The use of stabilization techniques has increased significantly in recent decades to adopt cost-effective solutions, to achieve reductions in quantities of material used and etc. Soil stabilization by adding materials such as cement, lime, bitumen etc. is one of effective methods for improving the geotechnical properties of soils. Nanoparticles are one of the newest additives and many studies about using Nanoparticles in soil improvement have been done. The use of Nanotechnology in some talented fields such as soil improvement, seepage and grouting will offer great advantages in geotechnics. This paper review the major Nano additives used in stabilization of soil and their influence on the different properties of soil.
Key Words: Nano Additives, Soil Stabilization, Nano-Clay, Nano-Silica
Soil is one of nature’s most abundant construction materials. Almost all construction is built with or upon soil. When unsuitable construction conditions are encountered, a contractor has four options; finding a new construction site, redesign the structure, remove the poor soil and replace it with suitable soil or improving the engineering properties of the site soils. Improving an on-site (in situ) soil’s engineering properties is referred to as soil stabilization. Soil stabilization is seen as a means of enhancing aspects of engineering and other elements, including the conductivity of hydraulics, compressibility, strength, and the density. There are two primary methods of soil stabilization used today: Mechanical and Chemical or Additives. A proper understanding of the geotechnical properties of soils is a pre-requisite for its use in engineering construction works. Various stabilization techniques are available to improve soil properties like addition of materials like cement, lime, bitumen etc. Nanoparticles are one of the newest additives and researches are going on to find its effect in properties of soils. Nanotechnology is a rapidly emerging technology with vast potential to create new materials with unique properties and to produce new and improved products for numerous applications. In recent years Nanotechnology is also gaining popularity in the field of Civil and Geotechnical Engineering. The applications of Nanotechnology in geotechnical engineering in dealing with soil can be in two ways: 1) in studying soil structure at the Nanoscale and 2) in soil manipulation at the atomic or molecular level through the addition of Nanoparticles as an external factor to soil.
Many of the soil and rock minerals are Nanomaterial and their chemical reactions occur in Nano scale. As a result of this fact, there is a great potential of Nanotechnology’s application in soil mechanics. Mixture of soil with some special additive could improve the soil strength parameters, and this procedure has been performed in the past for stabilization and improvement of weak soils. The main strategy of Nanotechnology in geotechnical engineering is the improvement of soil parameters with the application of Nano materials. The presence of Nano material in the soil could influence significantly the physical and chemical behavior of soil due to a very high specific surface area of Nano materials, surface charges and their morphologic properties.
NANO ADDTIVIES IN STABILIZATION OF SOIL
Nano additives are been used directly with the soil or being an additive on the stabilization material, like cement. Cement is an important resource for stabilizing, it is been used widely in many construction applications. Even with using additives whether Nano or else, cement is acting like a binder and strength material. Some researchers studied the effect of adding Nano additives to the soil in presence of optimum cement content and study the effect of the added Nano additives; others used the Nano additives directly with the cement to enhance the behavior of cement on its own. Nano-Silica and Nano-Clay are the major additives that is been studied in soil stabilization, few researches studied other additives, like Nano-CuO, Carbon-Nanotube, Nano-MgO, or Nano-alumina.
Silica is known as pozzolanic material and has been used to stabilize and improve soil strength (Diaz-Rodriguez, 2004; Patricia et al., 2007). In recent years, Nanoparticles have attracted considerable scientific interest for many civil engineering applications. Nanoparticles of SiO2 have been used as additives with soil or cement-soil mixture.
Sayed Hessam Bahmani (2014) performed an experimental study to determine the effect of SiO2 Nanoparticles on consistency, compaction, hydraulic conductivity, and compressive strength of cement-treated residual soil. There were no apparent changes in the liquid limit (LL) of the Nano- SiO2-treated soil at any cement level. However, the plastic limit (PL) of the specimens initially increased at Nano- SiO2 content of 0.2% but then decreased at higher Nano- SiO2 contents. In general, the PL index of samples with 15 nm silica particles were lower than that of samples with 80 nm silica particles at cement dosages of 4% and 6%. As the cement percentage increased to 8%, the PL of samples with 80 nm silica particles decreased to a level below that of 15 nm silica particles. Consequently, lower loads of Nano-silica of up to 0.2% resulted in the lowest PI for all samples with the cement levels of 4 and 6%. The Nano-silica dosage associated to the lowest PI increased to 0.4% at 8% cement level. The addition of Nanoparticles resulted in an increase in the optimum moisture content (OMC) and a slight decrease in the maximum dry density (MDD) of the specimens. A greater increase in the MDD was observed with the addition of Nanoparticles with an average diameter of 80 nm than that of 15 nm at all cement levels. It was also observed that an increase in the Nanomaterial content resulted in a decrease in the MDD but an increase in the OMC. Addition of Nano-silica increased the compressive strength of samples dramatically. However, lower loads of SiO2 Nano particle resulted in higher strengths. The maximum strength was 673 kPa, 1020 kPa, 1611 kPa for 4%, 6%, 8% cement, respectively when silica particles of 15 nm were used. The compressive strength of the samples without Nano-silica was 424 kPa, 450 kPa, and 515 kPa for 4%, 6%, 8% cement, respectively. The addition of higher percentage of the Nanoparticles led to a lower strength gain.
