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Soil–water characteristic curve of lime treated gypseous soil
CLAY-03172; No of Pages 11 Applied Clay Science xxx (2014) xxx–xxx
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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 b
Centre de Recherche sur la Matière Divisée CRMD-CNRS and Laboratoire PRISME, Université d’Orléans, Polytech’Orléans,Orléans, France Mosul University, College of Engineering, Civil Engineering Department, Al-Majmooah street, Mosul, Iraq
a 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 2014 Available online xxxx Keywords: Gypseous soil Lime stabilization Curing conditions SWCC Micro structure
a b s t r a c t 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 was determined for the first time for lime treated gypseous soil, using tensiometric plate, osmotic membrane and 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 significant effect on the SWCC. All the experimental data fitted 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 characterized by soil suction, which plays a significant role in determining the performance of soil as foundation materials in terms of permeability, strength and volume change (Lin and Cerato, 2012). Further, many of 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 suction 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 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 influence 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
⁎ 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).
extremely labor intensive. One tool that has made the analysis of unsaturated soil data simpler and more practical is the soil-water characteristic curve (SWCC) (Fredlund and Rahardjo, 1993; Zhai and Rahardjo, 2012; Satyanaga et al., 2013; Li et al., 2014). SWCC is defined 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 unsaturated soil that can be used to determine the 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, analyzing suction in the context of the aforementioned geotechnical properties has become the subject of much research in the rapidly growing field 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 unsaturated 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 structures, roadways and infrastructures, to avoid damage due to gypsum
http://dx.doi.org/10.1016/j.clay.2014.09.024 0169-1317/© 2014 Elsevier B.V. All rights reserved.
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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dissolution. Lime stabilization is often performed in order to overcome such problems. The improvement in the geotechnical properties of gypseous 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, flocculation 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 calcium hydrates which contribute to flocculation 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 fills 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 significant 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 inducing 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 find safe and cost-effective solutions to the engineering problems that can occur with this type of soil. In the present study, the SWCC of lime treated gypseous soil (containing different amounts of gypsum) under different curing conditions (curing temperature 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 fitted using the Fredlund and Xing (1994) and Van Genuchten (1980) equations. 2. Materials and experimental methods 2.1. Materials The soil samples were a natural fine-grained soil, obtained from a borrow pit near Jossigny in the eastern part of Paris-France. The soil samples were collected at a depth between (1.5–2.0 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 be about 18.5%. The soil had a liquid limit of 29%, a plastic limit of 21%, and a plasticity index of 8%. The percentages of clay, silt and sand were 19, 64 and 17% respectively. The chemical analysis showed the presence of clay minerals (SiO2 = 68.8% and Al2O3 = 8.4%) and of calcite (CaO = 5.9%). The high amount of silica reflected the presence of quartz. The results of the chemical analysis correlated well with the results of the X-ray diffraction(Fig. 7): silica reflected the presence of quartz, alumina indicated the presence of clay mineral (kaolinite and illite) and calcium oxide indicated the presence of calcite mineral. The specific gravity of the soil was 2.66. The soil can be classified as sandy lean clay (CL) according to the Unified Soil Classification System (USCS). The quick lime used in this study, supplied by the French company LHOIST, 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 the Merck KGaA company, Germany, is a very fine gypsum and passes through an 80 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). 50
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Fig. 1. Experimental soil-water characteristics curve of soil samples cured at 20 °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
A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx Table 1 Volumetric water content with suction of soil samples at different curing temperature and time. Suction, kPa
10 100 1000 10,000 150,000
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
38 35.4 28.8 13.6 3.8
38.9 36.9 30.2 16.2 4.2
39.5 37.3 30.6 16.4 4
40.9 38.9 31.9 16.9 4.3
41.4 39.7 32.5 16.5 4.1
43.1 40.9 33.9 19.8 5.1
43 41.5 33.7 17.7 4.3
45.3 42.8 36.1 19.4 5.5
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 soil samples. To ensure the uniformity of the soil 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 Proctor optimum moisture content of the natural soil (i.e. 11%). During mixing, proper care was taken to prepare homogeneous mixtures.
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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 first 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/m3). The soil samples were 50 mm in diameter and 10 mm in height. After compaction, the samples were immediately wrapped in cling film and coated with paraffin 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–
Ettringite Ettringite 20°C 5%G
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Fig. 2. Microstructure changes and ettringite minerals formation during 180 days of curing at 20 °C and 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
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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 the soil 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 concentration and 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).
