1 Introduction

Disposal of waste materials is becoming a predominant issue for most countries in the world. The accumulation of these waste materials in huge quantities is causing both environment and financial problems. According to Awuchi (2019), the average plastic waste generation is estimated at 15.4 billion pieces per day. The most prevalent waste materials are plastic waste materials. These materials are the most usable material types in our daily life. Plastic wastes are produced in massive quantities such as plastic bottles, which are made from polyethylene terephthalate (PET), and plastic sack and carpet, which are made from polypropylene (PP). Polyethylene products are considered to be the major components of waste materials in Kurdistan/Iraq. Hanna and Mosa (2012) reported that in Duhok City, more than 5000 tons of polyethylene (500-cm3 bottle capacity) is produced as waste materials every year. Despite its benefits in our daily life, plastic pose negative impacts on the environment and health. Since the majority of plastics are not biodegradable and can sustain for many years, many countries are planning to decrease or prevent the effect of plastic materials through efficient recycling and reusing these materials in a wide range of fields.

Many researchers have carried out studies to find effective methods to reduce the pollution of these materials including recycling and reusing these materials in civil engineering applications as a solution to preserve the environment from the pollution of plastic waste materials. An effective method to utilise these materials is to be used as a soil stabiliser for road construction (Tatone et al. 2018). Traditional soil stabilisers such as cement and lime are widely used for improving the geotechnical properties of weak soils (Sherwood 1993; Yadav et al. 2018 and Yadav and Tiwari 2016). The effectiveness of these materials on improving the properties of soils is confirmed by various researchers (Bell 1996; Little 1995; Rout et al. 2012; Rasul et al. 2015; Rasul et al. 2016; Yadav and Tiwari 2017; Rasul et al. 2018). However, the high usage of these materials makes them non-cost-effective (Obo and Ytom 2014). Therefore, many researchers attempt to find alternative cost-effective soil stabilisers such as plastic, tyre chips, and rice husk.

Using plastic wastes for soil stabilisation can improve the foundation layers of pavement (Khattab et al. 2011). Thus, this can solve the problem of wastes by reducing the quantities and recycling these materials for enhancing the properties of soils. One method of using plastic for soil stabilisation is to use the plastic in the form of discrete fibres (Yetimoglu and Salbas 2003), because when plastic materials are merged with soils, they behave similar to fibre-reinforced soil. Several researches have been conducted to investigate the effectiveness of plastic waste materials in the form of discrete fibres on properties of soils (Ziegler et al. 1998; Babu and Chouksey 2011; Mondal 2012; Ahmadinia et al. 2012; Modarres and Hamedi 2014; Fauzi et al. 2015; Changizi and Haddad 2015; Rawat and Kumar 2016; Peddaiah et al. 2018; Salimi and Ghzavi 2019). These researchers found that using plastic waste materials for soil stabilisation will improve the properties of weak soils such as an increase in UCS, CBR, and Mr and a decrease in the soil plasticity.

This study investigates the effect of using plastic waste as fibre-shaped waste materials on geotechnical properties of soils. A number of standard geotechnical laboratory tests were performed in order to identify the effect of length and contents of fibres that are randomly distributed throughout the soil. These tests consisted of soil index properties, standard compaction test (proctor test), unconfined compressive strength test, California Bearing Ratio test, and resilient modulus tests.

2 Experimental Programme

2.1 Materials and Methodology

2.1.1 Soil

The soil used in this study was collected from the Hawshki area, West of Duhok City, Kurdistan Region, Iraq. The sample is divided into four equal parts by removing two diagonally opposite parts and then mixing two remaining parts properly. The physical properties of the soil are shown in Table 1; this soil is classified as clay soil according to the American Society for Testing and Materials (ASTM) with about 93% silt and clay.

Table 1 Physical properties of the native soil
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2.1.2 Plastic Waste Material

In this study, polyethylene terephthalate (PE) (water bottles) and polypropylene (PP) (woven polypropylene bags) wastes were used as fibre stabilisers as shown in Fig. 1. Plastic fibres were prepared by cutting waste bottles and bags into two sizes in lengths of 1.0 and 2.0 cm and in widths of 2.5 to 3.0 mm each as shown in Fig. 2. The fibre contents were applied at 1%, 2%, 3%, and 4% of dry weight of the clayey soil.

