soil–water characteristic curve of lime treated gypseous soil

Applied Clay Science xxx (2014) xxx–xxx

CLAY-03172; No of Pages 11

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Applied Clay Science

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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

⁎ Corresponding author. Tel.: +33 2 38 25 78 81 (ou38255379; fax: +33 2 38255376 (Secr.).

E-mail addresses: muzahim.al-mukhtar@univ-orleans(M. Al-Mukhtar).

http://dx.doi.org/10.1016/j.clay.2014.09.0240169-1317/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Aldaood, A., et al., S10.1016/j.clay.2014.09.024

a b s t r a c t
a r t i c l e i n f o
Article history:Received 4 April 2014Received in revised form 14 September 2014Accepted 17 September 2014Available online xxxx

Keywords:Gypseous soilLime stabilizationCuring conditionsSWCCMicro structure

The determination of water holding capacity variations with environmental conditions, in particular relativehumidity (suction), is essential in the assessment of the behaviour of gypseous soil. The relationship betweensuction and moisture content is expressed by the soil-water retention curve (SWRC) or soil-water characteristiccurve (SWCC). This relationshipwas determined for thefirst time for lime treated gypseous soil, using tensiomet-ric plate, osmotic membrane and vapour equilibrium techniques, in the suction pressure range of (10–1,000,000kPa). Soil samples containing (0, 5, 15 and 25%) gypsum were treated with 3% lime and cured for 28, 90 and180 days at 20 °C and 40 °C. Results showed that the water holding capacity of the soil samples increased withincreasing gypsum content, curing period and curing temperature. The effect of gypsum content on SWCC wasgreater than the effect of curing conditions, although microstructural properties of the treated soil samplesshowed that curing conditions also had a significant effect on the SWCC. All the experimental data fitted wellto 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 significant role in determiningthe performance of soil as foundation materials in terms of permeabili-ty, strength and volume change (Lin and Cerato, 2012). Further,many ofthe geotechnical engineering problems, especially in arid or semiaridclimatic areas, are associated with unsaturated soils (Fredlund andRahardjo, 1993). Soil suction (total suction) has two components:matric and osmotic suction (Fredlund and Rahardjo, 1993). Total suc-tion is defined as the total free energy of the soil water per unit volume.Matric suction refers to a measure of the energy required to remove awater molecule from the soil matrix without the water changing state.It represents the difference between the pore air pressure and thepore water pressure. Osmotic suction arises from differences betweenthe salt concentration of the pore water and that of pure water. Thetotal soil suction is given by the sum of matric and osmotic suction.For low suction values, only a small influence of osmotic suction isobserved; for higher suction values, above 1500 kPa, the contributionof osmotic suction is negligible (Burckhard et al., 2000; Çokça, 2002).

Unlike tests in traditional soil mechanics, tests that directly measureunsaturated soil properties are not as easily accessible and are often

), +33 2 38 49 49 92, +33 2

.fr, [email protected]

oil–water characteristic curve

extremely labor intensive. One tool that has made the analysis of unsat-urated soil data simpler andmore practical 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 defined as therelationship between gravimetric water content, volumetric watercontent, degree of saturation and soil suction (or equivalent relativehumidity). The keys of the SWCC are air entry value AEV (Ψa), saturatedwater content (θs), residual water content (θr) and water entry value(Ψr) (Fredlund and Xing, 1994; Vanapalli et al., 1999). SWCC indirectlyallows for the determination of the geotechnical properties of unsatu-rated soil that can be used to determine the shear strength, permeabilityand volume change of soils. Further, the water retention ability of a soilis 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 growingfield of unsaturated soil mechanics (Delage et al., 1998; Al-Mukhtaret 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 semiaridzones in the world. These soils typically exhibit low strength, and highcollapse and settlement characteristics upon wetting. However, theproblems caused by gypseous soils are usually associated with climatebecause in arid and semiarid zones climatic conditions change overtime, and these climate changes causemoisture changes within unsatu-rated soils near the surface. Gypseous soils can be improved by variousmethods. Chemical stabilization of gypseous soils is very important formany geotechnical engineering applications such as pavement struc-tures, roadways and infrastructures, to avoid damage due to gypsum