Table 2 Pore size distribution of soil samples with curing conditions.
2.4. Mineralogical and microstructural tests
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 insignificant 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 increased pore size (flocculation). 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 28 days at 20 °C. It can be seen that increasing the curing period resulted in more macro pores centered on 6 μm and reduced the number of pores centered on 2 μm, while there was a slight and insignificant variation in the number of pores centered
Mineralogical and microstructural tests were conducted at the end 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 fix resin, gold coated and then scanned. Several digital images at different magnifications were recorded in order to examine the cementitious compounds and the formation of ettringite. A pore size distribution 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 μm. Soil samples were lyophilized using ALPHA 1–2 Ld Plus – GmbH apparatus before applying mercury tests to minimize 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 μm sieve to serve as samples for the 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
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Fig. 3. Pore size distribution of soil samples cured for 28 days at 20 °C and for 180 days 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
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on 0.06 μm. The increase in macro pores with curing period is attributed to the development of ettringite minerals. Lastly, the influence of the curing period may vary depending on the gypsum content because of the variations in time-dependent pore redistribution.
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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 the water
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Fig. 4. Experimental SWCC of soil cured at different curing temperature for (A) 28 days and (B) 180 days.
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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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 fixed 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 the acceleration of chemical reactions in the soil 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 the change in the pore size distribution of soil samples. The continuous reaction between soil, lime and gypsum with increased temperature, as well as the formation of CSH, CAH and ettringite minerals, caused the soil samples cured at 40 °C to have a finer 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%) reflected in the reduction of medium-sized pores. No changes were observed in large pores. In gypseous soil samples and for the same curing conditions, lime treatment reduced the number of small pores and increased the medium pores. Again no changes were observed
Table 3 SWCC keys of soil samples at different curing conditions. Temp. (°C)
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0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25
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Residual state
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θa (%)
Ψr (kPa)
θr (%)
190 200 200 200 210 160 210 210 230 210 170 190 200 180 200 200 200 210 190 240 180 190 200 190
33 35 38 39 33 36 38 40 33 37 39 41 34 36 39 40 34 39 40 40 34 38 40 42
90,000 60,000 80,000 100,000 120,000 120,000 170,000 130,000 150,000 190,000 180,000 150,000 100,000 110,000 110,000 110,000 110,000 110,000 90,000 80,000 190,000 165,000 150,000 120,000
2 6 6 5 2 3 3 2 2 4 4 5 3 5 6 7 2 4 6 7 2 4 5 6
in large pores. The changes in the pore space of soil samples with curing temperature are due to the pozzolanic reaction products. The pozzolanic products (CSH and CAH) not only enhanced the inter-cluster bonding strength but also filled the pore space. As a result, the water holding capacity of the soil samples significantly increased with an increasing curing temperature. Further, the ettringite mineral fills the pores within the soil matrix, thus leading to a decrease in the void ratio of the gypseous 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 influence of curing temperature was found to be more significant at low suction pressure (below 1500 kPa). The presence of ettringite may also influence the SWCC of soil samples. 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 influence 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, especially low pressure below 1500 kPa, a significant 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
Fig. 6. Typical SWCC showing the saturation, desaturation and residual zones (Vanapalli et al., 1999).
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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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 development of an osmotic gradient attracts more water into the gypsum-soil matrix; as a result, gypsum addition influences the SWCC. Also, the refinement of the pore structures of soil samples, especially those cured at 40 °C, as shown in Fig. (3) increases the volumetric water content 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 airentry value (AEV) and the corresponding volumetric water content. The AEV is an important parameter for unsaturated soils since the degree of saturation starts to drop rapidly when the suction pressure exceeds 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 corresponding residual suction pressure. In general, it can be observed that the AEV of soil samples did not change significantly with curing conditions (curing periods and curing temperature), while the θa increased slightly with gypsum content but was not affected by curing conditions. Further, as the curing period and temperature increased, the (Ψr) values increased and also increased slightly with gypsum content. The variation in saturated and de-saturated (residual) states with curing conditions reflects the mineralogical and microstructural changes in soil samples, as shown in Figs. (7 and 8). 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 kaolinite is exhausted by the pozzolanic reaction, and is consistent with the pozzolanic behaviour of kaolinite. Curing conditions had an insignificant effect on the mineralogical changes in soil samples. In other words, no new reflections 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 reflections seemed to be more pronounced, 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
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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].
