The influence of nanomaterials on collapsible soil treatment

Engineering Geology 205 (2016) 40–53

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Engineering Geology journal homepage: www.elsevier.com/locate/enggeo

The influence of nanomaterials on collapsible soil treatment Behnam Iranpour a,⁎, Abdolhosein haddad b a b

Geotechnical Engineering, Semnan University, Semnan, Iran Department of Civil Engineering, Semnan University, Semnan, Iran

a r t i c l e

i n f o

Article history: Received 21 May 2015 Received in revised form 16 February 2016 Accepted 26 February 2016 Available online 3 March 2016 Keywords: Collapsible soil Nanoclay Nanosilica Treatment Collapse potential

a b s t r a c t This paper presents an experimental study aiming to understand the impacts of nanomaterials on collapsible soil Behavior. Collapsible soils are among problematic soils from the perspective of geological and geotechnical engineering. These soils under constant stress present volume decrease related to the increase in moisture content. The reason is due to the weakened bond between particles under a constant stress. Collapsible soils exist in different parts of the world. In this study, samples used in experiments were collected by opening a trial pit from the sub-tropical areas of Iran. The soil samples were examined using standard geotechnical tests. The assessment of collapsibility potential of soil samples was carried out using the standard ASTM (2003). An appropriate sample with collapsible potential was employed to assess the effects of nanomaterials selected. Samples were treated with four types of nanomaterials (nanoclay, nanocopper, nanoalumina, and nanosilica) and combined under different percentages of the total dry weight of the soil. Soil tests were carried out in natural water content and density. The results showed that using various nanomaterials had different effects on the behavior of collapsible soils. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Some unsaturated soils are often relatively stiff and suffer from only small consolidation under normal foundation loads. However, these soils show volume shrinkage related to the increase in moisture content under constant stress without any change in the external forces. The strain induced by change moisture content is a typical behavior of a phenomenon called collapse (Dudley, 1970; Barden et al., 1973; Tadepalli et al., 1992; Rollins and Rogers, 1994). The main characteristics of collapsible soil are their porous and metastable structure and some moisture content lower than the necessary amount for its saturation (approximately 60–70% saturation). (Feda, 1966; Houston et al., 2001; Rao and Revanasiddappa, 2002; Pereira et al., 2005; Zeng and Meng, 2006; Zorlu and Kasapoglu, 2009). Collapse behavior has been observed in different parts of the world, especially in the tropical regions. Based on Clemence and Finbarr (1981); Rogers et al. (1994) and Gao (1996), collapsible soils distribution covers about 17% of the United States, 17% of Europe, 15% of Russia and Siberia, and large areas of China. Collapsible soils also exist in South America (i.e., Argentina and Uruguay) and southern Africa (Nouaouria et al., 2008). Cases of collapse have been documented extensively in many parts of the world and are typically associated with saturation by water, broken water pipes, other kinds of artificial flooding from the surface, or upward water saturation from perched ⁎ Corresponding author at: Division of Geotechnical Engineering, Department of Civil Engineering, Semnan University, Semnan, Iran. E-mail addresses: [email protected] (B. Iranpour), [email protected] (A. haddad).

http://dx.doi.org/10.1016/j.enggeo.2016.02.015 0013-7952/© 2016 Elsevier B.V. All rights reserved.