of lime treated gypseous soil, Appl. Clay Sci. (2014), http://dx.doi.org/

http://dx.doi.org/10.1016/j.clay.2014.09.024

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2 A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx

dissolution. Lime stabilization is often performed in order to overcomesuch problems. The improvement in the geotechnical properties of gyp-seous soil and the chemical stabilization process using lime, take placethrough two basic chemical reactions: short and long term reactions.The short-term reactions include cation exchange, flocculation andagglomeration; these processes are primarily responsible for modifyingengineering properties such as workability and plasticity reduction(Little, 1995; Bell, 1996; Al-Mukhtar et al., 2010a). The long termreactions, called pozzolanic reactions, lead to the creation of new calci-um hydrates which contribute to flocculation by bonding adjacent soilparticles together and as curing occurs they strengthen the soil (Inglesand Metcalf, 1972). Pozzolanic reactions are time and temperaturedependent and thus strength develops gradually over a long period(Al-Mukhtar et al., 2010a,b, 2012).

Many collapsible soils, such as loess, loosely compacted fills orgypseous soils can undergo substantial settlement as the materials arewetted at relatively large overburden pressures, bringing about damageto the overlying structures. Future climate changes (especially relativehumidity), which could potentially cause significant changes in thesoil moisture regime for many areas of the world, as well as rapiddevelopments in many arid areas and the tropics, will be factors induc-ing further problems associated with unsaturated soils. The behaviourof unsaturated lime treated gypseous soils in general appears to becomplex due to the large number of physical and chemical phenomenainvolved, in particular gypsum dissolution and ettringite formation. Asound understanding of the unsaturated behaviour (especially thesoil-water characteristic curve) of lime treated gypseous soil is thusrequired, in order to find safe and cost-effective solutions to theengineeringproblems that can occurwith this type of soil. In thepresentstudy, the SWCC of lime treated gypseous soil (containing differentamounts of gypsum) under different curing conditions (curing temper-ature and curing periods) were measured. The SWCC of soil sampleswere studied in the suction range of (10–1,000,000 kPa) using threedifferent techniques: tensiometric plates, osmotic membrane and

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Fig. 1. Experimental soil-water characteristi

Please cite this article as: Aldaood, A., et al., Soil–water characteristic curve10.1016/j.clay.2014.09.024

vapour equilibrium. The experimental test results were fitted usingthe Fredlund and Xing (1994) and Van Genuchten (1980) equations.

2. Materials and experimental methods

2.1. Materials

The soil sampleswere a naturalfine-grained soil, obtained fromabor-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. Aftersampling the soil was homogenized and kept in plastic bags thentransported to the laboratory for testing. The natural water content insituwas found to be about 18.5%. The soil had a liquid limit of 29%, a plas-tic limit of 21%, and a plasticity index of 8%. The percentages of clay, siltand sandwere19, 64 and17% respectively. The chemical analysis showedthe presence of clay minerals (SiO2 = 68.8% and Al2O3 = 8.4%) and ofcalcite (CaO = 5.9%). The high amount of silica reflected the presenceof quartz. The results of the chemical analysis correlated well with theresults of the X-ray diffraction(Fig. 7): silica reflected the presence ofquartz, alumina indicated the presence of clay mineral (kaolinite andillite) and calcium oxide indicated the presence of calcite mineral. Thespecific gravity of the soil was 2.66. The soil can be classified as sandylean clay (CL) according to theUnified Soil Classification System (USCS).

The quick lime used in this study, supplied by the French companyLHOIST, is a very fine lime and passes through an 80 μm sieve opening.The activity of the lime used was 94%.

The gypsum (CaSO4.2H2O) used in this study, supplied by theMerckKGaA company, Germany, is a very fine gypsum and passes through an80 μm 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).