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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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].
Ψ Ψr
therefore X-ray reflections are greatly weakened. Second, it is possible that reflections from these phases overlap with both those of primary minerals of natural soil and/or with the reflections formed during 28 days. These observations confirmed SWCC key parameters, as shown in Table (3).
a n
3.5. Modeling of SWCC
m
In this study two model equations (Van Genuchten, 1980; Fredlund and Xing, 1994) were used to fit the experimental results of SWCC. In 1994 Fredlund and Xing proposed a model using a three-parametric continuous function as shown below:
e
3 Ψ ln 1 þ 6 7 Ψr 7 θ ¼ θs 6 41− 1000000 5 ln 1 þ Ψr 2
!m 1 ln e þ Ψa n
where: θ θs
volumetric water content at desired suction. saturated volumetric water content.
ð1Þ
soil suction (kPa). soil suction (kPa) corresponding to the residual water content, θr. soil parameter related to the air entry value of the soil (kPa). soil parameter controlling the slope at the inflection point in the soil-water characteristic curve. soil parameter related to the residual water content of the soil; and natural number, 2.71818……….
Van Genuchten (1980) proposed a closed-form equation for the entire range of suction, given by: θ ¼ θr þ
ðθs −θr Þ ½1 þ ðαψÞn m
ð2Þ
Where the parameters θ, θs and Ψ are as in the Fredlund and Xing equation, θr α
residual volumetric water content, parameter related to the air entry value.
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
A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx
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0% G 5% G 15% G 25% G
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Suction Pressure (kPa)
Volumetric w/c (%)
9
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0
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1000
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Suction Pressure (kPa)
Fig. 9. Experimental and Modeling SWCC with Fredlund and Xing equation and Van Genuchten equation of soil samples cured at 20 °C.
50 0% G 5% G 15% G 25% G
40
30
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Fredlund and Xing
Volumetric w/c (%)
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0 10
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10
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Suction Pressure (kPa)
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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10
n m
A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx
parameter related to the pore size distribution of soil 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 figures 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 SWCC shown in this figure represent the best fit SWCC using Fredlund and Xing or Van Genuchten equations, while the points represent the experimental SWCC. In general, the fit 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 the Fredlund and Xing and Van 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 fitted and experimental values, as evidenced by the coefficient of determination which was more than or equal to 0.99 for the two models. However, more data are necessary to define 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
Gypsum content (%)
28 days at 20 °C
0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25
90 days at 20 °C
180 days at 20 °C
28 days at 40 °C
90 days at 40 °C
180 days at 40 °C
Fredlund equation
Van genuchten equation
n
m
SSR⁎
α
n
SSR
1.5 1.7 1.45 1.28 1.35 1.37 1.3 1.8 1.8 1.9 2 3.3 1.8 1.2 1.2 1.1 1.1 1.1 1.1 0.88 0.83 1.1 1.25 1.3
0.9 0.78 0.76 0.8 0.93 0.85 0.84 0.74 0.75 0.64 0.63 0.48 0.76 0.83 0.76 0.8 1.07 0.91 0.82 0.95 1.1 0.85 0.76 0.76
20 18 18 23 20 21 32 35 45 67 85 62 32 30 34 42 29 44 28 49 51 66 77 85
0.016 0.018 0.016 0.016 0.015 0.018 0.016 0.014 0.017 0.018 0.02 0.016 0.015 0.018 0.015 0.017 0.018 0.016 0.017 0.018 0.02 0.019 0.016 0.019
1.46 1.376 1.374 1.36 1.43 1.373 1.376 1.41 1.396 1.34 1.33 1.35 1.421 1.346 1.34 1.33 1.42 1.37 1.331 1.33 1.37 1.335 1.34 1.33
22 22 27 40 27 32 47 47 53 87 99 104 37 44 55 66 38 63 50 81 75 88 103 111
⁎ SSR = summation of squared error.