water (Lutenegger and Hallberg, 1988). Based on these studies, it seems that the collapsing phenomenon is a complex combination, which includes fabric and initial moisture content, initial dry density, and stress conditions. Among many collapse controlling factors, stress and hydraulic features are of considerable significance (Langroudi and Jefferson, 2013). According to Jennings and Knight (1975), the soil deposits that are most likely to collapse are: (a) loose fills, (b) altered wind-blown sands, (c) wind-blown silt (i.e., loess), and (d) decomposed granite and other acid igneous rocks. The collapsible soil, due to wetting, could result in settlements of 2–6% of their thickness (Beckwith and Hansen, 1989). Massive settlements such as the irrigation canal of 4.5 m have been reported in the West Central Part of the San Joaquin Valley in California (Bull, 1964). Over the last 20 years, especially in the recent decade, nanotechnology, as an interdisciplinary area, has witnessed much growth. While nanotechnology is a new trend, it is not a unique combination of chemistry, physics, biology, and engineering (Wilson et al., 2008). The field of nanotechnology is defined as the production and use of materials at the nanoscale, which is normally characterized as smaller than 100 nm in one dimension (Nel et al., 2006). Nanotechnology is reformation and processing of materials into nanoscale to develop materials with the better performance. Therefore, nanotechnology is a novel approach in all sciences. Such an approach can be used in geotechnical engineering in two ways: (1) studying the soil structure in nanometer scale to enhance our understanding of soil nature, as well as studying the performance of soils treatment with different nanostructures, and (2) conducting soil exploitation at the atomic or molecular scale. This is facilitated by the addition of nanomaterial as an external prop pant to soil (Majeed and Taha, 2013). Although

B. Iranpour, A. haddad / Engineering Geology 205 (2016) 40–53 Table 1 Physical characteristic of the soils.

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Table 3 Properties of alpha aluminum oxide (alumina, Al2O3), copper oxide (CuO) nanopowders, and silicon dioxide (SiO2) nanopowders.

Characteristics

Values and descriptions

Soil sample Specific gravity Natural moisture content (Ω) (%) Liquid limit (%) Plasticity index (%) Passing no. 200 sieve (%) Clay content (b 2 μm) (%) Unified soil classification system (USCS) Dry unit weight (γd)(kN/m3) Initial void ratio (e0) Chemical composition compound SiO2 (%) Al2O3 (%) Fe2O3 (%) MgO (%) CaO (%) SO3 (%) K2O Other

S1 2.593 4.4 28 7 80 28 CL-ML 13.7 0.85

S2 2.705 5 25 9 53 22 CL 15.2 0.74

S3 2.682 4 33 11 93 15 CL 13.2 0.93

S4 2.604 3 26 10 43 19 SC 15.6 0.63

S5 2.580 3.8 24 9 75 7 CL 13.8 0.78

63.87 27.81 5.63 0.51 0.03 0.05 2.10

57.93 16.06 4.86 3.50 9.80 0.03 1.32 6.50

47.78 15.39 6.44 5.64 13.42 0.02 1.61 9.70

61.90 28.01 5.48 1.48 1.23 1.90

55.46 13.21 9.45 1.06 2.56 0.03 1.34 16.80

limited research has been conducted to evaluate the benefits of adding nanomaterials to the soil, there are many deficiencies in empirical tests and operational data in this field. Yonekura and Miwa (1993) studied silica nanomaterial to increase sand compressive strength. Also, Noll et al. (1992) investigated the use of silica nanomaterial for soil stabilization and reduction in soil permeability. Gallagher and Lin (2005) studied silica nanomaterial for increasing soil's cohesion/adhesiveness and decreasing its permeability. It was found that the behavior of the sand was improved by nanomaterials analyzed in cyclic loading conditions. It is thought that the improvement mechanism of colloidal silica is bonding between the gel and the individual sand particles. Colloidal silica solutions gel can be formed by inter-particle siloxane bonds due to collisions between the nanoparticles in the dispersion. The gel encapsulates the individual sand particles. It is thought that this bonding and encapsulation maintains the soil structure during dynamic loading (Langroudi et al., 2014). As a result, it was indicated that cohesion/adhesiveness depended on the percentage of nanomaterial increase. Gallagher et al. (2007), in the United States, used nanomaterials practically in a place where soil was of sand type with high permeability, reporting 40% improvement in settlement after applying artificial earthquake and evaluating the yielded settlement. Zhang et al. (2004), on the other hand, indicated that the existence of nanostructure in soil caused an increase in Atterberg limits. The

Parameter

Value

Formula Particle density (g/cm3) Specific surface area (m2/g) Average particle size (nm) Solubility in water (%) Melting point (°C) Composite Al2O3 CuO SiO2 Color Crystal structure

Al2O3 3.6 ≥15 80 Insoluble 2072

CuO 6.4 ~20 40 Insoluble 1326

SiO2 2.4 170–200 20–30 Soluble 1600

99.9% N/A N/A White Rhombohedral

N/A 99.9% N/A Black N/A

N/A N/A ≥99.5% white Amorphous

main objective of this research was to determine the effect of the type of nanomaterial on soil collapse behavior by analyzing the results of experiments in different samples. For this aim, the undisturbed soil samples from a field with high collapse potential were selected according to the collapse potential test results. The collapse potential test on the remoulded samples was re-applied on those selected samples under initial moisture content and dry unit weight and with no added nanomaterials. Then the results were compared whit those of collapse potential test on remoulded soil samples combined with nanomaterials.

