Abstract
Dispersive and expansive soils are considered problematic, and these soil properties cause serious problems for many engineering structures. For many years, comprehensive studies have been carried out with the aim of improving the swelling and dispersive qualities of soils by using additives. Each feature in the literature associated with the improvement of the dispersive and swell properties of clay soil with additives was separately evaluated. In this study, the effect of cement and natural zeolite additives on the characteristics of dispersibility and swelling potential of clay soils were investigated. A fixed percentage of cement (3%) plus different percentages of natural zeolite (1%, 3%, 6%, 10%, 15%, and 20%) were mixed with four different clay soil samples. In this context, first, the physical and chemical properties of the soil samples were determined. Next, the swell percentage, swell pressure, crumb, pinhole and unconfined compressive strength tests at different curing times were performed on samples with and without the additive by compressing the sample to achieve particular compaction characteristics. Significant strength value increases depended on curing time, and the properties were improved with the mixture of cement and zeolite additives, depending on the sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) values of clay soil samples with different plasticity characteristics that exhibit dispersive and swell properties. This study not only showed that a mixture of cement and zeolite additives improved the dispersive and swell properties of clay soil samples with four different plasticity characteristics, depending on their SAR and ESP values, but also significant increases in strength values were observed.
1 Introduction
In geotechnical engineering applications, soils in a project field may not have the desired features and qualities. These soils can have a weak, inflatable, dispersive, highly compressible nature and can be highly permeable. Building on these types of soils requires the development of techniques, such as soil stabilization, to improve the soil. In many parts of the world, swelling soils are frequently encountered, especially in arid and semi-arid regions. Possible damages can be reduced or completely prevented by early identification of the swelling behavior and the associated factors. Significant research on the damages resulting from the swelling behavior of clay soils and design criteria have been conducted [1–4].
Another problem of clay soils is that they have dispersive properties. Soils that are dislodged easily and rapidly in flowing water of low salt concentration are called dispersive soils. Previously, clays were considered to be non-erosive, but it is now clear that erosive clay soils do exist [5]. These structurally unstable soils can have a high degree of erosion and can be easily dispersed. Erosion caused by the dispersibility of clay depends on the amount and content of the dissolved salts in the water and soil gaps. Many earth dams have been damaged and collapsed as a result of piping caused by dispersive soils [6]. Piping is the main reason for the damage and collapse of 25% of the 214 earth dams that collapsed between 1885 and 1951 [7]. In each case, the soil containing readily dispersive clay particles went easily into suspension in flowing water. Problems associated with dispersive soils are reported from many parts of the world. A number of earth dams, hydraulic structures, and roadway embankments have failed owing to erosion problems. The tendency of clays to disperse or deflocculate depends on clay type and soil chemistry [5].
The structure of clay, which strongly influences its dispersion and expansion mechanisms, is quite complex. Soil expansion triggers soil dispersion. Whether swelling or dispersion dominates depends on the degree to which soil swelling weakens the bond between the clay particles; swelling dominates when sufficiently strong net attractive forces prevent the separation of the particles, and dispersion dominates otherwise. The effect of a water leak in an embankment constructed of dispersive clays depends on the leak velocity. If the leak velocity is low, the clay surrounding the flow channel can swell, preventing seepage. If the leak velocity is high, dispersed clay particles are carried away. A flow rate faster than the swelling rate of the clay causes the flow channel to enlarge and leads to the collapse of the piping [8]. Thus, the swell potential of the embankment soil is an additional factor in this collapse mechanism.
Comprehensive studies have been conducted to improve the properties of swelling and dispersive soils using additives with different contents because of the damage such soils incur to engineered structures [9, 10]. In comprehensive studies by Bell [11], Cokca [12], Bhuvaneshwari et al. [13], Zha et al. [14], Yilmaz and Civelekoglu [15] and Turkoz et al. [16], additives such as lime, cement, industrial wastes, gypsum, fly ash and magnesium chloride solution were used. Experimental studies (e.g., Nelson and Miller [17]; Feng [18]; Al-Rawas et al. [19]) have shown that adding cement to clay soils generally decreases the liquid limit, plasticity index and swelling potential and increases the shear strength of the soils. The cement content used in these studies ranged from 3% to 20%. The effects of cement additives on the shear strength, consistency limits and swelling potential of soils with similar characteristics were also evaluated in these studies. In the present study, the effects of cement additives mixed with different proportions of natural zeolite on swelling potential, dispersibility and strength characteristics of natural clay soils with different characteristics were investigated.
