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Full Research Article

Innovative Agricultural Soil Management: Geotechnical Utilization of Recycled Plastic Waste to Improve Soil Strength, Stability and Resistance to Desertification

I
Iman H. Gatea1,*
A
Atyaf K. Hameed1
J
Jameelah W. Ogaili1
A
Anwar L. Mohammed2
Z
Zainab F. Nadhim1
H
Hisham K. Auda1
A
Ameena G. Abid1
A
Adaweya B. Sabir1
F
Faihaa F. Ahmed2
A
Alaa I. Hamed2
1Scientific Research Commission, Baghdad, Iraq.
2Ministry of Construction, Housing and Public Municipalities, Baghdad, Iraq.

ABSTRACT

Background: Muthanna Governorate in southern Iraq is characterized by Gelic Andosols that are highly susceptible to drought and erosion. The accumulation of polyethylene terephthalate (PET) bottles, which are rarely discarded, poses an environmental concern due to their persistence and greenhouse gas emissions. Addressing both soil weakness and plastic waste management is therefore critical. The Samawah desert soil, being saline, is unsuitable for construction purposes. This study investigates the potential enhancement of Samawah desert soil with PET strips as a reuse strategy.

Methods: PET bottles were cut into 15 mm × 20 mm strips and incorporated into soil at weight ratios of 5%, 10% and 15%. Laboratory tests, including shear strength, plastic limits, Proctor compaction and California bearing ratio (CBR), were conducted to evaluate the performance of the reinforced soils and determine the effectiveness of PET strips in improving soil properties.

Result: The findings indicate that PET strips enhanced soil behaviour by increasing shear strength and specific gravity, while reducing swelling and desiccation cracking. Compaction characteristics were improved, with a marginal increase in maximum dry density and a slight decrease in optimum moisture content, reflecting improved load bearing capacity. Evidence suggests that recycled PET strips are an inexpensive and environmentally friendly alternative for reinforcing weak soils and combating erosion in arid regions such as Samawah. Additionally, their use provides a practical pathway for reducing plastic waste accumulation.

INTRODUCTION

For thousands of years, soil stabilization, an engineering technique, has been used to enhance weak soils that hinder the movement of people, goods and infrastructure. Soil is indispensable natural resource that plays a relevant role in improving crop production however, inadequate information about the potential uses are lacking (Ojobor et al., 2026). Ancient civilizations noted that mixing soil with a stabilizer like lime could improve its strength. Soil stabilization is a process aimed at modifying the soil’s engineering properties such as strength, compressibility and water resistance and can be achieved using various methods, including physical techniques, chemical and biological or combination of all or any is done (White, 2005). The soil weakness is assessed with the help of some property. Selecting a proper stabilizing agent or technique soil stabilization is essential to enhance the performance and properties of soil. Historically, stabilization methods using cement and lime have been quite popular and effective. However, in the last few decades, due to the ever-increasing cost of cement and lime additives and the cement production process emits approximately one ton of CO2 for every ton of cement produced, the use of pozzolanic materials has been declining. Due to such restraints, researchers are looking for other stabilizing agents such as plastic waste, wood ash, fly ash, glass fibre and other industrial (Abhishek et al., 2016; Iftikhar et al., 2023). Use of solid waste material can be a good candidate for soil stabilization process. Solid waste means unwanted scraps and materials that are generally disposed of. It includes non-hazardous by-products that arise from productive activities along with municipal waste.  These wastes are found in solid, semi solid, liquid, or containerized gaseous forms (Alavi et al., 2010; Mason et al., 2018). Plastic waste is one of the major environmental problems now. Since about 1940, plastics and synthetic polymers have played a prominent role in modern living for over fifty years. The use of plastic is ubiquitous in packaging, buildings, medicine and electronics. Although plastics have become useful materials, they are challenging to manage and dispose of. If you improperly dispose of your plastic in landfill where it is exposed to sunlight and violent, it releases methane and ethylene. The most major environmental problem of twenty first century is Climate change and Global warning caused by Carbon Footprints (Geyer et al., 2017). Climate change is a long-term alteration in global or regional climate. A rise in the temperature of Earth’s atmosphere is happening due to the addition of greenhouse gases to Earth’s atmosphere according to (Al-Ani and Sarapää, 2008; Kader et al., 2017). Global warming has already started to present issues like storms happening more frequently and becoming more intense, rising sea levels and massive flooding (Hulme, 2002; Atalar, 2004). The engineering of sustainable solutions can effectively utilize the challenge of plastic waste management (Shah and Hayat, 2026) and soil stabilization. Plastic waste can be used as a stabilizing agent and implementing it is a great solution to deal with plastic waste disposal. It also enhances soil’s properties. This may be done particularly with weak sandy soils.
       
Soils can have their strength and durability improved through the addition of appropriate binding materials, their accommodation to changing environmental conditions and their useful employ to engineering works (Holtz and Kovacs, 1981; Madhavi, 2011). Soil stabilization is cost-effective. Thus, it reduces reliance on standard “dig and dump” methods which saves time and money (Chen et al., 2012). Also, it improves the moisture behaviour by enhancing the drying behaviour of moist soil (Raman et al., 1998; Ganeshi et al., 2023). Stabilization is used to prevent erosion from dust deposition and soil especially in arid and semi arid regions (Ahmadzadeh and Khosravi, 2023; Naghdi et al., 2024). In the same way, it helps to enhance durability in facilitating workability and long term performance of soil-aggregate mixtures. In addition, backer, at the base, is essential for slope stabilization (Ahmed and Hussain  2023). Further, slope stabilization is made successful by effectively reinforcing sloping lands with stability all over (Arbanas and Arbanas, 2014). In addition, minimizing the changes in volume of soil with temperature or moisture enhance the volume stability (Deshpande et al., 2022). In this context, current research aims to evaluate the cost-effective and eco-friendly stabilization of soil using plastic waste. Including plastic waste in sandy soil improves its  characteristics . Similarly, this process will provide a solution to lessen the amount of plastic waste and help with soil stabilization.

MATERIALS AND METHODS

Sample collection
 
Soil samples were collect from the Al Simawa desert. Each soil sample was placed in a properly labeled plastic bag, air-dried, crushed and passed through a 2-mm sieve for laboratory testing (Sabijon, 2026).
 
Specific gravity test
 
Soil specific gravity refers to the weight of the solid particles of soil to the weight of water of an equivalent volume. The specific gravity of the collected soil samples was determined according to ASTM D854 in a simple experiment using volumetric flask.
 
Particle size distribution
 
Sieve analysis as per ASTM D422M was done to determine the particle size distribution of the soil (Kakuturu et al., 2019).
 
Plastic material preparation
 
The locally obtained high-density bottle plastic widely used for packaging liquids such as water was used in our study. The material properties are compiled in the Table 1.

Table 1: Properties of high density bottle plastic (HDPE).


 
Plastic material preparation
 
We used high density bottle plastic available locally in the market. This plastic is used for storing liquids like water. Table 1 summarizes the material properties.
       
The bottles were cut into strips measuring 15 mm in width and 20 mm in length. The strips were mixed with dry soil in proportions of 5, 10 and 15 with 100 g soil.
 
Minimum and Maximum unit weights
 
The maximum dry density d.max and minimum dry density d.min are determined. Table 2 of IS2720 gives soil classification on relative density (Murthy, 2002).

Table 2: Division of sandy soil (Murthy, 2002).