Table 2 Properties of the nanoclay. Parameter

Value

Physical properties Particle density (g/cm3) Specific surface area (m2/g) Particle size

2.35 750

Mechanical properties Hardness, shore D Tensile strength, ultimate Modulus of elasticity Flexural modulus Thermal properties Deflection temperature at 0.46 MPa (66 psi) Color

comments

b2.00 μm ≤6.00 μm ≤13.0 μm

10% 50% 90%

80 101 MPa 4.576 GPa 3.5 GPa

5% Cloisite-reinforced nylon 6 5% Cloisite-reinforced nylon 6 5% Cloisite-reinforced nylon 6 5% Cloisite-reinforced nylon 6

93.0 °C Off white

Fig. 1. Undisturbed soil sample removed from the sides of an examination pit.

Table 4 Classification of collapse potential (ASTM, 2003). Degree of collapse

Collapse potential (%)

None Slight Moderate Moderately severe Severe

0 0.1 to 2.0 2.1 to 6.0 6.1 to 10.0 N10

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Fig. 2. Results from tests on the specimens of S1–S5.

information provided by the company are shown in Table 2. The measured size was 5–15 nm in thickness and 20 nm to 10 μm in diameter.

2. Materials 2.1. Soil characteristics Soils in arid and semiarid areas mostly consist of aeolian sediments deposits. Clay and silt particle aggregation is common in such soils. These soils often present some collapsibility resulting from their metastable structure, which is guaranteed by cementations and matric suction since they are typically unsaturated soils (Benatti and Miguel, 2013). Soil samples were collected from arid and semi-arid regions of Iran, including southern and western city of Semnan and the southern region of Gorgan city. In order to determine the collapsibility potential of the soil under the consideration area, 5 soil samples were taken and prepared from different locations. At site, top soil was removed for about 0.5 m and then undisturbed soil samples were collected at 0.5 m depth, by opening 1 m deep trial pits. The test pits were dug to a depth of 1.5 m below the ground surface. It was critical not to use water or slurry mud to support the sides of the borehole during sampling. Collapsible soils in the extracted samples were carefully sealed and handled. Soil specimens were referred to as S1 to S5. Table 1 shows the results related to the physical properties testing of the undisturbed samples: dry unit weight (γd), initial void ratio (e0), and natural or field moisture content (Ω). Furthermore, Table 1 shows the liquid limits (LL) and plasticity index (PI) of the soil samples based on ASTM (1998).

2.2.2. Nanocopper, nanoalumina, and nanosilica The nanoalumina material applied in this study was alpha phase ultrafine alumina(α-Al2O3) powder with the purity of 99.99%, nanocopper oxide with the purity of 99.99%, and nanosilica with the purity of 99.5%, as supplied by US Research Nano-materials, Inc., Houston, USA. The general properties are shown in Table 3. 3. Experimental methods 3.1. Preparation of soil–nanomaterial mixtures In order to get the combination of soil–nanomaterials, 0.1%, 0.2%, 0.4%, and 0.6% of nanomaterials with respect to the dry weight of soil were used. Specific surface area of nanocopper and nanoaluminum was close, but as can be seen in Table 3, the density of nanocopper was more than the other nanomaterials used. Based on Tables 2 and 3, nanoclay and nanosilica density and specific surface area (SSA) were nearly the same. SSA is an important parameter. At nanoscale, there exists the higher ratio of surface to volume. Therefore, the interaction between particles and solution is very strong, such that very minute

2.2. Nanomaterials Four types of nanomaterials were applied in this study, i.e., nanoclay, nanoalumina, nanocopper, and nanosilica. The main properties of the nanomaterials are discussed below. 2.2.1. Nanoclay The nanoclay montmorillonite applied in this study was supplied by Southern Clay Products, Gonzales, TX, USA. The specifications and all the Table 5 Results from tests on the specimens of S1–S5.