- In the lime treated gypsum soil, the water holding 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 accelerated the chemical reactions (i.e. pozzolanic 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 modifications 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 specificity of the type of soil and the progress of the reaction between lime and the clay during curing. Finally, as this study is the first to address the SWCC of lime treated gypseous soils, more tests are needed to determine the general features of the SWCC corresponding to field 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 properties such as shear strength, volume change and permeability. References
Table 4 Equations parameters of modeling SWCCs of soil samples. Curing condition
conditions) that influence the SWCC of lime treated gypsum soil. Theoretical equations were used to evaluate their performance in fitting experimental data. The main conclusions that can be drawn from this study are:
Al-Mukhtar, M., Lasledj, A., Alcover, J.F., 2010a. Behaviour and mineralogy changes in lime-treated expansive soil at 50 °C. Appl. Clay Sci. 50 (2), 199–203. Al-Mukhtar, M., Qi, Y., Alcover, J.-F., Bergaya, F., 1999. Oedometric and water retention behaviour of highly compacted unsaturated smectites. Can. Geotech. J. 36 (4), 675–684. Al-Mukhtar, M., Lasledj, A., Alcover, J.F., 2010b. Behaviour and mineralogy changes in lime-treated expansive soil at 20 °C. Appl. Clay Sci. 50, 191–198. Al-Mukhtar, M., Khattab, S., Alcover, J.F., 2012. Microstructure and geotechnical properties of lime-treated expansive clayey soil. Eng. Geol. 139, 17–27. Al-Mukhtar, M., Lasledj, A., Alcover, J.-F., 2014. Lime consumption of different clayey soils. Appl. Clay Sci. 95, 133–145. Bell, F.G., 1996. Lime stabilization of clay minerals and soils. Eng. Geol. 42, 223–237. Burckhard, S.R., Pirkl, D., Schaefer, V.R., Kulakow, P., Leven, B., 2000. A Study of soil waterholding properties as affected by TPH contamination. Proceedings of the 2000 Conference on Hazardous Waste Research, pp. 356–359. Çokça, E., 2002. Relationship between methylene blue value, initial soil suction and swell percent of expansive soils. Turk. J. Eng. Environ. Sci. 26, 521–529. Delage, P., Howat, M.D., Cui, Y.J., 1998. The relationship between suction and swelling properties in a heavily compacted unsaturated clay. Eng. Geol. 50, 31–48. Eades, J.L., Grim, R.E., 1966. A quick test to determine lime requirements for soil stabilization. Highw. Res. Rec. 139, 61–72. Fredlund, D.G., Rahardjo, H., 1993. Soil mechanics for unsaturated soils. John Wiley and Sons, Inc., USA. Fredlund, D.G., Xing, A., 1994. Equations for the soil-water characteristic curve. Can. Geotech. J. 31 (4), 521–532. Guan, G.S., Rahardjo, H., Leong, E.C., 2010. Shear strength equations for unsaturated soil under drying and wetting. J. Geotech. Geoenviron. 136 (4), 594–606. Ingles, O.G., Metcalf, J.B., 1972. Soil stabilization principles and practice. Butterworth, Sydney. Li, X., Li, J.H., Zhang, L.M., 2014. Predicting bimodal soil–water characteristic curves and permeability functions using physically based parameters. Comput. Geotech. 57, 85–96. Lin, B., Cerato, A.B., 2012. Investigation on soil-water characteristics curves of untreated and stabilized highly clayey expansive soils. Geotech. Geol. Eng. 30, 803–812.
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A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx Little, D.N., 1995. Handbook for stabilization of pavement sub grade and base courses with lime. Kendall Hunt Publishing Company, Iowa, USA (by National Lime Association). Melinda, F., Rahardjo, H., Han, K.K., Leong, E.C., 2004. Shear strength of compacted soil under infiltration condition. J. Geotech. Geoenviron. 130 (8), 807–817. Satyanaga, A., Rahardjo, H., Leong, E.C., Wang, J.Y., 2013. Water characteristic curve of soil with bimodal grain-size distribution. Comput. Geotech. 48, 51–61. Sheng, D., Zhou, A., Fredlund, D.G., 2011. Shear strength criteria for unsaturated soils. Geotech. Geol. Eng. 29, 145–159.
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Thyagaraj, T., Rao, S.M., 2010. Influence of osmotic suction on the soil-water characteristics curves of compacted expansive clay. J. Geotech. Geoenviron. 136 (12), 1695–1702. Van Genuchten, M.Th, 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892–898. Vanapalli, S.K., Fredlund, D.G., Pufahl, D.E., 1999. The influence of soil structure and stress history on the soil water characteristic curve of a compacted till. Geotechnique 49 (2), 143–159. Zhai, Q., Rahardjo, H., 2012. Determination of soil–water characteristic curve variables. Comput. Geotech. 42, 37–43.