       
Relative density is calculated using the very own formula:

 
Dr = Relative density (%)
ρd = Natural dry density (g/cm3)
ρdmax = Maximum dry density (g/cm3)
ρdmin = Minimum dry density (g/cm3).
 
Compaction test
 
The proctor compaction test is performed per ASTM D698 and the maximum dry density (MDD) and optimum moisture content (OMC) of the soil samples are obtained (Akayuli et al., 2013).
 
Proctor compaction test standard
 
Compaction tests of the reinforced plastic-soil mix were conducted as per standard D698 ASTM. The experimental setup was made up of a rammer, a collar and a mould.  The rammer has a weight of 2.5 kg with a collet and detachable base plate. Besides, the collar has an effective height of 5 cm. Finally, the metal mould has an internal diameter and height of 10.15 cm and 11.7 cm respectively. The above arrangements will assist in the laboratory determination of compaction characteristics. Soil compaction process is the method of increasing the bulk density of soil and removing air from voids. Soil was filled in each mold in three layers and compacted with almost 25 blows of the rammer for each layer (Temel et al., 2005).
 
Water content
 
Oven drying method for determining water content of soil samples was carried out as per ASTM D2216. During this operation, wet soil is dried completely at 105°C for a specific duration of 18 to 24 hours. The moisture content is determined by the ratio of the weight of water lost to the dry soil solids weight. The weight of water equals the weight of dry soil solids (Kakuturu et al., 2019).
 
Plastic limit
 
The soil plastic limit was conducted as per the rules of ASTM D4318. The water content at which the soil no longer possesses plasticity is known as the plastic limit of soil. A medium size of fine grained soil is rolled into threads until they reach a diameter of 3 mm during this test. The time at which soil pulverises and quantity of water soil content is determined to determine plastic limit (Imtiyaz et al., 2023).
 
California bearing ratio (CBR)
 
The CBR test is a laboratory method to determine the bearing capacity or mechanical strength of the subbase/subgrade of a road or highway. The test was carried out according to ASTM D1883 (Yildirim and Gunaydin, 2011).

RESULTS AND DISCUSSION

Chemical characteristics of Al-Samawa sandy soil
 
As shown in Table 3, Al-Samawa sand dune soil is mostly siliceous with SiO2  at 77.76%. Because of this, it essentially very rich in quartz. This composition is typical for desert sand, which displays low natural cohesion and weak bonding between particles. CaO (9.27%) and Fe2O3 (3.92%) act as minor cementing agents. These alone have an insufficient effect on the loading mechanical strength.

Table 3: Results of chemical tests for sand dune.


       
The mechanical reinforcement is justified due to the chemical characteristics of the sandy soil. This type of sandy soil has low bearing capacity, erosion and excessive deformation. Desert type applications are subgrade or embankment. According to the soil classification map in Fig 1 for Al Muthanna Governorate where Al Samawah is situated, it can be seen that Gelic Andosols dominate large areas of the desert with Eutric Regosols prominent near river systems. Soils of the type that are geotechnically problematic, are expansive clays and loose.

Fig 1: Soil classification map of Al Muthanna Governorate showing Gelic Andosols and Eutric Regosols in Al Samawah area.


       
Sands that are low in clay are common in central and south Iraq. The first step of the criteria was the sampling of representative soil from the desert area in Al Samawah in this regional context. The essential features of this soil were established and recorded to serve as a baseline for the further test. After that, a series of controlled laboratory experiments were conducted to assess the effect of plastic strip and desert soil mixture. The study aimed to examine the influence of plastic reinforcement on specific gravity, compaction, moisture content and bearing capacity of soil to mitigate soil map geotechnical problems. The specific gravity of sandy soil may vary between 2.63 and 2.67. The density of soil particles is denser than that of water is the specific gravity. The weight volume relationship of soil is an important parameter in soil mechanics. Soil that is evaluated and tested for engineering purposes is very useful. The addition of plastic strips in sandy soil was used for construction in the study by (Al Shafian et al., 2023).
       
Prior research has shown that plastic waste, such as strips from used plastic bottles, can improve soil performance. For instance, shear strength can be improved, desiccation cracking can be reduced, etc. The specific gravity of the soil was not influenced by plastic strips to a great extent. However, the strips enhanced the mechanical properties of the soil to a great level especially the compaction behaviour and bearing capacity of the soil.  In past findings, binders provided better structural performance in concrete and bituminous pavements, which reiterates this finding. This stabilization method not only enhances engineering but also contributes to plastic waste. A sustainable and cost effective solution for construction applications is provided by management. The soil’s specific gravity is seen to increase as the percentage of plastic strips increases as seen in Fig 2. The specific gravity was found to be 2.65 at 0% plastic content while it achieved the maximum of 2.75 at 15% plastic content. The slight increase shows the increase in soil density after laying the strips but not drastically altering it.

Fig 2: Effect of plastic percentage of specific gravity.


       
Bioengineered strips do enhance the soil engineering properties. Fig 3 displays the moisture-density relationship of dune sand determined by Proctor compaction test along with the OMC and MDD. The environmental impacts were also considered here. Process strengthens the sandy soils and disposal of plastic waste and also balances the environment by reusing plastic waste in soil stabilization. Increasing the percentage of plastic strips from 0-15% increases specific gravity from 2.65 to 2.75. Although the addition is not considerable, the use of plastic strips does not greatly change the soil density. The soil’s mechanical performance is enhanced positively. Same basic properties but improved soil performance can be achieved using plastic strips. Soil that used strips of waste plastics showed better compaction and bearing capacity. It can offer a sustainable alternative to plastic waste. Soils with high specific gravity are strong enough to be used for making roads, foundations and other construction works. The specific gravity of a soil is one of the soil quality parameters. Density of soil particles refers to water. Roy and Dass (2014) state that higher specific gravity increases cohesion and angle of shearing resistance, which are two of the shear strength parameters of soil. In addition, the (CBR) will be enhanced due to the improved specific gravity of the subgrade material. The subgrade strength is commonly measured using a CBR test (Ramasubbarao and Siva, 2013). Simultaneously, compacting the soil is extremely beneficial to the soil. The application of mechanical energy rearranges soil particles and diminishes the void ratio by the expulsion of entrapped air and increases dry density. Soil compaction enhances the engineering behaviour of soil irrespective of moisture contents. Benefits includes improved shear strength, lesser compressibility and lesser permeability. Two essential soil properties useful to assess the the ability of soils for engineering application are compaction and specific gravity. They are also useful to assess the stability and durability of soils.

Fig 3: The relationship of the dry density of soil with water content (control).


 
Maximum dry density (MDD)
 
Table 4 reports that the control sample displayed (sand + 0% strips) has an MDD of 16.3 kN/m3. Nonetheless, the MDD for the 5% strips fell slightly to 16.02 kN/m3. The initial addition of plastic disrupts the packing of particles, as such shows the outcome. When the compressive stripping marginal density was 10%, interlocking and compaction efficiency improved to about 16.4 kN/m3. As the plastic increased in 15% strips, the MDD decreases marginally to 16.25 kN/m3. This means that excess use of plastic hinders achieving maximum compaction.

Table 4: Summary of proctor compaction test results.