Δe at vertical stress 200 kPa Collapse potential (%) Degree of collapse

S1

S2

S3

S4

S5

0.047 2.60 Moderate

0.011 0.73 Slight

0.100 5.18 Moderate

0.028 1.67 Slight

0.087 4.94 Moderate

Fig. 3. Microstructure observation of undisturbed soil sample S5 at the initial intact state.

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Fig. 4. Results from tests on the remoulded specimens of S3 and S5.

amounts of them may lead to considerable effects on the physicchemical behavior and engineering properties of soil. Due to the difference in the density and SSA of nanomaterials, to study the effect of these parameters in combination with soil, the same percentages for various nanomaterials were used. According to Taha and Taha (2012), in order to get homogeneous soil samples, dry materials have to be firstly mixed, and then water should be added to the mixture. Based on this method, mixing was accomplished in two phases. Initially, it was premixed or hand mixed, the quantity of soil was divided into five layers, and each layer was sprayed with the required amount of nanomaterial. Each layer was mixed alone, and then they were put in the pot and mixed by a horizontal cylindrical mixer for at least 3 h (Jones et al., 2007). All mixing was done manually, and care was exercised to prepare homogeneous mixtures at each stage. Furthermore, distilled water was used for preparing the samples. 3.2. Test procedures The existing criteria for estimating collapse potential do not completely capture the collapse behavior of the soils, and most of them are not a direct measurement of collapse potential. They may be used to gain an initial assessment of the degree of collapsibility of a soil but cannot be considered a substitute for laboratory determination of the collapse potential. However, double consolidation test is an effective method to evaluate collapse potential (El Howayek et al., 2011).This criterion has been defined based on the double consolidation test according to ASTM (2003), using unsaturated samples at their natural water content. A collapse study is usually carried out using this criterion, without considering the influence of matric suction on soil (Jennings and Knight, 1975; Medero et al., 2009; Zorlu and Kasapoglu, 2009). The specimen used in these tests was molded from the curling of a metal ring in undisturbed soil blocks removed from the sides of an examination pit shown in Fig. 1. To minimize the changes in the soil structure, the crimping was made. The specimen was placed in the loading device immediately after determining the initial wet mass and the height of the specimen following compaction or modification. Loads increments were applied each hour at natural water content. Load

increments were applied under 5 levels of graduated stress from 25 kPa to 200 kPa. Then the specimen was flooded with water 1 h after loading to the 200 kPa and recording the deformation. The soil specimen was inundated by distilled water for saturation and left for 24 h (ASTM, 2003). When the specimens were immerged, deformations were recorded within the defined time intervals according to ASTM (2011) consolidation test. Also, the time duration for additional load was a day (24 h), according to ASTM (2011) test. The collapse potential (Cp) was measured by the ratio of the change in the height of the soil specimen to the initial height of soil specimen. The laboratory collapse potential (Cp) values were calculated using the formulation proposed by Jennings and Knight (1975) as presented in Eq. (1): CP ¼ Δe=ð1 þ e0 Þ ¼ ðΔH=H0 Þ

where Δe is the change in void ratio resulting from wetting, e0 is the natural void ratio, ΔH is the change in specimen height resulting from wetting, and H0 is the initial height of the specimen. Cp is classified in Table 4. Collapse potential (Cp) is used to evaluate settlement that may occur in a soil layer at a particular site. Cp is estimated from Eq. (1) using a predetermined applied vertical stress and fluids applied to a soil specimen taken from the soil layer. Settlement of a soil layer for the applied vertical stress is determined by multiplying Cp by H/100, where H is the thickness of the soil layer (ASTM, 2003). First, the undisturbed soil samples resulting from the field were passed through the collapse potential test. Then those samples with a high amount of collapse potential were picked. The collapse potential

Table 6 Results from tests on the remoulded specimens of S3 and S5.