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Table 1Volumetric water contentwith suction of soil samples at different curing temperature andtime.

Suction, kPa Soil with 5% gypsum Soil with 25% gypsum

28 daysof curing

180 daysof curing

28 daysof curing

180 daysof 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 45.3100 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 36.110,000 13.6 16.2 16.4 16.9 16.5 19.8 17.7 19.4150,000 3.8 4.2 4 4.3 4.1 5.1 4.3 5.5

3A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx

An experimental programwas performed on soil samples with varyingpercentages of gypsum (0, 5, 15 and 25%) of the dry weight of soil. Astandard Proctor compaction effort (ASTM D-698) was adopted in thepreparation of soil samples. To ensure the uniformity of the soil samples,only soil passing through a 4 mm sieve opening was used. The soil wasinitially oven-dried for 2 days at 60 °C. The required amount of soil wasmixed with gypsum under dry conditions. Water was added to the soilsamples to reach the standard Proctor optimummoisture content of thenatural soil (i.e. 11%). During mixing, proper care was taken to preparehomogeneous mixtures.

20°C5%G

Ettringite

Ettringite

Ettringite

20°C15%G

20°C25%G

Fig. 2.Microstructure changes and ettringite minerals for


The soil mixtures were then stored in plastic bags for a period of24 hours before compaction for moisture equalization. For lime treatedgypseous soil samples, the mixtures were prepared first by thoroughmixing of dry predetermined quantities of soil, gypsum and lime toobtain a uniform color. Then the required amount of water (11%) wasadded and again mixed to obtain a uniform moisture distribution. Themixture was then placed in plastic bags and left for 1 hour mellowingtime. After that, the soil samples were statically compacted to themaximum dry unit weight of the natural soil (17.7 kN/m3). The soilsamples were 50 mm in diameter and 10 mm in height. After compac-tion, the samples were immediately wrapped in cling film and coatedwith paraffin wax to reduce moisture loss. In order to study the effectof curing periods on the SWCC, the compacted soil samples werecured 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) werecarried out using three complementary techniques: tensiometric plates,osmotic membrane and vapour equilibrium techniques. The SWCC oflime treated soil samples were determined after 28, 90 and 180 daysof curing. The SWCC in the suction range of 10–20 kPa was measuredusing tensiometric plates. A period of 21 days was required for soilsamples to reach equilibrium. The SWCC in the suction range of 100–

Ettringite

Ettringite

Ettringite

40°C25%G

40°C15%G

40°C5%G

mation during 180 days of curing at 20 °C and 40 °C.




Table 2Pore size distribution of soil samples with curing conditions.

Temperature Curingperiod

Gypsum Smallporesb0.1 μm

Mediumpores0.1–10 μm

LargeporesN10 μm

Porosity

(°C) Day % % % % %

20 28 0 22 76 2 265 38 59 3 26

25 30 66 4 3240 180 0 39 59 2 28

5 28 69 3 2825 21 73 6 34


1500 kPa was determined using the osmotic membrane technique. Thesoil samples were placed inside a semi-permeable membrane, then thesoil sample and membrane were submerged in a polyethylene glycol(PEG) solution with different concentrations to impose various suctionvalues (i.e. 100–1500 kPa). A period of 28 days was required for the soilsamples to reach equilibrium. The SWCC in high suction ranges (over1500 kPa) was determined using the vapour equilibrium technique.This technique is based on the observation that the relative humidityin the airspace above a salt solution is unique to the concentrationand chemical composition of that solution. The soil samples inside thedesiccators will absorb or desorb themoisture until suction equilibriumis reached (this takes more than 4 weeks). All three techniques weregenerated under null stress and at room temperature (20 °C).

2.4. Mineralogical and microstructural tests

Mineralogical andmicrostructural testswere conducted at the end of28 and 180 days of curing at 20 °C and 40 °C for all soil samples withvarious amounts of gypsum. Microscopic observations were performedto explain soil behaviour along with SWCC and to evaluate the presenceof pozzolanic compounds and ettringite minerals in the samples.