 
Optimum moisture content (OMC)
 
The (OMC) was 13.1% and this value increased progressively with the addition of plastic: reaching 17.5% at a 5% plastic content, 18.9% at 10% and 19.4% at 15% (Table 4). The soil’s moisture retention capacity increases with the addition of plastic strips, as these strips create additional voids and facilitate a uniform distribution of moisture within the soil. The slight increase in (MDD) at a 10% plastic content indicates an optimal level of reinforcement, where maximum contact between soil particles and plastic strips is achieved. It is worth noting that soils requiring a higher OMC necessitate the addition of water to ensure adequate compaction; this enhances the reinforced soil’s resistance to desiccation cracking and improves its durability under field conditions. The use of plastic waste in soil stabilization involves utilizing plastic strips for improving geotechnical properties of soil. Further, the use of plastic strips helps manage the plastic waste effectively. Studies show that plastic strips enhance the compaction behavior and shear strength of soils. Use only required plastic strips as excess amount disturbs arrangement of soil particles and makes it less dense.
       
Research has consistently highlighted the beneficial role of waste plastic (PET) strips in soil stabilization. (Tejeswini, 2013) reported that their inclusion enhances compaction characteristics, while (Bozyigit et al., 2021) demonstrated that the addition of plastic strips improves overall earth stability, showing promise in subgrade reinforcement. Similarly, (Patidar and Mahiyar, 2014) observed that compaction curves shift noticeably when soils are treated with plastic strips, with the optimal reinforcement level producing the highest dry density and reduced swelling. Collectively, these findings indicate that incorporating PET strips at an ideal proportion of 10% yields significant improvements in dry density and moisture content. More water may be required by higher percentages but may reduce compaction. According to a recent geotechnical study, the technique is economically feasible and environmentally friendly for an effective soil stabilization. By adopting this approach, sandy soils gain improved engineering properties while simultaneously contributing to plastic waste management. he compaction curve of dune sand (Fig 3), which plots dry density against moisture content, clearly identifies the optimum moisture content corresponding to the maximum dry density, in accordance with Proctor compaction theory. Incremental additions of water enhance soil strength by lubricating particles, allowing efficient rearrangement and higher density. Once the optimum point is reached, however, surplus water begins to occupy soil voids and displace air, leading to a decline in density. As emphasized by Roy and Dass (2014), these parameters are fundamental in evaluating soil suitability for construction. The figure illustrates this relationship, highlighting how moisture facilitates particle rearrangement and compaction until the optimum condition is achieved, beyond which excess water reduces effectiveness.
       
Collaboration has focused on recently published research works concerning Sikkim. The soil’s (MDD) makes it less likely to destabilize the foundation or pavement due to settlement. When the void ratio decreases, the soil particle’s interlocking increases soil’s shear strength. In addition, good compaction reduces permeability and compressibility that confirm the reliability of dune sand for geotechnics. According to the study, dune sand at OMC can work as subgrade material when compacted. The suggestion was made to use plastic strip reinforcement. The study shows that proctor test gives the upper limit of MDD and OMC i.e. moisture density relation in the soil due to compaction rearrangement of soil particles filling in the voids region which improves their engineering properties like shear strength, compressibility, etc.
       
In addition, preventing the settlement, cracking and instability of embankments, subgrades and foundations is the practical significance of compaction. The crucial importance of all these aspects rests in maintaining one’s durability and stability of the project.  As shown in Fig 3, the relationship between moisture and density of dune soil is such that the dry density initially increases with increase in water content. This is caused by the effect of moisture, which acts as a lubricant and allows particles to reorganize. Soil’s optimum moisture content is 13% to 14%. A maximum dry density of 16.4 kN/m3 is achieved at a specific moisture content. Adding excess water after this moisture causes the voids to fill while the air is pushed out. Therefore, this decreases the dry density.  This sandy soil shows classical compacting behaviour. In sample 1, which contains 5% plastic strips, MDD slightly decrease while OMC slightly increase as shown in Fig 4. The plastic material has a low specific gravity which causes a reduction in dry density. As a result of this, a lighter component has replaced part of the soil mass in the bulking agent. Chu et al. (2021) reported similar observations with a difference of opinion regarding lower packing efficiency being responsible for lower dry density values. At this reinforcement level, the plastic strips are uniformly arranged and serve as randomly dispersed tensile reinforcements and enhance the inter-particle restraint without significant disturbance of the skeleton soil. Tang and colleagues found that these fibers could also enhance ductility and increase tensile resistance through similar reinforcement. In Sample 2 having 10 plastic strips Fig 5, there was a steep decline in MDD and an apparent increase in OMC. When the content of plastic increases, the void ratio increases which disturbs the soil to soil contact making it less compact. Plastic or polymeric inclusion’s increasing in soil matrices will usually lead to a reduction in dry density and an increase in moisture demand because of enhanced surface area and less interlocking of particles. With a drop in MDD, soil’s tensile resistance and deformation control enhanced. This indicates suitable balancing between compaction characteristics and reinforcement benefits. The research of Sivakumar (Buzzi et al., 2011) found that by moderate inclusion of fiber soil performance improved considerably without causing much degradation of density.

Fig 4: Relation between soil’s dry density and water content (sample 1).



Fig 5: Relationship between soil’s water content and dry density (sample 2).


       
The proctor test indicates that MDD and OMC are the guideline values of soil (Roy and Dass, 2014) and compaction rearranges the particle structure of soil and improves its engineering properties. Adequate compaction is critical to prevent settlement, cracking and instability in embankments and foundations (Ran et al., 2024), aligning with the present study. The compaction behavior of sandy soil reinforced with recycled plastic strips was evaluated using the Standard Proctor test (ASTM D698, BS1377 Part 4, IS 2720 Part 7). Results show that varying reinforcement percentages influence (MDD) and (OMC). The typical bell shaped curve of unreinforced sandy soil reflects cohesionless behavior under standard energy. Efficient particle rearrangement accounted for the control soil’s compaction response. However, Sample 3, reinforced with 15% plastic strips, exhibited the lowest MDD and highest OMC (Fig 6). At such a high percentage of plastic inclusion, the granular structure of sandy soil gets disturbed. Overlap and clustering of plastic strips reduced effective stress in soils and increased internal voids and thus poor packing efficiency. If reinforcement exceeds the optimal level, it adversely affects compaction behavior and load-carrying capacity.    

Fig 6: Relationship between soil’s water content and dry density (sample 3).


       
Adding more plastic to the mix boosts ductility and crack resistance but it also lowers dry density significantly making it unsuitable for highly compacted and strong applications. Findings of the study concluded that with increase in plastic strip content maximum dry density goes on decreasing. Also Increase in plastic strip content and optimum moisture content has direct relationship which has also been reported in earlier studies on waste plastic-reinforced soils (Deng et al., 2025). The research found that with 5% to 10% content of plastic strip there obtained optimum range for reinforcement benefit without losing compaction efficiency too much. If the level of plastic material exceeds the range, engineering performance is adversely affected. Recycled plastic strips can be applied as soil reinforcement material to an extent and it is said to enhance the geotechnical performance of soil. Also, it provides a way to use plastic waste appropriately (Hamard et al., 2016). In the following figure, the MDD of the natural Al-Samawa sand dune soil, soil reinforced with 5% (sample one), 10% (sample two) and 15% (sample three) plastic strips are shown. As per study results, it was found that the plastic strips can noticeably impact the densification potential of sandy soil and, subsequently, the (CBR) and shear strength behaviour. The MDD obtained from the test is approximately 16.4 kN/m3. This is due to the loose structure of sand dune soil (Fig 7), which is held together by insufficient bonding. A soil with such characteristics will generally exhibit low values of CBR and shear resistance. Hence, it would not be appropriate to use it as a pavement subgrade and as a foundation layer without improvement.