Δe at vertical stress 200 kPa Initial void ratio (e0) Collapse potential (%) Degree of collapse ðCp e0 Þ

S3

S5

0.084 0.915 4.35 Moderate 4.75

0.066 0.76 3.75 Moderate 4.93

ð1Þ

Fig. 5. Microstructure observation of remoulded soil sample S5.

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Fig. 6. Results from tests on the specimens of S3 mixed with nanoclay.

Fig. 7. Results from tests on the specimens of S5 mixed with nanoclay.

test on remoulded samples was re-applied on those selected samples under initial moisture content and dry unit weight and with no added nanomaterials. The collapse potential test was also conducted on the samples combined with nanomaterials to learn the effect of nanomaterials on the collapse potential. 4. Results and discussion In order to determine the collapsibility, soil specimens were tested through the criterion of ASTM (2003). Fig. 2 show results from test on soil samples, and the results obtained from specimens have been brought in Table 5. The presented results in Table 5 showed that specimens S1, S, and S4 had low collapsible potential, while specimens S3 and S5 had relatively high collapsible potential. As shown in Fig. 2, the strain rate of samples was not significantly changed at each load increment (up to 200 kPa). At the vertical stress of 200 kPa and the immersion time, samples showed changes in strain, as a result of collapsible potential. During the 200 kPa compression and the saturation of samples, severe strain changes occurred. These strain changes were due to the loss of the resistance factor binding the soil. Therefore, samples S3 and S5 had collapse potential, or, to put it another way, had a collapsible structure. The scanning electron microscopy (SEM) observation of sample S5, as presented in Fig. 3, showed that the largest pores of collapsible

soil were dry inter-grains pores with an average entrance diameter of 8 μm, with their location being between clean silt grains of 10 μm average diameter. The fine-grained fraction of the soil existed as the bonding material for the larger-grained particles. Observation of the SEM images suggested that the resistance of the microstructure could not be homogeneous based on the irregular position of existing clay aggregations between a metastable arrangement of silt grains. Clearly, collapse under wetting should occur by the densification of the areas where grains are clean with large pores around them. The zones in which the porosity is filled by clay aggregation should be more resistant and locally less sensitive to collapse. The collapse potential test was also carried out on the remoulded samples of S3 and S5 to better understand the effect of the type of

Table 7 Results from tests on soil samples S3 and S5 with nanoclay. Soil sample S5

Soil sample S3 Percent of nanoclay (%)

Δe at 200 kPa

Initial void ratio (e0)

Δe at 200 kPa

Initial void ratio (e0)

0 0.1 0.2 0.4 0.6

0.084 0.003 0.008 0.074 0.003

0.915 0.914 0.913 0.914 0.914

0.066 0.004 0.026 0.048 0.040

0.760 0.760 0.763 0.763 0.764

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Fig. 8. The effect of nanoclay on the collapse potential (Cp).

24%, respectively, in comparison to the undisturbed sample. The scanning electron microscopy (SEM) observation of sample S5, as presented in Fig. 5, showed that the largest pores of collapsible soil were dry intergrains pores, with their location being between clean silt grains. The fine-grained fraction of the soil existed as the bonding material for the larger-grained particles. Clearly, collapse under wetting should occur by the densification of the areas where grains are clean with large pores around them. 4.1. The effect of nanoclay on the collapsible soil

Fig. 9. Sketch of collapsible soil structure (Casagrande, 1932).

nanomaterials on soil collapse behavior by analyzing the results of experiments. The remoulding process was passed through the ASTM (2003) and ASTM (2011) standards in natural water content and density. The results of the experiment conducted to determine the collapse potential for remoulded samples without any nanomaterial have been brought on Fig. 4 and Table 6. As can be seen, the collapse potential reduction that occurred for both samples could be anticipated due to the high number of factors affecting collapse potential like soil structure. The collapse potential of samples S3 and S5 was decreased by 17% and