The high resolution scanning electronmicroscope (SEM) equipmentPHILIPS XL 40 ESEM, was used. The fractions of soil samples wereinjected with epoxy fix resin, gold coated and then scanned. Severaldigital images at different magnifications were recorded in order toexamine the cementitious compounds and the formation of ettringite.

A pore size distribution assessmentwas carried out to determine thefabric of the soil samples by using a Pore Size Porosimeter (9320), inwhich the mercury pressure was raised continuously to reach morethan 210 MPa, and to measure the apparent pore diameter in therange 3.6 nm to 350 μm. Soil samples were lyophilized using ALPHA1–2 Ld Plus – GmbH apparatus before applying mercury tests to mini-mize micro-cracks due to thermal drying. Only soil samples cured for28 days at 20 °C and those cured at the higher temperature (40°) for180 days were tested.

For the X-Ray diffraction test (XRD), fractured samples produced oncompletion of the desired curing periods for all soil mixes werepowdered and sieved through a 400 μm 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. Thediffraction patterns were determined using Cu-Kα radiation with aBragg 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 gypsumcontents are presented in Fig. (1). These curves were determined after

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28, 90 and 180 days of curing at 20 °C. During curing periods, soilsamples experience continuous changes in micro structure, whichshould induce considerable variations in SWCC. This means that theexperimental results composing the SWCC of samples that undergovariable curing periods cannot be determined in the same conditions.Curing periods have an insignificant effect on the shape of SWCC ofsoil samples for all gypsum contents (i.e. all curves have an S-shapedcurve). For the same gypsum content, it can be seen that despite theslight difference between the SWCC obtained, the overall trend of theSWCC is similar.

In general, the soil samples cured for 180 days have a higher waterholding capacity than samples cured for 28 and 90 days. The effect ofcuring time is more visible at 180 days than at 90 days in comparisonwith water content at 28 days. The kinetics of lime–clay reactions islow as the tested soil contains kaolinite and illite and these reactionsdepend on the mineralogy of clayey soils (Al-Mukhtar et al., 2014).Table 1 presents the values of volumetric water content with suctionof soil samples at different curing temperatures (20 °C and 40 °C)and curing times (28 days and 180 days). The effects of curing periodson SWCC are greater at low suction pressure than at high suctionpressure (N10,000 kPa). The difference in the SWCC of soil sampleswith curing period is attributed to the formation of cementitiousmaterials. During lime treatment many clay particles are chemicallybound together and form coarser aggregates, resulting in an increasedpore size (flocculation). As the curing periods increase, the pore spacedecreases due to the increase in hydration products and the formationof more cementitious materials. At the same time, the presence ofgypsum leads to the formation of ettringite minerals, as shown inFig. (2).

Cementitious materials and ettringite minerals cause changes in thepore space of the soil samples. Fig. (3) and Table (2) show the pore sizedistribution of soil samples cured for 28 days at 20 °C. It can be seen thatincreasing the curing period resulted in more macro pores centered on6 μm and reduced the number of pores centered on 2 μm, while therewas a slight and insignificant variation in the number of pores centered

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or 28 days at 20 °C and for 180 days at 40 °C.





on 0.06 μm. The increase inmacro poreswith curing period is attributedto the development of ettringite minerals. Lastly, the influence of thecuring period may vary depending on the gypsum content because ofthe variations in time-dependent pore redistribution.