Fig 7: Effect of additive to sand dune on maximum density.


 
Effect of 5% plastic strips (Sample one)
 
MDD was reduced slightly when compared to natural sand dune soil with 5% of plastic strips. The unit weight is reduced because of the lesser unit weight of plastic which only partially replaces soil particle while slightly increasing voids. There is a slight reduction in density; however, this does not indicate a reduction in performance from CBR  and shear strength angles. Plastic strips function as tensile reinforcement that improves abilities to hinder particle movement. As a result, there would be slight improvement in shear strength and CBR and slight decrease in dry density due to frictional resistance and re-distribution of stress (Choudhary et al., 2010).
 
Effect of 10% plastic strips (Sample two)
 
Among the soils tested, the maximum dry density of sample 2 is highest ≈17.0 kN/m3. The addition of 10% plastic strips improves the characteristics of sandy soil of Al Samawa. The use of strips of plastic material here restrict particle movement and lead to better packing and densification during the compaction process. The CBR value and shear stress strength increase as the maximum dry density of a material increases. Higher density reduces compressibility and improves inter particle contact while plastic strips offer additional tensile resistance. Thus, sample two must show better bearing capacity, superior load distribution and the highest values of shear strength parameters, which makes it most suitable for pavement sub-grade and construction of embankments.
 
Effect of 15% plastic strips (Sample three)
 
The maximum dry density of 15% plastic strips is lower than Sample two. Reduced samples show that an increase in plastic content fractures the soil skeleton which increases void ratio and reduced packing efficiency but is still higher than sample one. According to CBR and shear strength, if a large number of plastic strips are added to the soil, the strain of soil particles will not be transferred properly and consequently the strength will not increase although ductility will be increased. The resistance to deformation of the intensified soil use may improve but its lesser density may not enhance the CBR and shear strength in that proportion and they may actually drop in comparison to the ideal 10%. Relationship between density, CBR and shear strength. Primarily confirming a geotechnical criterion, trend shown in Fig 6. The maximum dry density of sandy soils significantly influences other index properties.
       
It is evidenced from the experimental results that when Al Samawa sandy soil was reinforced with recycled plastic strips, its compaction behavior, moisture sensitivity and strength behaved like a fibre reinforced soil. The native sand dune soil’s very low density and weak bonding contributed to its poor CBR and shear strength which suggest such soils cannot be used for engineering applications. By adding 5% plastic strips there is moderate improvement and 10% reinforcement has shown maximum density and maximum strength this percentage can be considered optimum. If the inclusions are more than 15%, the packing of the particles will be disturbed, which will lower the dry density and strength efficiency; thus always require a reinforcement in an effective range. The researchers said that dry density decreased early due to use of plastic strip for reinforcing. Afterwards, the tensile restraint leads to a strength improvement.
       
Furthermore, the fibers have a positive impact  because they provide ductility enhancement. Moreover, they assist in postponing the breakdown of the material. Observed maximum behaviour is at 10% reinforcement which agrees with statement given by Banerjee et al., (2002) moderate addition of fibre produces the best compaction efficiency and strength. The reduction in performance at 15% reinforcement reinforces (Wang and Yejiao, 2024) findings that excessive quantity of polymeric inclusions increases voids quantity but decreases effective stress transfer capacity. In addition to the mechanical impacts, the environmental effects are also significant. The use of discarded PET bottles can solve two current problems soil stabilization and plastic waste problem. The present study shows that the use of plastic ensures better strength and durability of soil and offers an eco-friendly solution to environmental pollution. The use of plastic strips made from recycled materials found in sandy soils is a practical and sustainable solution for constructing infrastructures in dry regions like Samawah in Iraq. According to the results, 10% plastic strip reinforcement is an effective addition to the Al-Samawa sandy soil as it improves its compaction and strength characteristics. Application of 10% plastic strip is recommended to improve the sandy soils at Al-Samawa according to relationship between maximum dry density, CBR and shear strength. The enhancement level upgrade improves carrying power and shear resistance, while also having good compaction characteristics. It can be used below roads, in embankments and deserts. In sandy soil of Al Samawa, the mechanical efficacy of the plastic strip utilized increased with the rise in inclusion content from 5% to 10% and then declined again at 15%. Literature has similar observations (Mardani et al., 2025) found that random distribution of plastic fibres increases the CBR and increases the shear strength significantly up to some optimum content after which the improvement remains constant. For the 5% strips, the interlocking and surface shear have started to mobilise along the inclusions as can be seen in Fig 8. This leads to a moderate increase CBR and peak shear over the unreinforced sand. The study by Wei et al., (2018) found that when small amounts of synthetic fibres are introduced into sandy matrices, the effect is an increase in the apparent cohesion. Also, this action further stabilizes the failure zone of the soil mass and delays the process of localization inside the soil matrix.  The improvements which are observed in Al Samawa sand are at the early stage. Raising the strip content to 10% usually brings the system close to optimum. Restraint networks become continuous, lateral dilation is efficiently curtailed and stresses are better distributed. According to Bezih et al., (2025), the presence of fibers at intermediate contents leads to an increase in the maximum value of the (CBR) and the shear strength of sandy soils. This improvement happens because of the fiber components’ effect on both the apparent cohesion and slight enhancement on friction angle. In Iraqi sand, PET strip reinforcement yield peak strength around 10% inclusion, confirming optimal behaviour, because 100% PET reinforcement produced much weaker sands. When the density reaches 15%, the soil structure and function of the compacted soil are disrupted, leading to micro voids and uneven load paths. Even though the  performance continue to remain higher than the baseline, the CBR and peak shear tends to saturate or decrease with respect to the 10% level. Excess fiber content can disrupt soil structure and reduce strength, while moderate reinforcement enhances interfacial shear and ductility. In the present study, reinforcing Al Samawa sandy soil with recycled plastic strips significantly improved compaction, moisture sensitivity and strength. The soil, being predominantly siliceous with low natural cohesion, benefited from the mechanical reinforcement of PET/HDPE strips, which increased specific gravity, shear strength and CBR values while reducing swelling and cracking. Compaction tests showed that dry density decreased at 5%, peaked at 10% and dropped again at 15%, confirming 10% as the optimum reinforcement level for effective particle interlock. Higher contents beyond this threshold disrupted packing, increased voids and reduced efficiency. Moisture density curves further indicated that reinforcement raised the optimum moisture content, enhancing resistance to drying cracks. Overall, 10% strip inclusion provided the best balance of strength, durability and compaction, addressing both soil instability and plastic waste management.

Fig 8: Effect of additive to sand dune on minimum density.

CONCLUSION

The changes of  properties of recycled plastic strip (PET and HDPE) reinforced Al-Samawa sandy soil were studied in this investigation. Adding plastics enhances shear strength and California Bearing Ratio (CBR), decreases cracking and swelling. The result is that the percentage of strip content of 10% is the best balance for soil stability. Beyond this percentage soil density reduction occurs. In the end, recycled plastic proves to be a cheap and effective option to stabilize weak desert soils while preventing waste.

ACKNOWLEDGEMENT

The authors acknowledge with thanks the Ministry of Housing and Construction and the Centre for Environment, Water and Renewable Energy for their guidance. The laboratory staff is also acknowledged for technical assistance, while academic colleagues and peer reviewers are recognized for their input towards the research.

CONFLICT OF INTEREST

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this manuscript.