Fig. 1
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Plastic waste materials used in the study

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Fig. 2
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Sample of the fibres used in this study

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2.2 Laboratory Tests

In this study, to examine the strength and mechanical properties of both native and stabilised soils, different laboratory tests were conducted including the determination of index properties of unstabilised soil and standard proctor compaction, UCS, CBR (soaked), and Mr for stabilised and unstabilised soils. All tests were conducted according to ASTM, except resilient modulus, which is conducted in accordance with AASHTO T307. For all tests, the adopted content of fibres was first manually mixed into the air-dried soil in small increments. Considerable care was taken to achieve homogeneous mixture during the mixing process. Then, the required water was added as shown in Fig. 3.

Fig. 3
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Mixing the fibres with soil

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2.2.1 Compaction Test

The laboratory compaction tests were performed in accordance with the ASTM D698 procedure. The purpose of this test is to determine the effect of plastic fibre on optimum moisture content (OMC) and maximum dry density (MDD) of stabilised soils and to use OMC that was obtained in this test for preparing samples for UCS, CBR, and Mr tests, dry density, and optimum moisture content.

2.2.2 Unconfined Compressive Strength Test

This test was conducted to investigate the effect of inclusion of plastic waste pieces on the unconfined compressive strength of soils. The test was performed in accordance with ASTM D2166. All the UCS test specimens were prepared at their respective MDD and OMC. All prepared specimens were air-dried in the open laboratory environment and were cured for 7 days.

2.2.3 California Bearing Ratio Test

For the CBR test, cylindrical specimens were prepared using their MDD at OMC in a rigid metallic cylinder mould with an inside diameter of 150 mm and a height of 175 mm. A mechanical loading machine equipped with a movable base that moves at a uniform rate of 1 mm/min and the calibrated proving ring is used to record the load. For this, static compaction is carried out through keeping the mould assembly in the compression machine and compacting the soil by pressing the displacer disc till the level of the disc reaches the top of the mould. This test was carried out in accordance with ASTM D1883. In this study, the soaked specimens were made at OMC as determined from the standard compaction test.

2.2.4 Resilient Modulus (Mr)

The repeated load from a triaxial test has also been conducted in this study to investigate the effect of plastic material stabilisers on resilient modulus values that represent the mechanical property of the soils. This parameter is significantly important to assess the performance of material under the repeated load of moving vehicles. In this test, a series of repeated loads are applied to the soil samples including the rest period. This test is conducted by using an ELE triaxial device and in accordance with the AASHTO T307 testing protocol as this standard is mostly used for determining resilient modulus in the laboratory. The haversine type of load is used with duration of a 0.1-s and 0.9-s recovery period. The sample dimensions are 100 mm in diameter and 200 mm in height. The curing period for all samples were 7 days. AASHTO T307 standards recommend three confining stresses in three cycles (41.36, 27.57, and 13.78 kPa) and five deviatoric stresses (13.79, 27.58, 41.39, 55.161, and 68.95 kPa).

The details of the sequence and cycles can be seen in Table 2.

Table 2 Mr test loading sequence
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3 Results and Discussion

3.1 Compaction Test

Figures 4, 5, 6, 7, 8, 9, 10, and 11 show the results of the compaction tests for the native soils and soils stabilised with PE and PP stabilisers of 1.0 cm and 2.0 cm in length at different stabiliser content ratios. For the native soil, the OMC was 21% and the MDD was 1645 kg/m3.

Fig. 4
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Proctor test results for 1-cm length of PE at different stabiliser contents

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Fig. 5
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Proctor test results for 2-cm length of PE at different stabiliser contents

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Fig. 6
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Effect of the PE ratio on OMC

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Fig. 7
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Effect of the PE ratio on MDD

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Fig. 8
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Proctor test results for 1-cm length of PP at different stabiliser contents

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Fig. 9
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Proctor test results for 2-cm length of PP at different stabiliser contents

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Fig. 10
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Effect of the PP ratio on OMC

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Fig. 11
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Effect of the PP ratio on MDD

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As it can be seen in the compaction test results, the OMCs and MDDs decrease for the stabilised soils compared to the native soils. The decrease in both of these two properties can be noticed as the fibre content increases. These can be seen clearly in Figs. 6 and 7. This consistent trend of effect of fibre content increase cannot be seen for the stabilisation with PP fibres (see Figs. 8, 9, 10, and 11).

For the soils stabilised with PE at various fibre contents of 1.0 cm and 2.0 cm in length, the results demonstrated a similar trend in terms of reduction in OMC. The results revealed that by increasing the PE percent, OMC was decreased. The reduction value for 1.0-cm and 2.0-cm-length PE is from 20% for 1% PE content to 17% for 4% PE content.