The hydraulic conductivity increased with an increase in content of both sizes of Nanoparticles (15 and 80 nm), however, it was seen that the least conductivity was observed with addition of 0.4% Nano-silica. The effect of 15 nm Nano-silica particles on decreasing the hydraulic conductivity was greater compared with the effect of the 80 nm Nanoparticles. Inclusion of the Nano-silica to the soil reduced the intensity of the peaks related to the calcium hydroxide. Moreover, the FTIR spectrum of the treated soil showed a broad group of Si–O–Si band in the region of 600–1500 cm. The differences in the transmittance percentages and positions of the peaks in the untreated soil, the cement-treated soil and the soil–cement mixture with Nano-silica may reveal that the nature and amount of the C–S–H phase has changed and may confirm the additional formation of C–S–H gel. SEM images also showed formation of a very dense matrix in which pores were filled to a great extent.
Seyedi et al., (2013) studied the use of Nano-Silica for stabilizing the weak soil and improving the shear strength parameters. Where adding Nano-Silica in the mixture of soil-lime (5% lime) caused decreasing the maximum dry density of the mixture and increasing the optimum moisture content. The Unconfined compressive strength (UCS) of the soil-lime mixture (5% lime) with adding 3% of Nano-Silica, increased up to 3 times (300%), after 28 days curing. The same comparison made for UCS of soil and soil-lime-Nano-Silica mixture, and results showed an increase of 7.5 times (750%) in UCS of soil for 3% Nano-Silica after 28 days curing.
Clays are layered minerals with space in between the layers where they can adsorb positive and negative ions and water molecules. Clays undergo exchange interactions of adsorbed ions with the outside too. Nanoclays are Nanoparticles of layered mineral silicates. Depending on chemical composition and Nanoparticle morphology, Nanoclays are organized into several classes such as montmorillonite, bentonite, kaolinite, hectorite, and halloysite.
Zaid Hameed Majeed, (2014) studied the effect of addition of three Nanomaterials (i.e., Nano CuO, Nano MgO and Nano-clay) on the geotechnical properties of soft soil. The liquid limit, plastic limit, linear shrinkage, compaction parameters and shear strength of the soil were determined. Addition of each of the Nanomaterials decreased the liquid limit, plastic limit, plasticity index and linear shrinkage of the soil. The dry density increased with increasing Nanomaterial percentage. On the other hand, the optimum moisture contents of the soil-Nanomaterials mixtures decreased with increase dosages of Nanomaterial. In addition, when the Nanomaterial percentage exceeded the optimum contents, there was evidence of particles agglomeration which in turn affected the mechanical properties of soils negatively. As well, the compressive strength of the soil increased with Nanomaterial addition. Moreover, the addition of a small amount of Nanomaterials, i.e., not more than 1%, leads to enhancement of soil geotechnical properties by increasing the compressive strength for all tested soils.
Norazlan Khalid, (2014) conducted laboratories studies to determine the properties of Nano-kaolin mixed with kaolin. The raw of white kaolin sample was used in this study. Meanwhile, the Nano-kaolin was produced using pulverization process through high energy milling process from raw kaolin into Nano size. Only 3% Nano-kaolin was used in this study to mix with kaolin due to the limited amount produced from the milling process. The main objectives of this study to determine the geotechnical properties of 3% Nano-kaolin mixed with kaolin. The geotechnical properties regards to its compaction characteristics, liquid limit, plastic limit, plasticity index, pH value, and shrinkage value, hydraulic conductivity value, microstructure and Nanostructure of kaolin and Nano-kaolin. Based on the experimental study, the presence of Nano-kaolin were improved the kaolin properties, event at a small quantity or percentage of Nano-kaolin were used. The presence of Nano-kaolin showed the dramatically influence to the engineering and basic properties of kaolin. It shows that the values of liquid limit, plastic limit and maximum dry density slightly higher after addition 3% Nano-kaolin. Meanwhile the plasticity value, optimum moisture content, and hydraulic conductivity value showed decreasing after addition of 3% Nano-kaolin. The application on small amount of Nano-kaolin was significant improved the geotechnical properties of kaolin.