Cement consists of numerous minerals and is manufactured by combining cement clinker (a sintered material of limestone and clay) with gypsum. Cement mixed with water forms calcium silicate hydrate and calcium hydroxide [Ca(OH)2]. Calcium silicate hydrate, generally referred to as CSH gel, forms on the surfaces of the cement particles, and because it has a strongly cementing effect it binds the soil together and increases its strength. Because the hydraulic reaction takes place considerably faster than the pozzolanic reaction, cement-stabilized soil normally attains higher strength than lime-stabilized soil, particularly in the first few months.
Because some Ca(OH)2 is formed during cement stabilization, pozzolanic reactions will also take place, although to a lesser extent than in lime stabilization. Hence, in cement stabilization, in addition to the cementation reaction, the same strength-enhancing reaction products are formed as in lime stabilization in about one-fifth of the quantity [20]. In other words, when cement is added to clayey soils in the presence of water, a number of reactions occur, leading to the modification of soil properties. These reactions include cation exchange, flocculation, carbonation and pozzolanic reaction [19].
Pozzolanic reaction, which is time dependent, involves interactions between soil silica and/or alumina and cement to form various types of cementitious products, thus enhancing the strength. A pozzolan is a siliceous or siliceous/aluminous material that, when mixed with cement and water, forms a cementitious compound. Zeolites as a pozzolan are aqueous aluminum silicates containing alkali and alkaline earth elements. Their structure is made up of a framework of SiO4 and AlO4 tetrahedrons linked to each other’s corners by sharing oxygen atoms. The substitution of Si4+ by Al3+ in tetrahedral sites results in more negative charges and a high cation exchange capacity [21].
The use of natural zeolites in industrial processes dates back to the 1940s. Zeolites have been used in many applications because they can function as a molecular sieve owing to their ion-exchange ability, adsorption and absorption properties, crystal structure [22] and silica content, as well as their lightweight, porous structures [23, 24]. Millions of tons of zeolite tuff exist in Turkey, especially clinoptilolite tuffs, which are widely available. Whereas clinoptilolite and analcime species are widely available, other very rare zeolite species can be observed in Turkey only in volcanic sedimentary formations. Turkey’s most important zeolite deposits have been detected in Manisa-Gordes and Balikesir-Bigadic, and these zeolites are easily operated on [25]. It is estimated that the total zeolite reserve is 50 billion tons throughout Turkey. Studies on zeolite with pozzolanic properties, which is used in many areas, have focused on using it as an additive to increase the strength of concrete [26].
Yukselen-Aksoy [27] examined the usage of zeolites obtained from Gordes and Bigadic. The engineering characteristics of zeolites from Gordes and Bigadic have been obtained through tests of grain-size distribution, specific gravity, compressibility, hydraulic conductivity, swell and shear strength. From these tests, zeolites have been found to be mechanically stable and suitable as filling material for landfill applications. Hossain et al. [28] utilized volcanic ash (VA) from natural resources of Papua New Guinea. Several tests of compaction and unconfined compressive strength have been conducted to study the influence of VA, finely ground natural lime, cement and a combination of ash, cement and lime [29]. The percentages of VA were 0%, 2%, 4%, 5%, 10%, 15% and 20%, whereas the percentages of both lime and cement were 0%, 2% and 4%. A very limited number of studies on the stabilization of problematic soils using zeolite (clinoptilolite), a natural pozzolan [29], have been conducted.
In this study, the improvement of four different clay soil samples determined to have high swelling potential and dispersible properties [with a low cement content (3%) based on the literature] was investigated using a range of additive mixtures with different zeolite percentages (0%, 1%, 3%, 6%, 10%, 15% and 20%). Swelling percentage and swelling pressure tests for evaluating the swelling potential of the samples, pinhole and crumb tests for determining dispersibility properties and unconfined compressive strength (UCS) tests for evaluating strength characteristics were conducted, and the results of the experiments on samples with and without additives were compared.