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Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Soil–water characteristic curve of lime treated gypseous soil Abdulrahman Aldaood a,b, Marwen Bouasker a, Muzahim Al-Mukhtar a,⁎ a b
Centre de Recherche sur la Matière Divisée CRMD-CNRS and Laboratoire PRISME, Université d’Orléans, Polytech’Orléans,Orléans, France Mosul University, College of Engineering, Civil Engineering Department, Al-Majmooah street, Mosul, Iraq
a 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 2014 Available online xxxx Keywords: Gypseous soil Lime stabilization Curing conditions SWCC Micro structure
a b s t r a c t 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 was determined for the first time for lime treated gypseous soil, using tensiometric plate, osmotic membrane and 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 significant effect on the SWCC. All the experimental data fitted 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 characterized by soil suction, which plays a significant role in determining the performance of soil as foundation materials in terms of permeability, strength and volume change (Lin and Cerato, 2012). Further, many of 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 suction 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 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 influence 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
⁎ 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).
extremely labor intensive. One tool that has made the analysis of unsaturated soil data simpler and more practical is the soil-water characteristic curve (SWCC) (Fredlund and Rahardjo, 1993; Zhai and Rahardjo, 2012; Satyanaga et al., 2013; Li et al., 2014). SWCC is defined 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 unsaturated soil that can be used to determine the 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, analyzing suction in the context of the aforementioned geotechnical properties has become the subject of much research in the rapidly growing field 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 unsaturated 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 structures, roadways and infrastructures, to avoid damage due to gypsum
http://dx.doi.org/10.1016/j.clay.2014.09.024 0169-1317/© 2014 Elsevier B.V. All rights reserved.
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A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx
dissolution. Lime stabilization is often performed in order to overcome such problems. The improvement in the geotechnical properties of gypseous 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, flocculation 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 calcium hydrates which contribute to flocculation 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 fills 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 significant 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 inducing 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 find safe and cost-effective solutions to the engineering problems that can occur with this type of soil. In the present study, the SWCC of lime treated gypseous soil (containing different amounts of gypsum) under different curing conditions (curing temperature 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 fitted using the Fredlund and Xing (1994) and Van Genuchten (1980) equations. 2. Materials and experimental methods 2.1. Materials The soil samples were a natural fine-grained soil, obtained from a borrow pit near Jossigny in the eastern part of Paris-France. The soil samples were collected at a depth between (1.5–2.0 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 be about 18.5%. The soil had a liquid limit of 29%, a plastic limit of 21%, and a plasticity index of 8%. The percentages of clay, silt and sand were 19, 64 and 17% respectively. The chemical analysis showed the presence of clay minerals (SiO2 = 68.8% and Al2O3 = 8.4%) and of calcite (CaO = 5.9%). The high amount of silica reflected the presence of quartz. The results of the chemical analysis correlated well with the results of the X-ray diffraction(Fig. 7): silica reflected the presence of quartz, alumina indicated the presence of clay mineral (kaolinite and illite) and calcium oxide indicated the presence of calcite mineral. The specific gravity of the soil was 2.66. The soil can be classified as sandy lean clay (CL) according to the Unified Soil Classification System (USCS). The quick lime used in this study, supplied by the French company LHOIST, 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 the Merck KGaA company, Germany, is a very fine gypsum and passes through an 80 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). 50
50
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Fig. 1. Experimental soil-water characteristics curve of soil samples cured at 20 °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
A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx Table 1 Volumetric water content with suction of soil samples at different curing temperature and time. Suction, kPa
10 100 1000 10,000 150,000
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
38 35.4 28.8 13.6 3.8
38.9 36.9 30.2 16.2 4.2
39.5 37.3 30.6 16.4 4
40.9 38.9 31.9 16.9 4.3
41.4 39.7 32.5 16.5 4.1
43.1 40.9 33.9 19.8 5.1
43 41.5 33.7 17.7 4.3
45.3 42.8 36.1 19.4 5.5
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 soil samples. To ensure the uniformity of the soil 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 Proctor optimum moisture content of the natural soil (i.e. 11%). During mixing, proper care was taken to prepare homogeneous mixtures.