Similarly, the MDD for both lengths at all fibre contents showed reduction in a value with the increase in the PE percent. The highest decrease was obtained at 4% fibre content for both lengths by 55 kg/m3. These data clearly state that by adding more PE percent, both OMC and MDD values decrease; also, if the length of PE increases from 1.0 to 2.0 cm, the OMC decreases more as shown in Figs. 6 and 7, except for MDD at 3% showing an odd trend. These results were similar to those obtained by Bala (2013), Nsaif (2013), Dhatrak and Konmare (2015a, b), and Kumar et al. (2018). Kumar et al. (2018) cut the PE into 1.0 cm, 2.0 cm, and 3.0 cm lengths at various fibre contents of 0%, 0.2%, 0.5%, and 0.8% and 1.0% of the dry weight of soil. They found that as the length and content of the plastic increase, the value of MDD decreases. They concluded that the highest reduction was found when plastic content was 1% of the dry weight of the soil and the optimum length of plastic strip inclusion was 3.0 cm. Nsaif (2013) investigated the effect of the plastic wastes that were made from plastic bottles with fibre contents of 0, 2, 4, 6, and 8% of the dry weight of the soil on the behaviour of the stabilised soil. Nsaif (2013) found that the increase in plastic content decreases both OMC and MDD of soils. The highest decrease was found at 8% plastic content.

Soil stabilised with PP at various fibre contents of 1.0 cm and 2.0 cm in length showed different behaviour in terms of OMC and MDD. It was found that for 1.0-cm length, MDD at 1% and 2% fibre content was decreased by 35 and 20 kg/m3, respectively. However, after that, by increasing the fibre content with 3% and 4%, MDD was increased by 20 and 30 kg/m3, respectively. For 2.0-cm fibre length, MDD was decreased with the increase in PP contents. The highest decrease was at 4% fibre content by 25 kg/m3. Taha et al. (2020) investigated the effects of polypropylene fibre (PF) 12.0 mm in length on the mechanical behaviour of clayey soils. They mixed the soil with 0%, 1.5%, 2.25%, and 3% of PF content by the soil weight. Their study concluded that the increase in fibre content results in an increase in MDD and a decrease in OMC with an optimum fibre content of 3%.

For 1.0-cm fibre length, 2% fibre content showed an increase in OMC with 2%. However, for other fibre contents, no significant changes in OMC occurred. For 2.0-cm fibre length at all fibre contents, OMC increased very slightly.

Generally, these data reveal that the inclusion of plastic waste materials in soils decreases the OMC. This is due to the fact that plastics are not absorbent materials compared to clay soils, which have high affinity to water due to its surface tension (Nsaif 2013). Regarding MDD, the results show no improvement in MDD after the addition of PE. This could be attributed to the low specific gravity of PE (Puppala and Musenda 2007; Gardete et al. 2019). This could be an advantage for using PE as a stabiliser to be one of the components for construction of embankments. However, improvements were found by the addition of PP with 3% and 4% fibre contents. This could be attributed to ease in mixing with soil, which behaves as multifilament fibre during the mixing action (Olgun 2013). Therefore, PP may be more efficient than PE in terms of its impacts on distribution and bonding of soil particles. In addition, the decrease in MDD will be due to the low specific gravity of fibres (Nsaif 2013).

3.2 Unconfined Compressive Strength (UCS) Test

Table 3 shows the results of UCS; as it can be seen, the addition of PE and PP fibre increased the stabilised soil strength considerably compared to the native soil strength, which was 148 kPa. However, this increase in UCS has an optimum based on the ratio of fibre content, as can be seen in Fig. 8, in which the increase in fibre content is up to 1%; thereafter, the curve flattens and continues at the same strength with the increase in fibre content. The comparison between PE and PP shows that the former has higher UCS values than the latter for 1.0-cm and 2.0-cm lengths of the fibre. In all cases, the 2.0-cm-length fibre results are higher than 1.0-cm-length fibre results.

Table 3 UCS test results for PE and PP fibres
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For soil stabilised with 1.0-cm and 2.0-cm-length PE fibre, the optimum fibre content was 1%. The value of UCS increased from 148 kPa (native soil) to 261 kPa (1.0-cm fibre length) and 291 kPa (2.0-cm fibre length) with an improvement by 76.4 and 96.6%, respectively. The lowest increase in UCS was at 4% fibre content for 1.0-cm fibre length and 2% fibre content for 2.0-cm fibre length.

