Nanotechnology has been applied to soil improvement techniques to offer an eco-friendly and cost-effective alternative to traditional techniques.
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The increase in urbanization and industrialization has challenged traditional soil improvement methods. Conventionally, chemical grouts (e.g., epoxy, acrylate, and sodium silicate) and cement are used to enhance soil strength. However, these techniques are extremely difficult to implement widely, can lead to intense disturbances, and create environmental pollution.
Nanomaterials for Soil Improvement
Nanomaterials are described as materials whose size does not exceed 100 nm and possess many biological, mechanical, optical, and electrical properties. In 2006, the US National Research Council (NRC) pointed out the introduction of nanotechnology in geotechnical materials with the creation of clay-sized particles of 0.002 mm. Over the years, nanotechnology applications for soil improvement techniques have improved.
Four common nanomaterials that are used to improve soil are carbon nanotube (CNTs), colloidal silica, laponite, and bentonite. Other nanomaterials used to improve soil properties are nanoscale zerovalent iron (nZVI), fullerenes, zinc oxide (ZnO), bimetallic nanoparticles, titanium dioxide (TiO2), and multiwalled carbon nanotubes (MWCNTs). Some of the common nanomaterials used in soil improvement techniques are discussed below:
CNTs are derived from graphene, a super thin sheet of carbon atoms in a hexagonal arrangement. These sheets are rolled into a tube to form CNTs and create a material that is stronger than steel but much lighter in weight.
Since CNTs have a high aspect ratio and elastic properties, it has a high potential to act as a filler within the cement grains to make the composite denser, tougher, and stronger.
Recently, the soil improvement sector has used 0.05–3 % CNTs by weight of the soil to improve the compressive strength of clayey sand soil. This shows that a very small amount of CNTs can significantly improve the compressive strength of the soil.
Colloidal silica is an aqueous dispersion of microscopic silica ranging between 7 and 22 nm in size. This nanomaterial is non-toxic and chemically inert, with excellent durability. Several studies have shown that the use of colloidal silica effectively improves the compressive strength of sand. A study has shown that sands stabilized by the addition of 20% colloidal silica, indicating its potential applications to similarly structured environments like soil.
Nanobentonite is a processed clay that is formed by the weathering of volcanic ash. Compared to natural bentonite, nanobentonite has better moisture-absorbing and swelling properties. In an aqueous medium, nanobentonite disperses and becomes gelatinous, possessing high viscosity, lubricity, and thixotropy. Owing to these properties, nanobentonite is used as a drilling additive to improve drilling, which in turn reduces fluid loss to the rock formation.
Laponite is composed of synthetic sheets of silicate nanoparticles, arranged in layers. This nanomaterial is 10 times smaller in size than bentonite. The structure of laponite is similar to that of a natural montmorillonite structure. In an aqueous medium, laponite disperses into a colorless suspension that has good rheological properties.
Underlying Mechanism of Nanomaterials for Improving Soil Strength
Soil is a three-phase medium, which is composed of soil particles, water, and air between the particles. The pore water pressure plays an important role in the extent of soil strength. Nanomaterials improve soil strength by modifying the pore fluid and enhancing the bond strength within the particles. Owing to their small size, nanoparticles can easily disperse into the pore space between the soil particles, particularly into fine soil particles that are not under high pressure.
When nanomaterials are added to the soil, their microstructure influences the soil strength. As discussed above, the two-dimensional structure of nanoparticles (e.g., CNT) differentially affects the soil, compared to three-dimensional nanoparticles (e.g., colloidal silica). Since some nanoparticles in soil interact in nanoparticle-water suspension, the rheological suspension properties influence their ability to improve soil strength.
The decrease in the particle size results in an increase in the nanoparticle’s specific surface area. This implies an increased atomic surface, which leads to a larger ion exchange capacity and elevated interaction with other particles. The presence of nanoparticles enhances the accumulation of pore water. Therefore, soil containing nanoparticles has higher plastic and liquid limits. This shows that quantum mechanics models are more relevant than classical mechanics to describe the motion state and energy of particles at the nanoscale level.
Advantages of Using Nanomaterials over Traditional Materials for Soil Improvement Techniques
As mentioned above, adding nanomaterials, such as CNTs and colloidal silica, significantly enhances soil strength, particularly its liquefaction resistance and compressive strength. Compared to traditional materials, such as grout, using nanomaterials is more environmentally friendly and cost-effective.
Traditional soil improvement techniques, such as mechanical reinforcements, are associated with high costs and environmental disturbances. Chemical grouting is another common technique to improve soil strength that involves the addition of engineered slurries on the targeted site. Subsequently, it is pressed by a pressure transmission system to strengthen the weak foundation. Common grouting materials used are cement and solution, such as epoxy, sodium silicate, and acrylate.
The addition of nanomaterials as stabilizers can significantly reduce environmental disturbance as it does not require high-pressure infusion. Importantly, this technique is effective in both bare lands and developed areas with buildings. Since the common nanomaterials used to improve soil are inert and non-toxic, it does not harm the soil and groundwater aquifers. Furthermore, since nanomaterials are required in very small quantities for effective soil strengthening, this approach is considered cost-effective.
References and Future Reading
Kannan, G. & Sujatha, E. R. (2022) A review on the Choice of Nano-Silica as Soil Stabilizer. Silicon, 14, pp. 6477–6492. doi.org/10.1007/s12633-021-01455-z
Mirzababaei, M., et al. (2021) Effect of Nano-Additives on the Strength and Durability Characteristics of Marl. Minerals, 11(10), p. 1119. doi.org/10.3390/min11101119
Mahdia, O. B., et al. (2021) Application of Nanotechnology in Geotechnical Engineering: A Review. Saudi Journal of Engineering and Technology. 6(6), pp. 143-151. doi.org/10.36348/sjet.2021.v06i06.005
Correia, A. A. S. & Rasteiro, M. G. (2016). Nanotechnology Applied to Chemical Soil Stabilization. Procedia Engineering, 143, pp.1252-1259. doi.org/10.1016/j.proeng.2016.06.113
Huang, Y. & Wang, L. (2016). Experimental studies on nanomaterials for soil improvement: a review. Environmental Earth Sciences, 75(497). doi.org/10.1007/s12665-015-5118-8
Alireza, S. G. S., et al. (2013) Application of Nanomaterial to Stabilize a Weak Soil. International Conference on Case Histories in Geotechnical Engineering. [online] Available at: https://scholarsmine.mst.edu/icchge/7icchge/session_06/5