Geotechnical and environmental aspects in soil improvement using plastic waste materials

In recent years, the price of soil stabilizing additives (hydraulic binders) has increased significantly. This problem has motivated researchers to search for new methods of soil improvement. One of the newly discovered methods is the improvement of soil with plastic waste materials, such as polyethylene terephthalate (PET). This paper studies the behaviour of clayey samples improved with various percentages of plastic waste materials under varying water content and temperature conditions, with the aim to check the behaviour of plastic waste materials − polyethylene terephthalate (PET) − in the analysed samples and the structure of the samples at the end of the tests. Environmental aspects and the variation of soil’s pH are also discussed. Because plastic waste pollution has reached an alarming level in recent years, the subject of this study is very debated and of great interest.

INTRODUCTION

The integration of sustainable materials in geotechnical applications is a subject of growing interest due to the increasing frequency of challenging foundation soils and the impact of plastic waste materials pollution on the entire planet. As manufacturing processes of plastic materials result in additional carbon emissions, an environmentally friendly method of reusing these materials should be considered.

The reuse of plastic waste materials is a crucial component in creating a pristine environment. Therefore, to preserve the environment and find a solution to a pressing issue that is becoming more and more widespread every day, the reuse of water bottles (PET waste materials) in a variety of businesses should be taken into consideration (CHEN et al. 2010).

The concept of soil reinforcement with fibres was introduced by VIDAL in 1969, who observed an increase in the shear parameters of reinforced samples (VIDAL 1969). Plastic materials offer two major advantages: are less susceptible to chemical and biological degradation and have negligible weight. The improvement of soils through the addition of plastic waste materials has been studied in recent years. The results are encouraging, depending on the form of plastic waste materials and the amount added to the soil.

KUMAR et al. 2017 studied the influence of plastic waste materials on cohesive soils. Clay soil samples were used in which a percentage of 0%, 0.4%, 0.8%, and 1.2% plastic waste materials were added. The results of the laboratory tests indicated a real improvement in the shear strength of the soil due to an increase in the internal friction angle. However, the cohesion did not show consistent changes.

BABU et al. 2011 introduced a new form of soil stabilizer made from plastic waste materials − plastic flakes derived from shredded drink bottles (PET). A specific Indian soil (with a reddish colour) was used for the soil samples. Triaxial compression tests were performed in a consolidated-undrained (CU) system, and an uniaxial compression test was performed. The results indicated an increase in soil shear strength and significant reductions in compressibility.

For a better understanding of plastic fibres behaviour in soil, BHATTARAI et al 2016 studied their impact on an inorganic mud. It was armed with different percentages of plastic materials (0.25, 0.5 and 1% of the dry soil weight) with varying lengths (10 mm, 20 mm, 30 mm, and 40 mm). According to its parameters, the soil samples used can be characterized as a high plasticity inorganic mud with medium plasticity organic clay. The California penetration test results showed an improvement in soil’s parameters for an addition of up to 0.5% of plastic materials; after that percentage, the parameters decreased.

In Romania, studies on improving soils with plastic waste materials are not very widespread. In a paper, shear parameters of a clay improved with different percentages of PET were studied. Tests were performed on unimproved samples and on samples improved with 2%, 4% and 6% PET waste materials. The results indicate that a small amount of polyethylene terephthalate can improve the shear parameters of a clay. Results were influenced by the percentage of PET added, its shape and its distribution in the samples required to shear (TRIMBITAS et al. 2021).

Despite the numerous benefits that plastic waste materials can bring to soil improvement, there are several environmental aspects that have been little studied. These are the effects that plastic materials can have on the ecosystem over a long period of time in variable temperature and water content conditions. The composition of PET materials includes phthalates. Phthalates are the most prevalent plasticizers utilized globally and have been commercially available for several decades (GUO et al. 2005, HU et al. 2003). Phthalates or phthalic acid esters (PAEs) have been extensively used since the 1920s in the production and processing of plastic products as plasticizers. Currently, PAEs are employed in a diverse range of industrial applications (SERODIO et al. 2006, SIMONEIT et al. 2005).

The presence of phthalates in the environment is attributed to their high boiling points and low vapor pressures which result in strong adsorption to soils and sediments (MA et al. 2013). Phthalates and their metabolites and degradation products have demonstrated significant environmental hormonal effects and pose potential threats to ecosystems and human health. Upon entering the soil ecosystem, PAEs may also be conveyed to the edible portions of food crops and subsequently enter the human food chain (LIU et al. 2010, WANG et al. 2008, XU et al. 2008, ZENG et al. 2010, ZHANG et al. 2009).

One way phthalates can enter the environment is through the migration of plastic materials, such as polyethylene terephthalate into soil. Another way that phthalates can enter the environment is by leaching from plastic materials into water and soil. It has been found that this process is influenced by factors such as pH, temperature, water content, and the type of plastic materials.

This paper studies the behaviour of clayey samples improved with various percentages of plastic waste materials − polyethylene terephthalate (PET) − under varying water content and temperature conditions. Laboratory tests capture the behaviour of the samples and how PET influences the pH of the clay after variations in water content and temperature.

MATERIALS AND METHODS

The studied soil is a plastic silty clay with medium activity. Laboratory tests were carried out on unimproved clay and on samples improved with shredded PET coming from a local plastic waste landfill (fig. 1, 2).

The soil was improved with different percentages of PET: 2%, 4%, and 6% reported to the dry unit weight of the clay. Four samples were obtained: Cl, Cl2PET, Cl4PET and Cl6PET. Previous tests indicate that a small amount of polyethylene terephthalate can improve the shear parameters of the clay (TRIMBITAS et al. 2021).