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Fig. 4. Experimental SWCC of soil cured at different cur


3.2. Effect of curing temperatures on SWCC

The SWCC of lime treated soil samples cured for 28 and 180 days attwo curing temperatures of 20 °C and 40 °C (Fig. 4) shows that thewater

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A B

Fig. 5. SWCC of soil samples cured for 180 days (A) at 20 °C and (B) at 40 °C.


holding capacity of all soil samples, with or without gypsum, increasedwith increased curing temperatures. The results reported in Table 1show that for all suctions, the water content at a fixed curing time(28 days or 180 days) is higher for soil samples cured at 40 °C than forsamples cured at 20 °C. The difference in water content increasedwhen suction decreased in the samples. This behaviour is attributed tothe acceleration of chemical reactions in the soil samples. In fact, a highertemperature promotes the pozzolanic reaction within the mixture andthe formation of calcium silicate hydrate (CSH) and calcium aluminatehydrate (CAH) which act as cementitious materials, so that they inturn contribute to the change in the pore size distribution of soil samples.

The continuous reaction between soil, lime and gypsum with in-creased temperature, as well as the formation of CSH, CAH and ettringiteminerals, caused the soil samples cured at 40 °C to have a finer pore sizedistribution than samples cured at 20 °C, as shown in Fig. (3) andTable (2). In soil samples without gypsum, long term lime treatmentand a higher temperature increased the proportion of small pores (by22% to 39%) reflected in the reduction ofmedium-sized pores. No chang-es were observed in large pores. In gypseous soil samples and for thesame curing conditions, lime treatment reduced the number of smallpores and increased themediumpores. Again no changeswere observed

Table 3SWCC keys of soil samples at different curing conditions.

Temp.(°C)

Curingtime (day)

Gypsumcontent (%)

Saturation state Residual state

Ψa, AEV(kPa)

θa(%)

Ψr

(kPa)θr(%)

20 28 0 190 33 90,000 25 200 35 60,000 6

15 200 38 80,000 625 200 39 100,000 5

90 0 210 33 120,000 25 160 36 120,000 3

15 210 38 170,000 325 210 40 130,000 2

180 0 230 33 150,000 25 210 37 190,000 4

15 170 39 180,000 425 190 41 150,000 5

40 28 0 200 34 100,000 35 180 36 110,000 5

15 200 39 110,000 625 200 40 110,000 7

90 0 200 34 110,000 25 210 39 110,000 4

15 190 40 90,000 625 240 40 80,000 7

180 0 180 34 190,000 25 190 38 165,000 4

15 200 40 150,000 525 190 42 120,000 6


in large pores. The changes in the pore space of soil samples with curingtemperature are due to the pozzolanic reaction products. The pozzolanicproducts (CSH and CAH) not only enhanced the inter-cluster bondingstrength but also filled the pore space. As a result, the water holdingcapacity of the soil samples significantly increased with an increasingcuring temperature. Further, the ettringite mineral fills the pores withinthe 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 theSEM analysis (see Fig. 2). Ettringite was observed to have formed andprecipitated in the pores of the soil matrix, especially in samples witha higher amount of gypsum. Finally, the influence of curing temperaturewas found to be more significant at low suction pressure (below 1500kPa). The presence of ettringitemay also influence the SWCCof soil sam-ples. Depending on the curing conditions, the time-dependent changesin the properties of the soil samples, such as gypsum dissolution orlime hydration can considerably influence the SWCC.

3.3. Effect of gypsum content on SWCC

The results (Fig. 5) show the SWCC of soil samples cured during180 days at 20 °C and 40 °C. For the same suction pressure, especiallylow pressure below 1500 kPa, a significant change in volumetric watercontent occurs for all gypsum-containing samples. In general, the effectof gypsum on the SWCC becomes less noticeable for high suctionpressures (over 10,000 kPa), where all the volumetric water contentvalues were similar. The increase in the volumetric water content ofsoil samples at a low suction pressure as the gypsum content increasescan be attributed to the fact that increases in gypsum content will

Fig. 6. Typical SWCC showing the saturation, desaturation and residual zones (Vanapalliet al., 1999).