3
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 first 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/m3). The soil samples were 50 mm in diameter and 10 mm in height. After compaction, the samples were immediately wrapped in cling film and coated with paraffin 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–
Ettringite Ettringite 20°C 5%G
40°C 5%G
Ettringite Ettringite 20°C 15%G
40°C 15%G
Ettringite Ettringite 20°C 25%G
40°C 25%G
Fig. 2. Microstructure changes and ettringite minerals formation during 180 days of curing at 20 °C and 40 °C.
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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 the soil 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 concentration and 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).
Table 2 Pore size distribution of soil samples with curing conditions.
2.4. Mineralogical and microstructural tests
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 insignificant 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 increased pore size (flocculation). 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 28 days at 20 °C. It can be seen that increasing the curing period resulted in more macro pores centered on 6 μm and reduced the number of pores centered on 2 μm, while there was a slight and insignificant variation in the number of pores centered
Mineralogical and microstructural tests were conducted at the end 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 fix resin, gold coated and then scanned. Several digital images at different magnifications were recorded in order to examine the cementitious compounds and the formation of ettringite. A pore size distribution 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 μm. Soil samples were lyophilized using ALPHA 1–2 Ld Plus – GmbH apparatus before applying mercury tests to minimize 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 μm sieve to serve as samples for the 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
0.025
28 days
0% G
0.02
Incrimental Intrusion (mL/g)
Incrimental Intrusion (mL/g)
The SWCC of lime treated soil samples with different gypsum contents are presented in Fig. (1). These curves were determined after
5% G
20°C
25% G
0.015 0.01 0.005 0 0.001
0.01
0.1
1
Entrance Diameter (μm)
10
100
Temperature
Curing period
Gypsum
(°C)
Small pores b0.1 μm
Medium pores 0.1–10 μm
Large pores N10 μm
Porosity
Day
%
%
%
%
%
20
28
40
180
0 5 25 0 5 25
22 38 30 39 28 21
76 59 66 59 69 73
2 3 4 2 3 6
26 26 32 28 28 34
0.025
180 days
0% G
0.02
5% G
40°C
25% G
0.015 0.01 0.005 0 0.001
0.01
0.1
1
10
100
Entrance Diameter (μm)
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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A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx
on 0.06 μm. The increase in macro pores with curing period is attributed to the development of ettringite minerals. Lastly, the influence of the curing period may vary depending on the gypsum content because of the variations in time-dependent pore redistribution.
5
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 the water
50
50
40
28 days at 40°C
30 20 10
180 days at 20°C
Volumetric w/c (%)
Volumetric w/c (%)
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180 days at 40°C
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1000
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30 20 10
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40
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Suction Pressure (kPa)
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Suction Pressure (kPa) 50
50 40
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30 20 10
180 days at 20°C
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28 days at 20°C
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100000
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50
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180 days at 40°C
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50 40
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30 20
A 10
180 days at 20°C
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28 days at 20°C
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10000
Suction Pressure (kPa)
40
180 days at 40°C
30 20
B 10
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25% G 0
0 10
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1000
10000
100000
Suction Pressure (kPa)
1000000
10
100
1000
10000
100000
1000000
Suction Pressure (kPa)
Fig. 4. Experimental SWCC of soil cured at different curing temperature for (A) 28 days and (B) 180 days.