The results of the experiment conducted to determine the collapse potential for samples combined with nanoclay have been brought in Figs. 6 and 7 and Table 7. The amount of strain in sample S3 showed a considerable reduction with the addition of nanoclay, especially at 0.1%. Strain was increased with the addition of nanoclay in different compressions, before and after saturation, getting closer to the initial strain to a large extent. This increased strain rate with 0.4% of nanoclay become greater than the initial amount (Fig. 6). With the addition of nanoclay in 0.1% and 0.2%, the amount of strain for the sample S5 showed a considerable reduction before and after saturation. The amount of strain was increased with the increase in the nanoclay percentage in different compressions (Fig. 7). As can be seen in Fig. 8, adding the percentage of nanoclay reduced the collapse potential of both selected samples drastically. Collapse potential, due to the addition

Fig. 10. Located nanomaterials on the bonding material in the soil sample (S3) mixed with 0.6% nanoclay.

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Fig. 11. Results from tests on the specimens of S3 mixed with nanoalumina.

Fig. 12. Results from tests on the specimens of S5 mixed with nanoalumina.

Fig. 13. The effect of nanoalumina on the collapse potential (Cp).

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Fig. 14. Results from tests on the specimens of S3 mixed with nanocopper.

Table 8 Results from tests on soil samples S3 and S5 with nanocopper. Soil sample S5

Soil sample S3 Nanocopper (%)

Δe at 200 kPa

Initial void ratio (e0)

Δe at 200 kPa

Initial void ratio (e0)

0 0.1 0.2 0.4 0.6

0.084 0.040 0.049 0.058 0.086

0.915 0.913 0.912 0.912 0.913

0.066 0.027 0.038 0.046 0.056

0.760 0.763 0.764 0.764 0.764

Table 9 Results from tests on soil samples S3 and S5 with nanoalumina. Soil sample S5

Soil sample S3 Nanoalumina (%)

Δe at 200 kPa

Initial void ratio (e0)

Δe at 200 kPa

Initial void ratio (e0)

0 0.1 0.2 0.4 0.6

0.084 0.038 0.045 0.049 0.081

0.915 0.914 0.912 0.914 0.913

0.066 0.026 0.033 0.034 0.073

0.760 0.762 0.761 0.763 0.763

percentage of nanoclay in sample S3, showed a mild increase, up to 0.2%. Following this, a considerable increase in the collapse potential was observed, reaching its maximum with 0.4%. The increase in the percentage of nanoclay in sample S5 also led to the increase of the collapse potential. The maximum collapse potential occurred in this sample, by 0.4%. In both samples, with increasing the percentage of nanoclay to more than 0.4%, the collapse potential was decreased. According to the initial values of collapse potential for the samples, as brought in Table 6, it could be seen that the biggest reduction occurred for both samples with 0.1% of nanoclay. The collapse potential of sample S3 was around 96% and 93% for sample S5, thereby showing reduction, in comparison to the initial sample. Given the collapsible soil structure shown in Fig. 9, the fine-grained fraction of the soil existed as the bonding material for the larger-grained particles, and these bonds were located in the small gaps between the adjacent grains under enough total stress (Dudley, 1970; Barden et al., 1973; Mitchell, 1993). According to the presented structure, the collapsible soil could be achieved by strengthening the bonding material to reduce the collapse potential (Cp). Considering the existence of nanopores in the bonding material, nanoparticles embedded in these spaces could cause higher specific strength bonding material layer, resulting in a stable soil structure. Increasing the strength of the soil structure reduced the collapse potential caused. Drastic reduction of collapse potential in 0.1% nanoclay could be due to the nanomaterial in the bonding material. Fig. 10 shows nanoparticles in this area, resulting in soil structure strengthening. Based on Taha (2009), with

Fig. 15. Results from tests on the specimens of S5 mixed with nanocopper.

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Fig. 16. The effect of nanocopper on the collapse potential (Cp).

Fig. 17. Distribution of nanomaterial particles in soil sample (S3) mixed with 0.4% nanoalumina and EDS spectrum obtained from the surface.

increasing the percentage of nanoclay, the amount of water surrounding the particles was increased due to the high SSA of nanoclay. This increase in the amount of water in the bonding material led to reducing

the strength of this layer. The decay process of bonding material and the decrease in the stability of soil structure increased the collapse potential.