2 Materials
2.1 Soils
The soil samples used in this study were obtained from the Afyon and Urfa provinces in Central and Southeastern Anatolia, Turkey. Sieve analysis, hydrometer analysis (ASTM D 422-63), consistency limits (ASTM D 4318-00) and specific gravity (ASTM D 854-00) tests were performed to characterize the soil samples. The compaction characteristics of the samples, determined by experiments conducted at the standard Proctor energy level, were in compliance with the ASTM D-698 [30] standard. ASTM [30] standard methods were followed during the preparation of samples, sampling and testing. Based on the identification test results, the samples were classified according to the Unified Soil Classification System (USCS) (ASTM D 2487-00). The grain-size distributions, compaction curves, physical properties and chemical compositions of the soil samples are presented in Figures 1 and 2 and Tables 1 and 2, respectively. X-ray diffraction (XRD) patterns of the soil samples are presented in Figure 3.
Physical properties of the soils used in the study.
| Property | Soil samples | |||
|---|---|---|---|---|
| 1 | 2 | 3 | 4 | |
| Grain size | ||||
| Gravel (%) | 1.5 | 0 | 1.3 | 0 |
| Sand (%) | 18.1 | 25.6 | 6.0 | 13.1 |
| Silt (%) | 54.4 | 64.4 | 37.7 | 53.9 |
| Clay (%) | 26.0 | 10.0 | 55.0 | 33.0 |
| Atterberg limits | ||||
| Liquid limit, LL (%) | 43 | 30 | 68 | 68 |
| Plastic limit, PL (%) | 26 | 19 | 31 | 33 |
| Plasticity index, PI (%) | 17 | 11 | 37 | 35 |
| Specific gravity, Gs | 2.66 | 2.68 | 2.79 | 2.64 |
| Classification (USCS) | CL | CL | CH | MH |
| Activity, A | 0.65 | 1.10 | 0.67 | 1.06 |
| ρdmaks (Mg/m3)a | 1.500 | 1.608 | 1.487 | 1.448 |
| wopt (%)b | 24.1 | 18.2 | 25.2 | 23.2 |
aMaximum dry density.
bOptimum water content.
Chemical compositions of the soil used in the study.
| Soil sample | Conductivity (mmho/cm) | pH | TDS (meq/l) | Na (%) | SAR | ESP (%) |
|---|---|---|---|---|---|---|
| 1 | 2.460 | 8.85 | 22.45 | 89.09 | 18.65 | 33.15 |
| 2 | 13.190 | 10.72 | 105.64 | 98.77 | 129.42 | 44.94 |
| 3 | 0.806 | 8.14 | 7.45 | 8.05 | 0.33 | 1.18 |
| 4 | 20.600 | 8.55 | 143.19 | 92.30 | 56.36 | 33.85 |
TDS, total dissolved salt; Na, sodium percentage.
Grain-size distributions of the soil samples.
Compaction curves of the soil samples.
XRD patterns of the soil samples.
Table 1 presents a summary of the geotechnical properties of the soil samples used in the study. When standard Proctor compaction results are evaluated, compared to other samples, sample 2, which has the lowest clay content, has greater maximum dry density and lower optimum water content. It can be observed that, generally, as fine-grained content increased optimum water content increased, whereas maximum dry unit weight decreased.
The chemical compositions of soil samples used in the experiment are presented in Table 2. Compared to other soil samples, it is seen that the greatest Na (%), sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) (%) values, which are shown as the reason for dispersive soil behavior, are in sample 2, and the lowest Na (%), SAR and ESP (%) values are in sample 3.
2.2 Additives
The zeolite used as an additive in this study was provided by the Rota Mining Corporation in the Manisa-Gordes region. In this study, the ground zeolite was 40 μm in size, the same form as that used in industrial applications. Another additive, Portland cement (CEM I 42.5 R), which can also be obtained commercially, was also used. The physical and chemical properties of the additives used in this study are shown in Table 3
Physical and chemical composition of the additives.
