.017 1.331 50
25 0.88 0.95 49 0.018 1.33 81
180 days at 40 °C 0 0.83 1.1 51 0.02 1.37 75
5 1.1 0.85 66 0.019 1.335 88
15 1.25 0.76 77 0.016 1.34 103
25 1.3 0.76 85 0.019 1.33 111
⁎ SSR = summation of squared error.
10 A. Aldaood et al. / Applied Clay Science xxx (2014) xxx– xxx
Please cite this article as: Aldaood, A., et al., Soil–water characteristic curve of lime treated gypseous soil, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.024
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