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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 ( % )


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 ( % )


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 ( % )


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

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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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




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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.




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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 ( %

)


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 ( %

)


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 ( % )


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 ( % )


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 ( % )


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 ( % )


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 ( % )


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 ( % )


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.




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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 ( % )


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 ( % )


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


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].




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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


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].




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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 ( % )


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 ( % )


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 ( % )


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 ( % )


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 ( % )


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 ( % )


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 ( % )


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 ( % )


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.




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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.



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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/
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