| Property | Cement | Natural zeolite |
|---|---|---|
| Specific gravity (g/cm3) | 3.10 | 2.2–2.4 |
| Specific surface (cm2/g) | 3410 | – |
| CEC (meq/g) | – | 1.5–2.1 |
| SiO2 (%) | 19.52 | 65–72 |
| CaO (%) | 64.08 | 2.4–3.7 |
| MgO (%) | 1.72 | 0.9–1.2 |
| Al2O3 (%) | 4.84 | 10–12 |
| Fe2O3 (%) | 2.68 | 0.7–1.9 |
| SO3 (%) | 2.40 | – |
| Na2O (%) | 0.41 | 0.1–0.65 |
| K2O (%) | 0.61 | 2.1–3.5 |
| Cr2O3 (%) | – | 0–0.01 |
| Loss on ignition (%) | 3.70 | 9–14 |
aCEC, cation exchange capacity.
The chemical analysis indicated that the natural zeolite is principally composed of silica (65–72%) and calcium oxide (2.4–3.7%), whereas the main components of cement are calcium oxide (64.08%) and silica (19.52%). The amount of oxides of sodium and potassium, known as ‘alkalis’, is found to be higher in natural zeolite (mean, 3.18%) compared to cement (1.1% maximum).
3 Methods
3.1 Sample preparation
The soil samples used in the study were dried at 105°C in a drying oven and then ground and passed through a no. 4 sieve to obtain a uniform distribution. Different amounts of the zeolite (0%, 1%, 3%, 6%, 10%, 15% and 20% by dry weight of the soil) and cement (3% by dry weight of the soil) were added and mixed into the prepared soil samples. Soil-additive mixtures were prepared for each soil sample by mixing in an optimum amount of water, which was determined at the standard Proctor energy level. All mixing was performed manually, and special attention was paid to obtaining a homogeneous mixture in each step. Swell percentage, swell pressure, crumb, pinhole and unconfined compressive tests were performed on the samples prepared by compressing each sample with the optimum water content.
3.2 Swell tests
The swell percentage and swell pressure of clays are best determined through direct measurements [17]. These methods include evaluating the free swell, expansion index (EI) and potential volume change (PVC), as well as by performing odometry tests, under laboratory conditions.
The swell percentage and swell pressure tests were performed using direct methods. PVC equipment was used to determine the swell pressure. There is no standard procedure for the PVC meter test, so we used the method proposed by Lambe [31]. The PVC meter test involves determining the pressure arising from the inhibited swell deformation that develops after the compacted soil sample is saturated with water. A proving ring handle is placed above the sample, which is compacted and placed in the system. The sample is soaked in water, and values are periodically read from the proving ring, converted to loads in known units using a calibration curve or by multiplying them by the proving ring factor and recorded. The pressure is obtained by dividing the load by the sample area.
The EI test, used to determine the swell percentage, was performed following the ASTM D 4829 [30] standard. The swell percentage, another important component of the swell potential, is defined as the ratio between the starting length of the sample and the final deformation of the sample after being soaked in water under 7 kPa of pressure for 24 h or until swelling is complete. In our study, the EI measurement mold was adjusted so that the components of the swell potential could be measured using samples of the same dimensions. A weight was manufactured to place 7 kPa of pressure on the samples in 2-cm-high, thin-walled rings with 7-cm diameters. Immediately after the samples were soaked in water, the swell percentage and swell pressure were measured at a series of time intervals (0.5, 1, 2, 4, 8, 16, 32, 60, 120, 240, 360, …, 2880 min) using digital deformation meters connected to the data logger.
3.3 Dispersibility tests
To determine the dispersibility characteristics of the samples, pinhole and crumb tests were performed following the standard procedures of the US Bureau of Reclamation (USBR) [32, 33]. The pinhole test is the most reliable of these tests and provides physical, quantitative results regarding the dispersibility of clay soils. In the test, a 1.0-mm hole is created in a cylindrical soil sample 25 mm in length and 33 mm in diameter that has been compacted at the standard Proctor energy level. Distilled water is passed through this hole under forces of 50, 180 and 380 mm (hydraulic inclinations of approximately 2, 7 and 15, respectively). The flow rate and turbidity of the water are recorded. The pinhole test and the evaluation of its results were performed according to the USBR 5410 [33] standard, and the quantitative analysis of the test results was performed according to the method proposed by Acciardi [34].