S.V. Neethu, (2013) studied the engineering behavior of Nano-Clay stabilized soil and the effect of addition of Nano-clay on lateritic soil and kaolinite clay. To study the effect of addition of Nano-clay on lateritic soil and kaolinite clay a series of tests were conducted with varying concentrations of Nano-clay (0, 0.25, 0.5, 1, 1.5 and 2%). Optimum concentration of Nano-clay for both the soils was determined from the test results. For lateritic soil, by the addition of Nano-clay to lateritic soil, liquid limits and plastic limits got increased. From compaction studies optimum moisture content increased and maximum dry density decreased by the addition of various percentages of Nano-clay to the soil. Coefficient of permeability values decreased up to 99.8% at 1% optimum concentration of Nano-clay. Coefficient of permeability attained a value of 1x10-8 cm/sec and satisfied the range of 1x10-7 cm/sec which ensures that the material can be used as a landfill liner. Unconfined compressive strength also got increased by 104% at optimum (1%) concentration of Nano-clay.
For kaolinite clay, by the addition of Nano-clay to lateritic soil liquid limits and plastic limits are found to be increased. From compaction studies optimum moisture content increased as the Nano-clay content increases and maximum dry density increased at 0.25% of Nano-clay and then it decreases. Coefficient of permeability values decreased up to 96.53% at 1% optimum concentration of Nano-clay. Coefficient of permeability attained a value of 9.79x10-7 cm/sec and satisfied the range of 1x10-7 cm/sec which ensures that the material can be used as a landfill liner. Unconfined compressive strength also got increased by 87.5% at optimum (1%) concentration of Nano-clay.
Meisam Bahari, (2013) evaluated the physical and geotechnical properties improvement of silt stabilized with Nano-clay through performing laboratory investigations as Atterberg Limits, Compatibility, adhesion and internal friction angle for two types of silt (ML and MH). Results shown that Nano-clay because of absorbing water has a considerable influence on increasing the limits of liquid and plasticity, and increase in adhesion and internal friction angle. This increase attributes to coherency of Nano-clay when surrounded by water. This leads to growth of interlock forces between Nano-particles in the vicinity of moisture which can cause soil stabilization by fastening the particles together and filling pores.
M. Arabania, (2012) evaluated the laboratory results of unconfined compressive strength (UCS), indirect tensile strengths and California Bearing Ratio (CBR) of sand cement mixtures containing different amount of Nano-clay. The results indicated that the reaction of alumino-silicate elements in Nano-clay with calcium hydroxide in cement leads to the increasing in bond strength and consequently resulting in higher tensile strength of hardened cement paste. By adding Nano-clay, the structure of soil cement mixture has become denser and more uniform and even very small voids have been removed.
The addition of Nano-materials more than the optimum value causes agglomeration of particles that produce negative side effects on the mechanical properties of the soil. The additive of Nano-materials does not reduce the hydraulic conductivity of soils.
This paper provides a review of the Nano additives used in stabilization of soil, with different soil types and amounts, and their effects and results. Nano-silica and Nano-clay are the major used additives in the researches. Different nanostructures exhibit different properties. Due to their smaller dimensions, nanoparticles possess a very high specific surface and react more actively with other particles in the soil matrix. The existence of even a minute amount of these nanoparticles can result in extraordinary effects on the engineering properties soil. This review showed that nanoparticles influence the strength, permeability, compressibility, density, indices, and resistance properties of soil. That was according to laboratory studies for the effect of using it as a mixture with different percentages with and without cement, and this can be implemented in the field in many ways, on its own as a mixture and mixing it to the soil through mixing machines, or it can applied with other soil stabilization methods or reinforcement system, such as road sub-base, grouting,… etc.
Application of technology of strengthening of soil allows to receive the road of the highest quality with service life of 10-15 years, having reduced terms of construction in 3-5 times and having provided to 50% of economy of the budget.