increase the osmotic suction pressure. Like other salts, gypsum causesosmotic suction – the suction potential resulting from salts present inthe soil pore water (Fredlund and Rahardjo, 1993) – and the develop-ment of an osmotic gradient attracts more water into the gypsum-soilmatrix; as a result, gypsum addition influences the SWCC. Also, therefinement of the pore structures of soil samples, especially thosecured 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 andto analyze the effect of curing conditions (curing periods and curingtemperature) and gypsum content, these curves are presented interms of volumetric water content and suction. These key parameters(Table 3) were determined using the classical method proposed byVanapalli 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 thedegree of saturation starts to drop rapidlywhen the suction pressure ex-ceeds the AEV. The de-saturation zone, also known as the residual zone,

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is represented by the residual water content and the correspondingresidual suction pressure. In general, it can be observed that the AEVof soil samples did not change significantly with curing conditions (cur-ing periods and curing temperature), while the θa increased slightlywith gypsumcontent butwas 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 variationin saturated and de-saturated (residual) states with curing conditionsreflects the mineralogical and microstructural changes in soil samples,as shown in Figs. (7 and 8). XRD patterns showed that all the intensitiesof the kaolinite clay mineral peaks decreased with curing conditions forall gypsum contents. This behaviour is attributed to the fact that kaolin-ite is exhausted by the pozzolanic reaction, and is consistent with thepozzolanic behaviour of kaolinite. Curing conditions had an insignificanteffect on the mineralogical changes in soil samples. In other words, nonew reflections were observed on the XRD patterns of soil sampleswhen the curing period increased from 28 days to 180 days. Whenthe curing period increased, these reflections seemed to be more pro-nounced, which means that crystallization of these new Ca-hydrateshas taken place. As mentioned by (Al-Mukhtar et al., 2010a,b, 2012),newly formed Ca-hydrate cannot be observed by XRD because thephases formed do not have a well-organized crystalline structure, and

30 40 50 60θ (°)

30 40 50 60 (°)

; E: Ettringite; Q: Quartz; K: Kaolinite; I: Illite. C: Calcite; F: Feldspar].




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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].


therefore X-ray reflections are greatly weakened. Second, it is possiblethat reflections from these phases overlap with both those of primaryminerals of natural soil and/or with the reflections formed during28 days. These observations confirmed SWCC key parameters, asshown in Table (3).

3.5. Modeling of SWCC

In this study two model equations (Van Genuchten, 1980; Fredlundand Xing, 1994) were used to fit the experimental results of SWCC. In1994 Fredlund and Xing proposed a model using a three-parametriccontinuous function as shown below:

θ ¼ θs 1−ln 1þ Ψ

Ψr

� �

ln 1þ 1000000Ψr

� �2664

3775 1

ln eþ Ψa

� �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 inflection 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 theentire range of suction, given by:

θ ¼ θr þθs−θrð Þ

1þ αψð Þn½ �m ð2Þ

Where the parameters θ, θs and Ψ are as in the Fredlund and Xingequation,

θr residual volumetric water content,α parameter related to the air entry value.




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Fig. 9. Experimental and Modeling SWCC with Fredlund and Xing equation and Van Genuchten equation of soil samples cured at 20 °C.

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Fig. 10. Experimental and Modeling SWCC with Fredlund and Xing equation and Van Genuchten equation of soil samples cured at 40 °C.


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




n parameter related to the pore size distribution of soilm parameter related to the asymmetry of the model curve

(m= 1-n−1.)

The results presented in Figs. (9 and 10) are representative of whatwas obtained concerning the modeling of all the experimental SWCCdata. These figures illustrate the modeling SWCC of soil samples curedat 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 figure represent the best fit SWCC using Fredlundand Xing or Van Genuchten equations, while the points represent theexperimental SWCC.

In general, the fit with experimental data provided by both modelswas similar; however, the Fredlund and Xing equation gave bettersummation of squared error (SSR) values than the Van Genuchtenequation. Table (4) gives both the Fredlund andXing andVanGenuchtenequations parameters used to model the SWCC of soil samples. Theseparameters were determined automatically by a computer program inorder to minimize the SSR values (difference between experimentaland modeling values). There is a good agreement between the fittedand experimental values, as evidenced by the coefficient of determina-tionwhichwasmore than or equal to 0.99 for the twomodels. However,more data are necessary to define precisely the effect of gypsum contenton the parameters of these models. These models depend on the poresize and particle size distributions, which are unlikely to capture thecomplexities of pore and void distribution through the gypseous soilsamples, since the pores of the soil samples changed due to the curingconditions and the formation of cementitious materials and ettringiteminerals.