The pinhole tests used in this study were performed using a new pinhole test system developed within the scope of a project supported by the Scientific and Technological Research Council of Turkey (TUBITAK). In this system, the water forces and flow rates during the test are controlled by electronic equipment, and the obtained data can be stored on digital media [35].
The crumb test yields good qualitative results and is used to determine the potential erodibility of clay soils. A dispersive soil may be misclassified as nondispersive based on the results of this test, but a dispersive classification based on this test is a strong indication that the soil is actually dispersive.
The crumb test was developed to determine the field behavior of dispersive clays and is performed on soil samples with natural water content. The samples are cubic in shape with a 15-mm side length, or they may have another shape with an equal volume. The sample is carefully placed in distilled water in a 250-ml porcelain container. The reaction between the soil and water causes colloidal (<0.002 mm) particles to segregate and form a suspension. The classification is performed by recording observations at certain time intervals [32].
3.4 Unconfined compressive strength tests
Unconfined compressive strength tests on compacted specimens were conducted according to the ASTM D2166 [30]. All samples subjected to the strength test were prepared at their optimum water content. The samples were prepared in stainless steel tubes so that the ratio of their height to their diameter was 2 (100-mm height and 50-mm diameter). The samples were removed from the tubes, placed in plastic bags and cured for 7 and 28 days in vacuum desiccators. This procedure allowed the effects of both the additive contents and the curing time on the sample strength to be determined.
4 Test results and discussion
4.1 Swell percentage and pressure
The swell percentage vs. time relationships obtained from the swell tests performed on samples with different additive contents are shown in Figure 4, and the effects of the additive content on the swell percentage are presented in Figure 5.
Swell percentage vs. time plots for the soil samples mixed with different additive contents.
Relationship between additive content and swell percentage of the soil samples.
The parameters obtained from the sample identification tests were used in the description and classification of the swell potential. In general, higher soil plasticity indices and liquid limits imply larger swell potentials. Van der Merwe [36] developed a method based on plotting the plasticity index against the clay content (Figure 6). In addition, Chen [3] classified a plasticity index over 35 as a very high swell potential, 29–35 as high, 10–35 as moderate and 0–15 as low. On the basis of the physical properties obtained from the identification test results, the swell potentials of the samples were classified as low (sample 2), moderate (sample 1) or very high (samples 3 and 4) according to the definitions of Van der Merwe [36] and Chen [3]. The swell tests (Figure 5) showed that sample 1, which exhibited a moderate swell potential (Figure 6), had a higher swell potential value than sample 3, which exhibited a very high swell potential value (Figure 6). Similar evaluations were completed for samples 3 and 4, and similar results were observed with respect to the swell pressure values. The reason for these results is that the values of the ESP and the SAR of sample 1, which has low plasticity, compared with sample 3, which has high plasticity, were higher.
![Figure 6 Classification chart, proposed by Van der Merwe [36], used to determine the swelling potential.](/document/doi/10.1515/secm-2012-0104/asset/graphic/secm-2012-0104_fig6.jpg)
Classification chart, proposed by Van der Merwe [36], used to determine the swelling potential.
When the swell percentage-time relationships of the samples, depending on contribution percentages, were investigated (Figure 4), it was found that swell percentage values occur in a very short time with a low additive content compared with the values obtained in the absence of an additive content. Although a significant reduction in swell percentage values was observed at the 3% C+3% Z additive level, the expected effect was not observed when the additive percentages were increased (Figure 5).
The swell pressure vs. time relationships and the final swell pressures of the samples are presented in Figures 7 and 8, respectively. It can be observed in Figure 7 that swell pressure values take place in a shorter time and at a lower level compared with the values obtained in the absence of an additive content, similar to the swell percentage. A significant decrease in swell pressure values at the 3% C+6% Z additive level and an increase in the values with increasing additive percentages, particularly in high-plasticity clay soils, were observed (Figure 8). When the test results were evaluated as a whole, both the swell percentage and the swell pressure were found to decrease gradually as the additive percentage increased, depending on the plasticity characteristics of the samples (LL and PI) and their chemical properties (SAR and ESP). Although an increased amount of zeolite additive had more significant effects on the clays with high plasticity (samples 3 and 4), the expected effect was not observed when using zeolite additives at percentages higher than 6–10%. This result is due to zeolite being a pozzolan material, and there is not enough cement to activate the binding properties with increasing zeolite percentages. The decrease in the swelling potential may be explained by the pozzolanic and cation exchange reactions that occurred between the soil and the additives [12].