Fig. 18. Distribution of nanomaterial particles in soil sample (S3) mixed with 0.6% nanoalumina and EDS spectrum obtained from the surface.

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Fig. 19. Distribution of nanomaterial particles in soil sample (S5) mixed with 0.2% nanocopper and EDS spectrum obtained from the surface.

Fig. 20. Distribution of nanomaterial particles in soil sample (S5) mixed with 0.6% nanocopper and EDS spectrum obtained from the surface.

4.2. The effect of nanoalumina and nanocopper on the collapsible soil The results of experiment conducted to determine collapse potential on the sample combined with nanoalumina have been brought in Figs. 11 and 12. The amount of strain in sample S5 showed reduction with the addition of nanoalumina, especially at 0.1% and 0.2%. The ultimate strain (at 800 kPa compression) was 10.42% with 0.1% nanoalumina and 11.39% with 0.2%, showing the reduction of 30.15% and 29.16%, respectively, relative to the initial sample. Strain was increased with the addition percentage of nanoalumina and at 0.4%, in different compressions, was enhanced before and after saturation, getting

very close to the initial strain. When nanoalumina addition was 0.6%, the amount of strain was increased, more than the initial strain to some extent (Fig. 12). A similar trend was observed in sample S3 such that at the beginning, with the percentages of 0.1% and 0.2%, the amount of strain was less than that of the initial strain, thereby showing reduction. Further addition of nanoalumina, however, led to the increase of strain. With the percentage of 0.4%, strain showed increase in different compressions before and after saturation, more than the initial sample. The ultimate strain (800 kPa compression) of the sample with 0.6% nanoalumina was increased to 48.48% (Fig. 11).

Fig. 21. Results from tests on the specimens of S3 mixed with nanosilica.

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Fig. 22. Results from tests on the specimens of S5 mixed with nanosilica.

As can be seen in Fig. 13, in both samples, adding the percentage of nanoalumina first decreased the collapse potential, but later, with the addition of more nanoalumina, this parameter was increased. The collapse potential, with the percentage of 0.1%, was decreased by 2.17% for sample S3 and 1.92% for sample S5. In sample S3, the amount of strain in different compressions, before and after compression, was less than that of the initial sample. The ultimate strain (800 kPa) of the sample, with the percentages of 0.1%, 0.2%, and 0.4% nanocopper, was 31.17%, 10.88%, and 1.04%, respectively, relative to the initial sample (Fig. 15). Sample S5, at first, with the percentage of 0.1%, showed an amount of strain less than that of the initial sample, thereby showing decrease. The increase in the percentage of nanocopper made the amount of strain in different compressions, before and after saturation, more or close to that of the initial sample (Fig. 14). As can be seen in Fig. 16, in both samples, the addition of nanocopper at first reduced the collapse potential, but with the further increase in the percentage of nanocopper, the collapse potential was increased, reaching to 0.6% at its maximum. The results obtained from specimens have been brought in Tables 8 and 9. The collapse potential, with the percentage of 0.1% nanocopper, was 2.29% for sample S3 and 1.54% for sample S5, thereby showing a reduction of 51.70% and 58.91%, respectively, relative to the collapse potential of the initial samples.

The initial decrease in collapse potential induced the increase in soil density as a result of the additional percentage of nanocopper and alumina (Taha and Taha, 2012). Density growth can have a significant effect on Cp by reducing it (Huat et al., 2008). Adding the percentage of nanomaterial to more than the optimum limit may possibly result from agglomeration in nanomaterial particles, which, in turn, leads to the increase in the void ratio and then reduction in density. Based on Ferkel and Hellmig (1999), the agglomeration of nanoscaled powders increases the amount of space between particles and, consequently, decreases the density of materials. However, addition in the content of nanoalumina and nanocopper indicated the reduction of density. This was because when particles were agglomerated, the void ratio was normally increased and the density was decreased. This, in turn, caused an increase in the collapse potential (Figs. 17, 18, 19, 20). The increase in volume ratio due to the addition percentage of nanomaterial resulted in the growth of Cp. 4.3. The effect of nanosilica on the collapsible soil In sample S3, with the addition of nanosilica, strain was decreased. However, strain was increased with the further addition of nanosilica in different compressions, before and after saturation, getting close to the initial strain considerably. Thus, with the percentage of 0.6%

Fig. 23. The effect of nanosilica on the collapse potential (Cp).