Now, the aim of this study is to analyse the behaviour of clayey samples improved with various percentages of plastic waste materials under varying water content and temperature conditions.

In the laboratory, clay and clay-PET samples were created at an optimum water content (OWC) of 18%. From the Proctor apparatus, cylindrical samples with a diameter and height of 10 cm each have been collected (fig. 3).

To simulate the in-situ behaviour of PET-improved clay, samples were placed in a climate cabinet (CONTROLS / 10-D1428/A) and subjected to cycles of temperature and humidity variations. Fifteen freeze-thaw cycles were performed (fig. 4), with temperature variations ranging between -10°C and +40°C, and humidity increasing up to 90%.

After fifteen cycles of freeze-thaw, the test was stopped, and the samples were left at room temperature for 24 hours. Then the samples were visually analysed. First, the two circular faces of the soil cylinders were analysed. In the case of unimproved clay (Cl), the shrinkage behaviour of the soil could be observed. On the sample were slightly visible several shrinkage cracks (fig. 5). On the clay sample improved with 2% PET (Cl2PET), an increase in shrinkage cracks could be observed. These were more numerous, some having a considerable thickness (fig. 6). On the other hand, for the samples improved with 4% (Cl4PET) and 6% PET (Cl6PET), the shrinkage behaviour of the clay was reduced. The shrinkage cracks were considerably fewer and smaller for Cl4PET, while for Cl6PET, they have almost disappeared (fig. 7, 8).

The shrinkage behaviour of the clay was analysed for the four cylinders. For unimproved clay, shrinkage cracks could be observed over the entire height of the sample (fig. 9). The cracks were caused by the variation of humidity in the climatic cabinet. For the Cl2PET sample, the number of shrinkage cracks has increased. It can also be observed that part of the Cl2PET sample has detached (fig. 10). For the Cl4PET sample there were fewer shrinkage cracks, and they were very thin. On the surface of the cylinder, a slight peeling of the sample could be observed (fig. 11). For the Cl6PET sample, the shrinkage cracks were missing. The cylinder was integral, with no missing material parts (fig. 12).

After the climatic cabinet test was performed, and the samples have been analysed, clay’s pH value has been calculated. The results showed a proportional increase in pH value with the percentage of PET added to the samples. Also, migration of phthalates into the soil was considered. This migration of the phthalates into soil is determined by the percentage of PET added to improve the geotechnical parameters of the clay.

RESULTS AND CONCLUSIONS

In the specific literature, soil improvement by adding plastic waste materials is an intensively studied topic. The results obtained so far are encouraging and show an increase in the geotechnical parameters of soils improved with various plastic waste materials. However, there are few studies that analyse what happens to plastic materials when they are in situ, under varying water content and temperature conditions. Also, little is known about how the underground ecosystem is affected by the emissions that plastic waste materials may have.

This paper studied the behaviour of clay samples improved with various percentages of plastic waste materials (PET) under varying humidity and temperature conditions. Laboratory tests captured the behaviour of the samples and how PET influences the pH of the clay after variations in humidity and temperature.

The tests in the climatic cabinet showed an improvement in the behaviour of the clay in terms of geotechnical parameters. If for an addition of 2% PET, the number of shrinkage cracks increased compared to the unimproved clay, for an addition of 4%, and 6% PET, the number of shrinkage cracks and their thickness decreased significantly. The best behaviour to temperature and humidity variations simulated in the climatic cabinet was shown by the CL6PET sample. Shrinkage cracks were almost non-existent, with the Cl6PET sample behaving better than the Cl sample. There was a decrease in the shrinkage behaviour of the samples as the percentage of PET added was increased.

The climatic cabinet tests aimed to analyse samples improved with plastic waste materials (PET) under varying temperature and humidity conditions. Although an increase in the shear parameters of the studied clay by adding different percentages of PET was found (TRIMBITAS et al. 2021), it is essential to study also its behaviour under similar conditions to those in situ.

From the geotechnical point of view, the results of this study and those in the specific literature showed an improvement in the geotechnical parameters of soils improved with plastic waste materials.

Although this method of soil improvement could solve the problem of recycling plastic waste materials and the problem of soils with poor geotechnical characteristics, the effect of plastic materials on the underground ecosystem should also be studied.

For the studied clay, an increase in pH value was found due to the added plastic waste materials (PET). This increase was directly proportional to the percentage of PET added. Also, migration of PET phthalates into the clay was observed. The influence and migration of phthalates in a soil improved with plastic waste materials is a topic that needs further study. Improving soils by adding plastic waste material can be done up to the maximum limit of phthalates allowed in the underground ecosystem.

ACKNOWLEDGMENTS

This paper was financially supported by the Project “Network of Excellence in Applied Research and Innovation for Doctoral and Postdoctoral Programs / InoHubDoc”, a project co-funded by the European Social Fund financing agreement no. POCU/993/6/13/153437.

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Authors:

Ana-Maria TRIMBITAS (URIAN), Ovidiu NEMES, Nicoleta Maria ILIES, Andor-Csongor NAGY − Technical University of Cluj-Napoca, Romania

[Proceedings of the 17th Danube European Conference on Geotechnical Engineering (17DECGE), June 7-9, 2023, Bucharest, Romania – https://17decge.ro/]

…citeste articolul integral in Revista Constructiilor nr. 208 – noiembrie 2023, pag. 48-50, 52