Swell pressure vs. time plots for the soil samples mixed with different additive contents.
Relationship between additive content and swell pressure of the soil samples.
These results can be attributed to the presence of zeolite in the soil-cementitious mix. As a pozzolonic material, the zeolite consumes the Ca(OH)2 formed during the cement hydration to produce cement-like hydration products. It is well known that Ca(OH)2 is essential for swelling reaction [37]. Consequently, the addition of zeolite reduced the amount of swelling that could occur.
4.2 Pinhole and crumb tests
The results of the pinhole tests performed on samples with different additive contents are presented in Figure 9 in terms of time vs. flow rate. Finally, changes in the dispersive properties of the samples resulting from different additive percentages are presented in Table 4. From the physical dispersibility test results (pinhole and crumb), it can be observed that samples 1, 2 and 4 show dispersive (D1-D2; K4-K3) properties and sample 3 shows intermediate soil (ND3; K1) properties. The dispersive properties of each of the four samples improved as the additive amount increased, as shown in Table 4. Improvements in the dispersibility degree of the samples were seen with various additive percentages. The pinhole and crumb test results were in agreement, and the dispersive characteristics improved depending on the additive percentage (Table 4). Samples 1, 2, 3 and 4 exhibited nondispersive soil (ND2-ND1) behaviors with additive percentages of 3% C, 3% C+10% Z, 3% C and 3% C+10% Z, respectively.
Results of dispersibility tests performed on the soil samples mixed with different additive contents.
| Sample | Test | Additive content | |||||||
|---|---|---|---|---|---|---|---|---|---|
| 0% | 3% C | 3% C+1% Z | 3% C+3% Z | 3% C+6% Z | 3% C+10% Z | 3% C+15% Z | 3% C+20% Z | ||
| 1 | Crumb test class | K4 | K1 | K1 | K1 | K1 | K1 | K1 | K1 |
| Pinhole test class | D1 | ND1 | ND1 | ND1 | ND1 | ND1 | ND1 | ND1 | |
| 2 | Crumb test class | K3 | K2 | K2 | K2 | K2 | K1 | K1 | K1 |
| Pinhole test class | D1 | ND3 | ND4 | ND4 | ND3 | ND1 | ND1 | ND1 | |
| 3 | Crumb test class | K1 | K1 | K1 | K1 | K1 | K1 | K1 | K1 |
| Pinhole test class | ND3 | ND1 | ND1 | ND1 | ND1 | ND1 | ND1 | ND1 | |
| 4 | Crumb test class | K3 | K2 | K2 | K1 | K2 | K1 | K1 | K1 |
| Pinhole test class | D2 | D2 | ND3 | ND3 | ND4 | ND2 | ND1 | ND1 | |
D1 and D2, dispersive; ND3 and ND4, intermediate soil; ND1 and ND2, nondispersive soil; K3 and K4, dispersive; K2, intermediate soil; K1, nondispersive soil.
Pinhole test results of the soil samples mixed with different additive contents.
4.3 Unconfined compressive strength
The effects of the additive content and the curing time on the UCSs of the samples are presented in Table 5. When experimental tests of unconfined compression were carried out at different curing times (uncured, 7 and 28 days) and with different additive levels, the strength was found to increase up to a certain percentage, although the expected strength increases were not observed when the additive levels were increased. Another factor affecting the strength is cure time. An increase in strength was observed with an increase in curing time. However, while a rapid increase in strength values was observed for samples 1 and 2, which had low plasticity at 7 days of curing, the final strength was reached after 28 days of curing for samples 3 and 4, which had high plasticity. The effect of the duration of curing on strength values is more significant for high-plasticity clay soils than for low-plasticity clay soils.