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A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx
50
0% G 5% G 15% G 25% G
40
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Suction Pressure (kPa)
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, increased with increased curing temperatures. The results reported in Table 1 show 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 for samples cured at 20 °C. The difference in water content increased when suction decreased in the samples. This behaviour is attributed to the acceleration of chemical reactions in the soil 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 the change in the pore size distribution of soil samples. The continuous reaction between soil, lime and gypsum with increased temperature, as well as the formation of CSH, CAH and ettringite minerals, caused the soil samples cured at 40 °C to have a finer 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%) reflected in the reduction of medium-sized pores. No changes were observed in large pores. In gypseous soil samples and for the same curing conditions, lime treatment reduced the number of small pores and increased the medium pores. Again no changes were observed
Table 3 SWCC keys of soil samples at different curing conditions. Temp. (°C)
20
Curing time (day)
28
90
180
40
28
90
180
Gypsum content (%)
0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25
Saturation state
Residual state
Ψa, AEV (kPa)
θa (%)
Ψr (kPa)
θr (%)
190 200 200 200 210 160 210 210 230 210 170 190 200 180 200 200 200 210 190 240 180 190 200 190
33 35 38 39 33 36 38 40 33 37 39 41 34 36 39 40 34 39 40 40 34 38 40 42
90,000 60,000 80,000 100,000 120,000 120,000 170,000 130,000 150,000 190,000 180,000 150,000 100,000 110,000 110,000 110,000 110,000 110,000 90,000 80,000 190,000 165,000 150,000 120,000
2 6 6 5 2 3 3 2 2 4 4 5 3 5 6 7 2 4 6 7 2 4 5 6
in large pores. The changes in the pore space of soil samples with curing temperature are due to the pozzolanic reaction products. The pozzolanic products (CSH and CAH) not only enhanced the inter-cluster bonding strength but also filled the pore space. As a result, the water holding capacity of the soil samples significantly increased with an increasing curing temperature. Further, the ettringite mineral fills the pores within the soil matrix, thus leading to a decrease in the void ratio of the gypseous 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 influence of curing temperature was found to be more significant at low suction pressure (below 1500 kPa). The presence of ettringite may also influence the SWCC of soil samples. 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 influence 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, especially low pressure below 1500 kPa, a significant 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
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 development of an osmotic gradient attracts more water into the gypsum-soil matrix; as a result, gypsum addition influences the SWCC. Also, the refinement of the pore structures of soil samples, especially those cured at 40 °C, as shown in Fig. (3) increases the volumetric water content 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 airentry value (AEV) and the corresponding volumetric water content. The AEV is an important parameter for unsaturated soils since the degree of saturation starts to drop rapidly when the suction pressure exceeds 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 corresponding residual suction pressure. In general, it can be observed that the AEV of soil samples did not change significantly with curing conditions (curing periods and curing temperature), while the θa increased slightly with gypsum content but was not affected by curing conditions. Further, as the curing period and temperature increased, the (Ψr) values increased and also increased slightly with gypsum content. The variation in saturated and de-saturated (residual) states with curing conditions reflects the mineralogical and microstructural changes in soil samples, as shown in Figs. (7 and 8). 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 kaolinite is exhausted by the pozzolanic reaction, and is consistent with the pozzolanic behaviour of kaolinite. Curing conditions had an insignificant effect on the mineralogical changes in soil samples. In other words, no new reflections 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 reflections seemed to be more pronounced, 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
800
Intensity (counts/s)
600
400
200
0 0
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20
30
40
50
60
2θ (°) 800
Intensity (counts/s)
600
400
200
0 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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A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx
800
Intensity (counts/s)
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0 0
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60
2θ (°) 800
Intensity (counts/s)
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400
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0 0
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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].
Ψ Ψr
therefore X-ray reflections are greatly weakened. Second, it is possible that reflections from these phases overlap with both those of primary minerals of natural soil and/or with the reflections formed during 28 days. These observations confirmed SWCC key parameters, as shown in Table (3).
a n
3.5. Modeling of SWCC
m
In this study two model equations (Van Genuchten, 1980; Fredlund and Xing, 1994) were used to fit the experimental results of SWCC. In 1994 Fredlund and Xing proposed a model using a three-parametric continuous function as shown below:
e
3 Ψ ln 1 þ 6 7 Ψr 7 θ ¼ θs 6 41− 1000000 5 ln 1 þ Ψr 2
!m 1 ln e þ Ψa n
where: θ θs
volumetric water content at desired suction. saturated volumetric water content.
ð1Þ
soil suction (kPa). soil suction (kPa) corresponding to the residual water content, θr. soil parameter related to the air entry value of the soil (kPa). soil parameter controlling the slope at the inflection point in the soil-water characteristic curve. soil parameter related to the residual water content of the soil; and natural number, 2.71818……….
Van Genuchten (1980) proposed a closed-form equation for the entire range of suction, given by: θ ¼ θr þ
ðθs −θr Þ ½1 þ ðαψÞn m
ð2Þ
Where the parameters θ, θs and Ψ are as in the Fredlund and Xing equation, θr α
residual volumetric water content, parameter related to the air entry value.
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A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx
50
0% G 5% G 15% G 25% G
40
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Volumetric w/c (%)
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30 20
Fredlund and Xing
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Van Genuchten
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Fredlund and Xing
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9
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Van Genuchten
10
180 days
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0
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Suction Pressure (kPa)
Fig. 9. Experimental and Modeling SWCC with Fredlund and Xing equation and Van Genuchten equation of soil samples cured at 20 °C.