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    Full Research Article

    Innovative Agricultural Soil Management: Geotechnical Utilization of Recycled Plastic Waste to Improve Soil Strength, Stability and Resistance to Desertification

    I
    Iman H. Gatea1,*
    A
    Atyaf K. Hameed1
    J
    Jameelah W. Ogaili1
    A
    Anwar L. Mohammed2
    Z
    Zainab F. Nadhim1
    H
    Hisham K. Auda1
    A
    Ameena G. Abid1
    A
    Adaweya B. Sabir1
    F
    Faihaa F. Ahmed2
    A
    Alaa I. Hamed2
    1Scientific Research Commission, Baghdad, Iraq.
    2Ministry of Construction, Housing and Public Municipalities, Baghdad, Iraq.

    ABSTRACT

    Background: Muthanna Governorate in southern Iraq is characterized by Gelic Andosols that are highly susceptible to drought and erosion. The accumulation of polyethylene terephthalate (PET) bottles, which are rarely discarded, poses an environmental concern due to their persistence and greenhouse gas emissions. Addressing both soil weakness and plastic waste management is therefore critical. The Samawah desert soil, being saline, is unsuitable for construction purposes. This study investigates the potential enhancement of Samawah desert soil with PET strips as a reuse strategy.

    Methods: PET bottles were cut into 15 mm × 20 mm strips and incorporated into soil at weight ratios of 5%, 10% and 15%. Laboratory tests, including shear strength, plastic limits, Proctor compaction and California bearing ratio (CBR), were conducted to evaluate the performance of the reinforced soils and determine the effectiveness of PET strips in improving soil properties.

    Result: The findings indicate that PET strips enhanced soil behaviour by increasing shear strength and specific gravity, while reducing swelling and desiccation cracking. Compaction characteristics were improved, with a marginal increase in maximum dry density and a slight decrease in optimum moisture content, reflecting improved load bearing capacity. Evidence suggests that recycled PET strips are an inexpensive and environmentally friendly alternative for reinforcing weak soils and combating erosion in arid regions such as Samawah. Additionally, their use provides a practical pathway for reducing plastic waste accumulation.

    INTRODUCTION

    For thousands of years, soil stabilization, an engineering technique, has been used to enhance weak soils that hinder the movement of people, goods and infrastructure. Soil is indispensable natural resource that plays a relevant role in improving crop production however, inadequate information about the potential uses are lacking (Ojobor et al., 2026). Ancient civilizations noted that mixing soil with a stabilizer like lime could improve its strength. Soil stabilization is a process aimed at modifying the soil’s engineering properties such as strength, compressibility and water resistance and can be achieved using various methods, including physical techniques, chemical and biological or combination of all or any is done (White, 2005). The soil weakness is assessed with the help of some property. Selecting a proper stabilizing agent or technique soil stabilization is essential to enhance the performance and properties of soil. Historically, stabilization methods using cement and lime have been quite popular and effective. However, in the last few decades, due to the ever-increasing cost of cement and lime additives and the cement production process emits approximately one ton of CO2 for every ton of cement produced, the use of pozzolanic materials has been declining. Due to such restraints, researchers are looking for other stabilizing agents such as plastic waste, wood ash, fly ash, glass fibre and other industrial (Abhishek et al., 2016; Iftikhar et al., 2023). Use of solid waste material can be a good candidate for soil stabilization process. Solid waste means unwanted scraps and materials that are generally disposed of. It includes non-hazardous by-products that arise from productive activities along with municipal waste.  These wastes are found in solid, semi solid, liquid, or containerized gaseous forms (Alavi et al., 2010; Mason et al., 2018). Plastic waste is one of the major environmental problems now. Since about 1940, plastics and synthetic polymers have played a prominent role in modern living for over fifty years. The use of plastic is ubiquitous in packaging, buildings, medicine and electronics. Although plastics have become useful materials, they are challenging to manage and dispose of. If you improperly dispose of your plastic in landfill where it is exposed to sunlight and violent, it releases methane and ethylene. The most major environmental problem of twenty first century is Climate change and Global warning caused by Carbon Footprints (Geyer et al., 2017). Climate change is a long-term alteration in global or regional climate. A rise in the temperature of Earth’s atmosphere is happening due to the addition of greenhouse gases to Earth’s atmosphere according to (Al-Ani and Sarapää, 2008; Kader et al., 2017). Global warming has already started to present issues like storms happening more frequently and becoming more intense, rising sea levels and massive flooding (Hulme, 2002; Atalar, 2004). The engineering of sustainable solutions can effectively utilize the challenge of plastic waste management (Shah and Hayat, 2026) and soil stabilization. Plastic waste can be used as a stabilizing agent and implementing it is a great solution to deal with plastic waste disposal. It also enhances soil’s properties. This may be done particularly with weak sandy soils.
           
    Soils can have their strength and durability improved through the addition of appropriate binding materials, their accommodation to changing environmental conditions and their useful employ to engineering works (Holtz and Kovacs, 1981; Madhavi, 2011). Soil stabilization is cost-effective. Thus, it reduces reliance on standard “dig and dump” methods which saves time and money (Chen et al., 2012). Also, it improves the moisture behaviour by enhancing the drying behaviour of moist soil (Raman et al., 1998; Ganeshi et al., 2023). Stabilization is used to prevent erosion from dust deposition and soil especially in arid and semi arid regions (Ahmadzadeh and Khosravi, 2023; Naghdi et al., 2024). In the same way, it helps to enhance durability in facilitating workability and long term performance of soil-aggregate mixtures. In addition, backer, at the base, is essential for slope stabilization (Ahmed and Hussain  2023). Further, slope stabilization is made successful by effectively reinforcing sloping lands with stability all over (Arbanas and Arbanas, 2014). In addition, minimizing the changes in volume of soil with temperature or moisture enhance the volume stability (Deshpande et al., 2022). In this context, current research aims to evaluate the cost-effective and eco-friendly stabilization of soil using plastic waste. Including plastic waste in sandy soil improves its  characteristics . Similarly, this process will provide a solution to lessen the amount of plastic waste and help with soil stabilization.

    MATERIALS AND METHODS

    Sample collection
     
    Soil samples were collect from the Al Simawa desert. Each soil sample was placed in a properly labeled plastic bag, air-dried, crushed and passed through a 2-mm sieve for laboratory testing (Sabijon, 2026).
     
    Specific gravity test
     
    Soil specific gravity refers to the weight of the solid particles of soil to the weight of water of an equivalent volume. The specific gravity of the collected soil samples was determined according to ASTM D854 in a simple experiment using volumetric flask.
     
    Particle size distribution
     
    Sieve analysis as per ASTM D422M was done to determine the particle size distribution of the soil (Kakuturu et al., 2019).
     
    Plastic material preparation
     
    The locally obtained high-density bottle plastic widely used for packaging liquids such as water was used in our study. The material properties are compiled in the Table 1.

    Table 1: Properties of high density bottle plastic (HDPE).


     
    Plastic material preparation
     
    We used high density bottle plastic available locally in the market. This plastic is used for storing liquids like water. Table 1 summarizes the material properties.
           
    The bottles were cut into strips measuring 15 mm in width and 20 mm in length. The strips were mixed with dry soil in proportions of 5, 10 and 15 with 100 g soil.
     
    Minimum and Maximum unit weights
     
    The maximum dry density d.max and minimum dry density d.min are determined. Table 2 of IS2720 gives soil classification on relative density (Murthy, 2002).

    Table 2: Division of sandy soil (Murthy, 2002).