4. Conclusions

Gypseous soils are commonly treated with lime in order to improvetheir engineering behaviour against environmental conditions such ashumidity or wetness. Experimental results presented in this studyshow the effect of different parameters (gypsum content and curing

Table 4Equations parameters of modeling SWCCs of soil samples.

Curingcondition

Gypsumcontent (%)

Fredlund equation Van genuchtenequation

n m SSR⁎ α n SSR

28 days at 20 °C 0 1.5 0.9 20 0.016 1.46 225 1.7 0.78 18 0.018 1.376 22

15 1.45 0.76 18 0.016 1.374 2725 1.28 0.8 23 0.016 1.36 40

90 days at 20 °C 0 1.35 0.93 20 0.015 1.43 275 1.37 0.85 21 0.018 1.373 32

15 1.3 0.84 32 0.016 1.376 4725 1.8 0.74 35 0.014 1.41 47

180 days at 20 °C 0 1.8 0.75 45 0.017 1.396 535 1.9 0.64 67 0.018 1.34 87

15 2 0.63 85 0.02 1.33 9925 3.3 0.48 62 0.016 1.35 104

28 days at 40 °C 0 1.8 0.76 32 0.015 1.421 375 1.2 0.83 30 0.018 1.346 44

15 1.2 0.76 34 0.015 1.34 5525 1.1 0.8 42 0.017 1.33 66

90 days at 40 °C 0 1.1 1.07 29 0.018 1.42 385 1.1 0.91 44 0.016 1.37 63

15 1.1 0.82 28 0.017 1.331 5025 0.88 0.95 49 0.018 1.33 81

180 days at 40 °C 0 0.83 1.1 51 0.02 1.37 755 1.1 0.85 66 0.019 1.335 88

15 1.25 0.76 77 0.016 1.34 10325 1.3 0.76 85 0.019 1.33 111

⁎ SSR = summation of squared error.


conditions) that influence the SWCC of lime treated gypsum soil.Theoretical equationswere used to evaluate their performance in fittingexperimental data. The main conclusions that can be drawn from thisstudy are:

- In the lime treated gypsum soil, thewater holding capacity increasedwith gypsum content. This behaviour is characterized in the SWCCby increasing the volumetric water content at air entry and theresidual water content with gypsum content.

- The curing period did not modify the saturation parameters(volumetric water content and suction at air entry value) of theSWCC of the lime treated soils. However, residual parameters(suction and water content) increased with curing period andtemperature as the micro pore structure changes with the progressof the pozzolanic reactions.

- Curing temperature accelerated the chemical reactions (i.e. pozzola-nic reactions) and increased the water holding capacity mainly inthe low suction range (high relative humidity) of all soil samples,with or without gypsum.

- Mineralogical and microstructural investigations reveal changes inthe micro structure of the lime treated gypsum soil samples withcuring conditions and provide explanations for the modificationsin the key parameters of SWCC.

- Interesting agreements were obtained between the experimentaland modeled SWCC by using the well-known Fredlund and Xingand Van Genuchten equations. Both are able to reproduce the globalshape of the SWCC of lime treated gypseous soil. However, animprovement in these models is certainly necessary to take intoaccount the specificity of the type of soil and the progress of thereaction between lime and the clay during curing.

Finally, as this study is the first to address the SWCC of lime treatedgypseous soils, more tests are needed to determine the general featuresof the SWCC corresponding to field conditions of these problematicsoils. Future studies should also address the relationship between theSWCC, 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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soil–water characteristic curve of lime treated gypseous soil

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