Influence of additive contents and curing time on the UCS test results of the soil samples.
| Additive content | Unconfined compressive strength (kPa) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sample_1 (days) | Sample_2 (days) | Sample_3 (days) | Sample_4 (days) | |||||||||
| 0 | 7 | 28 | 0 | 7 | 28 | 0 | 7 | 28 | 0 | 7 | 28 | |
| 0% | 229.2 | – | – | 192.8 | – | – | 441.4 | – | – | 461.7 | – | – |
| 3% C | 339.9 | 624.6 | 704.4 | 291.4 | 490.0 | 685.8 | 441.4 | 612.1 | 930.5 | 540.4 | 733.3 | 850.7 |
| 3% C+1% Z | 427.5 | 705.1 | 774.5 | 466.4 | 879.3 | 778.0 | 495.4 | 622.4 | 934.8 | 545.0 | 815.9 | 867.0 |
| 3% C+3% Z | 508.4 | 794.3 | 863.3 | 479.0 | 868.8 | 943.3 | 644.4 | 653.2 | 889.4 | 548.9 | 700.2 | 981.7 |
| 3% C+6% Z | 396.0 | 602.6 | 651.4 | 534.3 | 871.6 | 948.2 | 678.7 | 758.4 | 895.2 | 657.0 | 808.7 | 992.5 |
| 3% C+10% Z | 363.3 | 573.6 | 586.4 | 661.9 | 911.0 | 1199.6 | 671.3 | 764.7 | 845.7 | 670.0 | 884.8 | 921.1 |
| 3% C+15% Z | 344.2 | 483.5 | 459.5 | 629.6 | 805.1 | 1103.1 | 586.2 | 655.2 | 957.8 | 694.0 | 895.1 | 923.7 |
| 3% C+20% Z | 352.9 | 395.6 | 482.8 | 559.8 | 838.0 | 893.0 | 575.7 | 629.7 | 754.3 | 705.9 | 920.0 | 996.1 |
The observed increase in strength obtained as a result of increased curing time and additive percentage is remarkable, especially for sample 2, compared with the strength increase obtained in the absence of additives. The observed behavior of sample 2 can be seen as more significant than the stress-strain relationship (Figure 10). As a result of the plastic behavior in the absence of additives, which is not shown in Figure 10, a strength value of 192.8 kPa was reached at a 6.09% axial strain. When the additive content and curing time were increased, larger strength values were obtained for lower axial strain values. For sample 2, the greatest strength was achieved, especially at the additive level of 3% C+10% Z, for all curing periods. Increases in strength values for the other samples occurred at different additive percentages depending on the curing time (Table 5). At very high zeolite contents (20%), the specimens suffer from less effective cementation due to increased granular nature and hence reveal low initial strengths due to tensile cracking upon uniaxial loading.
Influence of additives on the stress-strain curves of soil sample 2.
5 Conclusion
In this study, the following conclusions were reached:
The clay soils used in the study exhibit both swell and dispersive properties. Especially on the basis of swell potential classification, low-plasticity clays, which are expected to exhibit a low or moderate swell potential, exhibit a high swell potential when they have high ESP and low SAR values. This result demonstrates that highly dispersive clay soils can also have a high swell potential.
When the test results were compared in terms of swell potential and dispersibility, an improvement was seen in both the dispersive and the swell properties of the samples at zeolite additive levels of 6–10%, with 3% cement content. The expected effect was not observed when the additive percentage was increased due to the presence of free zeolites, which are unable to react with adequate cement.
When the results of UCS tests carried out at different additive levels and with different curing times (uncured, 7 and 28 days) were analyzed, the strength was found to increase up to a specific additive level, but the expected increase in strength with increased additive levels was not observed. The curing time is another factor affecting strength. Specifically, the strength was found to increase with increased curing time. Whereas a rapid increase in strength was observed in low-plasticity samples 1 and 2 after a 7-day curing period, a significant strength increase in high-plasticity samples 3 and 4 was observed only after a 28-day curing period. The positive impact of curing time on strength was more significant for high-plasticity clay soils than for low-plasticity clay soils.
This study showed that a mixture of cement and zeolite additives not only improved the dispersive and swell properties of clay soil samples with four different plasticity characteristics, depending on their SAR and ESP values, but significant increases in strength values were also observed.
Our results demonstrate that problematic clay soil properties, such as swelling and dispersibility, can be reduced, thus increasing the soil strength.
The authors acknowledge Dr. Hasan Savas for his kind contribution to the experimental study of this investigation.
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