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Fig. 24. Located nanomaterial on bonding material in the soil sample (S5) mixed with 0.6% nanosilica and EDS spectrum obtained from the surface.

Fig. 25. Comparison of the effect of nanomaterials on the collapse potential for soil sample S3.

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Fig. 26. Comparison of the effect of nanomaterials on the collapse potential for soil sample S5.

nanosilica, the amount of strain (800 kPa) was increased to 25.79%, showing growth relative to the strain of the initial sample (800 kPa) (Fig. 21). Also, the amount of strain for sample S5 was decreased with the addition percentage of nanosilica with the percentage of 0.1% and 0.2%, before and after saturation. The amount of strain in different compressions was increased with the increase in the percentage of nanosilica (Fig. 22). As shown in Fig. 23, by adding the percentage of nanosilica, the collapse potential of both samples was decreased. However, in the case of sample S3, it showed a mild increase, up to 0.2%. Following this increase in the collapse potential, it was observed that the maximum increase occurred with the percentage of 0.4%. Increasing the percentage of nanosilica in sample S5 also led to the increase of the collapse potential. The maximum collapse potential in this sample occurred with the percentage of 0.4%. In both samples, with the increase in the percentage of nanosilica to more than 0.4%, the collapse potential was decreased. According to the initial values of the collapse potential for the samples brought in the above Table 6, it could be seen that the biggest reduction in collapse potential for both samples occurred with 0.1% nanosilica. The collapse potential of sample S3 and S5 showed a reduction of 55.34% and 63.64% in comparison to the initial sample. The results obtained from specimens have been brought in Table 10. The initial decrease in the amount of collapse potential was as result of the existing nanosilica in bonding material (Fig. 24). SSA nanosilica was about 3 times smaller than nanoclay based on Tables 2 and 3. This difference caused less increase in the amount of water surrounding particles as compared with soil–nanoclay combined. Thus, with increasing nanosilica Cp, there was less growth versus nanoclay. According to Figs. 25 and 26, it can be seen that nanoparticles had a better effect on the soil collapse behavior in low percentages. Increasing nanoparticles to more than the optimum limit can have a negative effect on this behavior. As in nanoalumina, the lowest collapse was observed for both samples with 0.1% nanoclay.

5. Conclusions Soil collapse highly depends on the structure of soil; it is necessary to consider factors contributing to the strength in order to improve it. Enhancing the resistance of clay and silt bridges, as well as cementation factors in the structure of collapsible soils, can reduce the danger likelihood of collapse. The combination of soil and nanomaterials is very sensitive and the amount and type of nanomaterials added to the soil could have both positive and negative impact on desired attributes and using an appropriate percentage of nanomaterials would result in the improvement of soil specifications. Dry unit weight and the water content are among the factors influencing soil collapse potential. According to the analysis of samples S1, S3, and S5, which had relatively high collapse potential, it was found that they all also had water content of less than 5% and dry unit weight of 14 kN per cube meter. The important parameter in regard to nanomaterial is the specific surface. As can be seen, the nanoclay had a larger specific surface in comparison to the nanosilica, hence showing a more considerable reduction of collapse potential with its addition to the soil. The effect of this parameter is also noteworthy in the case of nanocopper and nanoalumina; however, this

Table 10 Results from tests on soil samples S3 and S5 with nanosilica. Soil sample S3

Soil sample S5

Percent of nanosilica

Δe at 200

Δe at 200

kPa

Initial void ratio (e0)

kPa

Initial void ratio (e0)

0 0.1 0.2 0.4 0.6

0.084 0.037 0.047 0.055 0.053

0.915 0.913 0.914 0.913 0.913

0.066 0.024 0.025 0.036 0.032

0.760 0.763 0.764 0.762 0.764

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