50 0% G 5% G 15% G 25% G
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Fredlund and Xing
Volumetric w/c (%)
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10
100
1000
10000
100000
1000000
Suction Pressure (kPa)
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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10
n m
A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx
parameter related to the pore size distribution of soil 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 figures 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 SWCC shown in this figure represent the best fit SWCC using Fredlund and Xing or Van Genuchten equations, while the points represent the experimental SWCC. In general, the fit 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 the Fredlund and Xing and Van 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 fitted and experimental values, as evidenced by the coefficient of determination which was more than or equal to 0.99 for the two models. However, more data are necessary to define 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
Gypsum content (%)
28 days at 20 °C
0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25 0 5 15 25
90 days at 20 °C
180 days at 20 °C
28 days at 40 °C
90 days at 40 °C
180 days at 40 °C
Fredlund equation
Van genuchten equation
n
m
SSR⁎
α
n
SSR
1.5 1.7 1.45 1.28 1.35 1.37 1.3 1.8 1.8 1.9 2 3.3 1.8 1.2 1.2 1.1 1.1 1.1 1.1 0.88 0.83 1.1 1.25 1.3
0.9 0.78 0.76 0.8 0.93 0.85 0.84 0.74 0.75 0.64 0.63 0.48 0.76 0.83 0.76 0.8 1.07 0.91 0.82 0.95 1.1 0.85 0.76 0.76
20 18 18 23 20 21 32 35 45 67 85 62 32 30 34 42 29 44 28 49 51 66 77 85
0.016 0.018 0.016 0.016 0.015 0.018 0.016 0.014 0.017 0.018 0.02 0.016 0.015 0.018 0.015 0.017 0.018 0.016 0.017 0.018 0.02 0.019 0.016 0.019
1.46 1.376 1.374 1.36 1.43 1.373 1.376 1.41 1.396 1.34 1.33 1.35 1.421 1.346 1.34 1.33 1.42 1.37 1.331 1.33 1.37 1.335 1.34 1.33
22 22 27 40 27 32 47 47 53 87 99 104 37 44 55 66 38 63 50 81 75 88 103 111
⁎ SSR = summation of squared error.
- In the lime treated gypsum soil, the water holding 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 accelerated the chemical reactions (i.e. pozzolanic 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 modifications 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 specificity of the type of soil and the progress of the reaction between lime and the clay during curing. Finally, as this study is the first to address the SWCC of lime treated gypseous soils, more tests are needed to determine the general features of the SWCC corresponding to field 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 properties such as shear strength, volume change and permeability. References
Table 4 Equations parameters of modeling SWCCs of soil samples. Curing condition
conditions) that influence the SWCC of lime treated gypsum soil. Theoretical equations were used to evaluate their performance in fitting experimental data. The main conclusions that can be drawn from this study are:
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A. Aldaood et al. / Applied Clay Science xxx (2014) xxx–xxx Little, D.N., 1995. Handbook for stabilization of pavement sub grade and base courses with lime. Kendall Hunt Publishing Company, Iowa, USA (by National Lime Association). Melinda, F., Rahardjo, H., Han, K.K., Leong, E.C., 2004. Shear strength of compacted soil under infiltration condition. J. Geotech. Geoenviron. 130 (8), 807–817. Satyanaga, A., Rahardjo, H., Leong, E.C., Wang, J.Y., 2013. Water characteristic curve of soil with bimodal grain-size distribution. Comput. Geotech. 48, 51–61. Sheng, D., Zhou, A., Fredlund, D.G., 2011. Shear strength criteria for unsaturated soils. Geotech. Geol. Eng. 29, 145–159.
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Thyagaraj, T., Rao, S.M., 2010. Influence of osmotic suction on the soil-water characteristics curves of compacted expansive clay. J. Geotech. Geoenviron. 136 (12), 1695–1702. Van Genuchten, M.Th, 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892–898. Vanapalli, S.K., Fredlund, D.G., Pufahl, D.E., 1999. The influence of soil structure and stress history on the soil water characteristic curve of a compacted till. Geotechnique 49 (2), 143–159. Zhai, Q., Rahardjo, H., 2012. Determination of soil–water characteristic curve variables. Comput. Geotech. 42, 37–43.
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