           
    Relative density is calculated using the very own formula:

     
    Dr = Relative density (%)
    ρd = Natural dry density (g/cm3)
    ρdmax = Maximum dry density (g/cm3)
    ρdmin = Minimum dry density (g/cm3).
     
    Compaction test
     
    The proctor compaction test is performed per ASTM D698 and the maximum dry density (MDD) and optimum moisture content (OMC) of the soil samples are obtained (Akayuli et al., 2013).
     
    Proctor compaction test standard
     
    Compaction tests of the reinforced plastic-soil mix were conducted as per standard D698 ASTM. The experimental setup was made up of a rammer, a collar and a mould.  The rammer has a weight of 2.5 kg with a collet and detachable base plate. Besides, the collar has an effective height of 5 cm. Finally, the metal mould has an internal diameter and height of 10.15 cm and 11.7 cm respectively. The above arrangements will assist in the laboratory determination of compaction characteristics. Soil compaction process is the method of increasing the bulk density of soil and removing air from voids. Soil was filled in each mold in three layers and compacted with almost 25 blows of the rammer for each layer (Temel et al., 2005).
     
    Water content
     
    Oven drying method for determining water content of soil samples was carried out as per ASTM D2216. During this operation, wet soil is dried completely at 105°C for a specific duration of 18 to 24 hours. The moisture content is determined by the ratio of the weight of water lost to the dry soil solids weight. The weight of water equals the weight of dry soil solids (Kakuturu et al., 2019).
     
    Plastic limit
     
    The soil plastic limit was conducted as per the rules of ASTM D4318. The water content at which the soil no longer possesses plasticity is known as the plastic limit of soil. A medium size of fine grained soil is rolled into threads until they reach a diameter of 3 mm during this test. The time at which soil pulverises and quantity of water soil content is determined to determine plastic limit (Imtiyaz et al., 2023).
     
    California bearing ratio (CBR)
     
    The CBR test is a laboratory method to determine the bearing capacity or mechanical strength of the subbase/subgrade of a road or highway. The test was carried out according to ASTM D1883 (Yildirim and Gunaydin, 2011).

    RESULTS AND DISCUSSION

    Chemical characteristics of Al-Samawa sandy soil
     
    As shown in Table 3, Al-Samawa sand dune soil is mostly siliceous with SiO2  at 77.76%. Because of this, it essentially very rich in quartz. This composition is typical for desert sand, which displays low natural cohesion and weak bonding between particles. CaO (9.27%) and Fe2O3 (3.92%) act as minor cementing agents. These alone have an insufficient effect on the loading mechanical strength.

    Table 3: Results of chemical tests for sand dune.


           
    The mechanical reinforcement is justified due to the chemical characteristics of the sandy soil. This type of sandy soil has low bearing capacity, erosion and excessive deformation. Desert type applications are subgrade or embankment. According to the soil classification map in Fig 1 for Al Muthanna Governorate where Al Samawah is situated, it can be seen that Gelic Andosols dominate large areas of the desert with Eutric Regosols prominent near river systems. Soils of the type that are geotechnically problematic, are expansive clays and loose.

    Fig 1: Soil classification map of Al Muthanna Governorate showing Gelic Andosols and Eutric Regosols in Al Samawah area.


           
    Sands that are low in clay are common in central and south Iraq. The first step of the criteria was the sampling of representative soil from the desert area in Al Samawah in this regional context. The essential features of this soil were established and recorded to serve as a baseline for the further test. After that, a series of controlled laboratory experiments were conducted to assess the effect of plastic strip and desert soil mixture. The study aimed to examine the influence of plastic reinforcement on specific gravity, compaction, moisture content and bearing capacity of soil to mitigate soil map geotechnical problems. The specific gravity of sandy soil may vary between 2.63 and 2.67. The density of soil particles is denser than that of water is the specific gravity. The weight volume relationship of soil is an important parameter in soil mechanics. Soil that is evaluated and tested for engineering purposes is very useful. The addition of plastic strips in sandy soil was used for construction in the study by (Al Shafian et al., 2023).
           
    Prior research has shown that plastic waste, such as strips from used plastic bottles, can improve soil performance. For instance, shear strength can be improved, desiccation cracking can be reduced, etc. The specific gravity of the soil was not influenced by plastic strips to a great extent. However, the strips enhanced the mechanical properties of the soil to a great level especially the compaction behaviour and bearing capacity of the soil.  In past findings, binders provided better structural performance in concrete and bituminous pavements, which reiterates this finding. This stabilization method not only enhances engineering but also contributes to plastic waste. A sustainable and cost effective solution for construction applications is provided by management. The soil’s specific gravity is seen to increase as the percentage of plastic strips increases as seen in Fig 2. The specific gravity was found to be 2.65 at 0% plastic content while it achieved the maximum of 2.75 at 15% plastic content. The slight increase shows the increase in soil density after laying the strips but not drastically altering it.

    Fig 2: Effect of plastic percentage of specific gravity.


           
    Bioengineered strips do enhance the soil engineering properties. Fig 3 displays the moisture-density relationship of dune sand determined by Proctor compaction test along with the OMC and MDD. The environmental impacts were also considered here. Process strengthens the sandy soils and disposal of plastic waste and also balances the environment by reusing plastic waste in soil stabilization. Increasing the percentage of plastic strips from 0-15% increases specific gravity from 2.65 to 2.75. Although the addition is not considerable, the use of plastic strips does not greatly change the soil density. The soil’s mechanical performance is enhanced positively. Same basic properties but improved soil performance can be achieved using plastic strips. Soil that used strips of waste plastics showed better compaction and bearing capacity. It can offer a sustainable alternative to plastic waste. Soils with high specific gravity are strong enough to be used for making roads, foundations and other construction works. The specific gravity of a soil is one of the soil quality parameters. Density of soil particles refers to water. Roy and Dass (2014) state that higher specific gravity increases cohesion and angle of shearing resistance, which are two of the shear strength parameters of soil. In addition, the (CBR) will be enhanced due to the improved specific gravity of the subgrade material. The subgrade strength is commonly measured using a CBR test (Ramasubbarao and Siva, 2013). Simultaneously, compacting the soil is extremely beneficial to the soil. The application of mechanical energy rearranges soil particles and diminishes the void ratio by the expulsion of entrapped air and increases dry density. Soil compaction enhances the engineering behaviour of soil irrespective of moisture contents. Benefits includes improved shear strength, lesser compressibility and lesser permeability. Two essential soil properties useful to assess the the ability of soils for engineering application are compaction and specific gravity. They are also useful to assess the stability and durability of soils.

    Fig 3: The relationship of the dry density of soil with water content (control).


     
    Maximum dry density (MDD)
     
    Table 4 reports that the control sample displayed (sand + 0% strips) has an MDD of 16.3 kN/m3. Nonetheless, the MDD for the 5% strips fell slightly to 16.02 kN/m3. The initial addition of plastic disrupts the packing of particles, as such shows the outcome. When the compressive stripping marginal density was 10%, interlocking and compaction efficiency improved to about 16.4 kN/m3. As the plastic increased in 15% strips, the MDD decreases marginally to 16.25 kN/m3. This means that excess use of plastic hinders achieving maximum compaction.

    Table 4: Summary of proctor compaction test results.


     
    Optimum moisture content (OMC)
     
    The (OMC) was 13.1% and this value increased progressively with the addition of plastic: reaching 17.5% at a 5% plastic content, 18.9% at 10% and 19.4% at 15% (Table 4). The soil’s moisture retention capacity increases with the addition of plastic strips, as these strips create additional voids and facilitate a uniform distribution of moisture within the soil. The slight increase in (MDD) at a 10% plastic content indicates an optimal level of reinforcement, where maximum contact between soil particles and plastic strips is achieved. It is worth noting that soils requiring a higher OMC necessitate the addition of water to ensure adequate compaction; this enhances the reinforced soil’s resistance to desiccation cracking and improves its durability under field conditions. The use of plastic waste in soil stabilization involves utilizing plastic strips for improving geotechnical properties of soil. Further, the use of plastic strips helps manage the plastic waste effectively. Studies show that plastic strips enhance the compaction behavior and shear strength of soils. Use only required plastic strips as excess amount disturbs arrangement of soil particles and makes it less dense.
           
    Research has consistently highlighted the beneficial role of waste plastic (PET) strips in soil stabilization. (Tejeswini, 2013) reported that their inclusion enhances compaction characteristics, while (Bozyigit et al., 2021) demonstrated that the addition of plastic strips improves overall earth stability, showing promise in subgrade reinforcement. Similarly, (Patidar and Mahiyar, 2014) observed that compaction curves shift noticeably when soils are treated with plastic strips, with the optimal reinforcement level producing the highest dry density and reduced swelling. Collectively, these findings indicate that incorporating PET strips at an ideal proportion of 10% yields significant improvements in dry density and moisture content. More water may be required by higher percentages but may reduce compaction. According to a recent geotechnical study, the technique is economically feasible and environmentally friendly for an effective soil stabilization. By adopting this approach, sandy soils gain improved engineering properties while simultaneously contributing to plastic waste management. he compaction curve of dune sand (Fig 3), which plots dry density against moisture content, clearly identifies the optimum moisture content corresponding to the maximum dry density, in accordance with Proctor compaction theory. Incremental additions of water enhance soil strength by lubricating particles, allowing efficient rearrangement and higher density. Once the optimum point is reached, however, surplus water begins to occupy soil voids and displace air, leading to a decline in density. As emphasized by Roy and Dass (2014), these parameters are fundamental in evaluating soil suitability for construction. The figure illustrates this relationship, highlighting how moisture facilitates particle rearrangement and compaction until the optimum condition is achieved, beyond which excess water reduces effectiveness.
           
    Collaboration has focused on recently published research works concerning Sikkim. The soil’s (MDD) makes it less likely to destabilize the foundation or pavement due to settlement. When the void ratio decreases, the soil particle’s interlocking increases soil’s shear strength. In addition, good compaction reduces permeability and compressibility that confirm the reliability of dune sand for geotechnics. According to the study, dune sand at OMC can work as subgrade material when compacted. The suggestion was made to use plastic strip reinforcement. The study shows that proctor test gives the upper limit of MDD and OMC i.e. moisture density relation in the soil due to compaction rearrangement of soil particles filling in the voids region which improves their engineering properties like shear strength, compressibility, etc.
           
    In addition, preventing the settlement, cracking and instability of embankments, subgrades and foundations is the practical significance of compaction. The crucial importance of all these aspects rests in maintaining one’s durability and stability of the project.  As shown in Fig 3, the relationship between moisture and density of dune soil is such that the dry density initially increases with increase in water content. This is caused by the effect of moisture, which acts as a lubricant and allows particles to reorganize. Soil’s optimum moisture content is 13% to 14%. A maximum dry density of 16.4 kN/m3 is achieved at a specific moisture content. Adding excess water after this moisture causes the voids to fill while the air is pushed out. Therefore, this decreases the dry density.  This sandy soil shows classical compacting behaviour. In sample 1, which contains 5% plastic strips, MDD slightly decrease while OMC slightly increase as shown in Fig 4. The plastic material has a low specific gravity which causes a reduction in dry density. As a result of this, a lighter component has replaced part of the soil mass in the bulking agent. Chu et al. (2021) reported similar observations with a difference of opinion regarding lower packing efficiency being responsible for lower dry density values. At this reinforcement level, the plastic strips are uniformly arranged and serve as randomly dispersed tensile reinforcements and enhance the inter-particle restraint without significant disturbance of the skeleton soil. Tang and colleagues found that these fibers could also enhance ductility and increase tensile resistance through similar reinforcement. In Sample 2 having 10 plastic strips Fig 5, there was a steep decline in MDD and an apparent increase in OMC. When the content of plastic increases, the void ratio increases which disturbs the soil to soil contact making it less compact. Plastic or polymeric inclusion’s increasing in soil matrices will usually lead to a reduction in dry density and an increase in moisture demand because of enhanced surface area and less interlocking of particles. With a drop in MDD, soil’s tensile resistance and deformation control enhanced. This indicates suitable balancing between compaction characteristics and reinforcement benefits. The research of Sivakumar (Buzzi et al., 2011) found that by moderate inclusion of fiber soil performance improved considerably without causing much degradation of density.

    Fig 4: Relation between soil’s dry density and water content (sample 1).



    Fig 5: Relationship between soil’s water content and dry density (sample 2).


           
    The proctor test indicates that MDD and OMC are the guideline values of soil (Roy and Dass, 2014) and compaction rearranges the particle structure of soil and improves its engineering properties. Adequate compaction is critical to prevent settlement, cracking and instability in embankments and foundations (Ran et al., 2024), aligning with the present study. The compaction behavior of sandy soil reinforced with recycled plastic strips was evaluated using the Standard Proctor test (ASTM D698, BS1377 Part 4, IS 2720 Part 7). Results show that varying reinforcement percentages influence (MDD) and (OMC). The typical bell shaped curve of unreinforced sandy soil reflects cohesionless behavior under standard energy. Efficient particle rearrangement accounted for the control soil’s compaction response. However, Sample 3, reinforced with 15% plastic strips, exhibited the lowest MDD and highest OMC (Fig 6). At such a high percentage of plastic inclusion, the granular structure of sandy soil gets disturbed. Overlap and clustering of plastic strips reduced effective stress in soils and increased internal voids and thus poor packing efficiency. If reinforcement exceeds the optimal level, it adversely affects compaction behavior and load-carrying capacity.    

    Fig 6: Relationship between soil’s water content and dry density (sample 3).


           
    Adding more plastic to the mix boosts ductility and crack resistance but it also lowers dry density significantly making it unsuitable for highly compacted and strong applications. Findings of the study concluded that with increase in plastic strip content maximum dry density goes on decreasing. Also Increase in plastic strip content and optimum moisture content has direct relationship which has also been reported in earlier studies on waste plastic-reinforced soils (Deng et al., 2025). The research found that with 5% to 10% content of plastic strip there obtained optimum range for reinforcement benefit without losing compaction efficiency too much. If the level of plastic material exceeds the range, engineering performance is adversely affected. Recycled plastic strips can be applied as soil reinforcement material to an extent and it is said to enhance the geotechnical performance of soil. Also, it provides a way to use plastic waste appropriately (Hamard et al., 2016). In the following figure, the MDD of the natural Al-Samawa sand dune soil, soil reinforced with 5% (sample one), 10% (sample two) and 15% (sample three) plastic strips are shown. As per study results, it was found that the plastic strips can noticeably impact the densification potential of sandy soil and, subsequently, the (CBR) and shear strength behaviour. The MDD obtained from the test is approximately 16.4 kN/m3. This is due to the loose structure of sand dune soil (Fig 7), which is held together by insufficient bonding. A soil with such characteristics will generally exhibit low values of CBR and shear resistance. Hence, it would not be appropriate to use it as a pavement subgrade and as a foundation layer without improvement.

    Fig 7: Effect of additive to sand dune on maximum density.


     
    Effect of 5% plastic strips (Sample one)
     
    MDD was reduced slightly when compared to natural sand dune soil with 5% of plastic strips. The unit weight is reduced because of the lesser unit weight of plastic which only partially replaces soil particle while slightly increasing voids. There is a slight reduction in density; however, this does not indicate a reduction in performance from CBR  and shear strength angles. Plastic strips function as tensile reinforcement that improves abilities to hinder particle movement. As a result, there would be slight improvement in shear strength and CBR and slight decrease in dry density due to frictional resistance and re-distribution of stress (Choudhary et al., 2010).
     
    Effect of 10% plastic strips (Sample two)
     
    Among the soils tested, the maximum dry density of sample 2 is highest ≈17.0 kN/m3. The addition of 10% plastic strips improves the characteristics of sandy soil of Al Samawa. The use of strips of plastic material here restrict particle movement and lead to better packing and densification during the compaction process. The CBR value and shear stress strength increase as the maximum dry density of a material increases. Higher density reduces compressibility and improves inter particle contact while plastic strips offer additional tensile resistance. Thus, sample two must show better bearing capacity, superior load distribution and the highest values of shear strength parameters, which makes it most suitable for pavement sub-grade and construction of embankments.
     
    Effect of 15% plastic strips (Sample three)
     
    The maximum dry density of 15% plastic strips is lower than Sample two. Reduced samples show that an increase in plastic content fractures the soil skeleton which increases void ratio and reduced packing efficiency but is still higher than sample one. According to CBR and shear strength, if a large number of plastic strips are added to the soil, the strain of soil particles will not be transferred properly and consequently the strength will not increase although ductility will be increased. The resistance to deformation of the intensified soil use may improve but its lesser density may not enhance the CBR and shear strength in that proportion and they may actually drop in comparison to the ideal 10%. Relationship between density, CBR and shear strength. Primarily confirming a geotechnical criterion, trend shown in Fig 6. The maximum dry density of sandy soils significantly influences other index properties.
           
    It is evidenced from the experimental results that when Al Samawa sandy soil was reinforced with recycled plastic strips, its compaction behavior, moisture sensitivity and strength behaved like a fibre reinforced soil. The native sand dune soil’s very low density and weak bonding contributed to its poor CBR and shear strength which suggest such soils cannot be used for engineering applications. By adding 5% plastic strips there is moderate improvement and 10% reinforcement has shown maximum density and maximum strength this percentage can be considered optimum. If the inclusions are more than 15%, the packing of the particles will be disturbed, which will lower the dry density and strength efficiency; thus always require a reinforcement in an effective range. The researchers said that dry density decreased early due to use of plastic strip for reinforcing. Afterwards, the tensile restraint leads to a strength improvement.
           
    Furthermore, the fibers have a positive impact  because they provide ductility enhancement. Moreover, they assist in postponing the breakdown of the material. Observed maximum behaviour is at 10% reinforcement which agrees with statement given by Banerjee et al., (2002) moderate addition of fibre produces the best compaction efficiency and strength. The reduction in performance at 15% reinforcement reinforces (Wang and Yejiao, 2024) findings that excessive quantity of polymeric inclusions increases voids quantity but decreases effective stress transfer capacity. In addition to the mechanical impacts, the environmental effects are also significant. The use of discarded PET bottles can solve two current problems soil stabilization and plastic waste problem. The present study shows that the use of plastic ensures better strength and durability of soil and offers an eco-friendly solution to environmental pollution. The use of plastic strips made from recycled materials found in sandy soils is a practical and sustainable solution for constructing infrastructures in dry regions like Samawah in Iraq. According to the results, 10% plastic strip reinforcement is an effective addition to the Al-Samawa sandy soil as it improves its compaction and strength characteristics. Application of 10% plastic strip is recommended to improve the sandy soils at Al-Samawa according to relationship between maximum dry density, CBR and shear strength. The enhancement level upgrade improves carrying power and shear resistance, while also having good compaction characteristics. It can be used below roads, in embankments and deserts. In sandy soil of Al Samawa, the mechanical efficacy of the plastic strip utilized increased with the rise in inclusion content from 5% to 10% and then declined again at 15%. Literature has similar observations (Mardani et al., 2025) found that random distribution of plastic fibres increases the CBR and increases the shear strength significantly up to some optimum content after which the improvement remains constant. For the 5% strips, the interlocking and surface shear have started to mobilise along the inclusions as can be seen in Fig 8. This leads to a moderate increase CBR and peak shear over the unreinforced sand. The study by Wei et al., (2018) found that when small amounts of synthetic fibres are introduced into sandy matrices, the effect is an increase in the apparent cohesion. Also, this action further stabilizes the failure zone of the soil mass and delays the process of localization inside the soil matrix.  The improvements which are observed in Al Samawa sand are at the early stage. Raising the strip content to 10% usually brings the system close to optimum. Restraint networks become continuous, lateral dilation is efficiently curtailed and stresses are better distributed. According to Bezih et al., (2025), the presence of fibers at intermediate contents leads to an increase in the maximum value of the (CBR) and the shear strength of sandy soils. This improvement happens because of the fiber components’ effect on both the apparent cohesion and slight enhancement on friction angle. In Iraqi sand, PET strip reinforcement yield peak strength around 10% inclusion, confirming optimal behaviour, because 100% PET reinforcement produced much weaker sands. When the density reaches 15%, the soil structure and function of the compacted soil are disrupted, leading to micro voids and uneven load paths. Even though the  performance continue to remain higher than the baseline, the CBR and peak shear tends to saturate or decrease with respect to the 10% level. Excess fiber content can disrupt soil structure and reduce strength, while moderate reinforcement enhances interfacial shear and ductility. In the present study, reinforcing Al Samawa sandy soil with recycled plastic strips significantly improved compaction, moisture sensitivity and strength. The soil, being predominantly siliceous with low natural cohesion, benefited from the mechanical reinforcement of PET/HDPE strips, which increased specific gravity, shear strength and CBR values while reducing swelling and cracking. Compaction tests showed that dry density decreased at 5%, peaked at 10% and dropped again at 15%, confirming 10% as the optimum reinforcement level for effective particle interlock. Higher contents beyond this threshold disrupted packing, increased voids and reduced efficiency. Moisture density curves further indicated that reinforcement raised the optimum moisture content, enhancing resistance to drying cracks. Overall, 10% strip inclusion provided the best balance of strength, durability and compaction, addressing both soil instability and plastic waste management.

    Fig 8: Effect of additive to sand dune on minimum density.

    CONCLUSION

    The changes of  properties of recycled plastic strip (PET and HDPE) reinforced Al-Samawa sandy soil were studied in this investigation. Adding plastics enhances shear strength and California Bearing Ratio (CBR), decreases cracking and swelling. The result is that the percentage of strip content of 10% is the best balance for soil stability. Beyond this percentage soil density reduction occurs. In the end, recycled plastic proves to be a cheap and effective option to stabilize weak desert soils while preventing waste.

    ACKNOWLEDGEMENT

    The authors acknowledge with thanks the Ministry of Housing and Construction and the Centre for Environment, Water and Renewable Energy for their guidance. The laboratory staff is also acknowledged for technical assistance, while academic colleagues and peer reviewers are recognized for their input towards the research.

    CONFLICT OF INTEREST

    The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this manuscript.

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