Similarly, for soil stabilised with 1.0-cm and 2.0-cm lengths of PP fibre, the optimum fibre content was 1%. The value of UCS increased from 148 kPa (native soil) to 233 kPa (1-cm fibre length) and 256 kPa (2.0-cm fibre length) with an improvement by 57.4 and 73%, respectively. The lowest increase in UCS was at 4% fibre content for 1.0-cm fibre length and 2% fibre content for 2.0-cm fibre length.

Muntohar (2009) stated that when soils were stabilised with fibres, the fibre transferred the applied load to the frictional interface between soil particles and fibres. As fibre contents increase, the interfaces between soil and fibres increase, and this leads to increasing the friction between soil particles and fibres (Olgun 2013). This makes it difficult for soil particles that are surrounding the fibres to change their position and thus enhance the soil cohesion between soil particles (Muntohar et al. 2013). In addition, the high tensile strength of fibre plays a significant role in making the soil to withstand more loads and increase its UCS (Tang et al. 2007a, b). It is known that PE has higher tensile strength than PP; therefore, the UCS of soils stabilised with PE is higher than that of soils stabilised with PP. AlAfandi (2015) used polyethylene waste material (water bottles) in the form of fibres with cement to enhance the compressive and tensile strength of clayey soils. The fibre contents consisted of 0.4%, 0.8%, and 1.2% of the soil dry weight, and the fibre lengths were 1.0 cm, 2.0 cm, and 3.0 cm. He found that soil stabilised with fibre was more efficient in the unconfined compressive strength (UCS) than in the tensile strength. The optimum fibre content and fibre length were 1.2% and 2.0 cm, respectively.

It can be seen from Table 4 that the increase in fibre content increases the UCS up to specific fibre content and then decreases. According to Naeini and Sadjadi (2008), the increase in fibre content above a specific percentage leads to slippage of fibre panels over each other and causes soil particles to separate, thus, it decreases the strength of the soil. The UCS results clearly showed that longer fibre lengths (2.0 cm) have higher increase in strength than shorter fibre lengths (1.0 cm). Many researchers have studied the effect of PE on UCS of soils with similar results (Puppala and Musenda 2007; Naeini and Sadjadi 2008; Ghorbani et al. 2018; Oliveira et al. 2018; Zukri et al. 2017; Pal et al. 2015; Sharma 2017; Franco et al. 2014; Louzada et al. 2019; Sai and Srinivas 2019). Their results revealed that the addition of PE and PP significantly increases the UCS of soil. However, the study shall optimise the percentage of fibre content; in this study for example, the strength decreases as the fibre content increases from 1% to 2% fibre content; therefore, a smaller percentage fraction can be tried such as 0.5% increment to find the optimum fibre content, which gives the highest UCS values (Fig. 12).

Table 4 CBR test results for PE fibres of 1 cm and 2 cm in length
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Fig. 12
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UCS test results for PE and PP with 1-cm and 2-cm lengths at different fibre contents

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3.3 California Bearing Ratio (CBR) Test

Table 4 shows the results of the CBR test. The CBR value for the native soil was 4%. As it can be seen, the addition of PE and PP fibre to the clayey soil increased the strength considerably compared to the native soil strength. In addition, this increase in CBR can be noticed with an increase in the fibre content ratio for both types of stabilisers and fibre lengths. The comparison between PE and PP shows that the first one has higher CBR values than the second one for 1.0-cm and 2.0-cm lengths of the fibre. Figures 13, 14, 15, and 16 show the profile of penetration versus the load in the CBR test increasing CBR by 55% (from 4.0 to 6.2) and 80% (from 4.0 to 7.2), respectively.

Fig. 13
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CBR test profile for 1-cm length of PE

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Fig. 14
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Test profile for 2-cm length of PE

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Fig. 15
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CBR test profile for 1-cm length of PP

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Fig. 16
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CBR test profile for 2-cm length of PP

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For the soil stabilised with PE, the addition of 1.0-cm and 2.0-cm lengths of PE fibre increased the CBR values by 55% (from 4.0 to 6.2) and 80% (from 4.0 to 7.2), respectively. And for the soil stabilised with PP, the addition of 1.0-cm and 2.0-cm lengths of PP fibre increased CBR by 42.5% (from 4.0 to 5.7) and 50% (from 4.0 to 6.0), respectively. These results clearly revealed that the fibre content and fibre length have significant effects on CBR values, and this was also confirmed by Ashraf (2011). The increase in the CBR value with the inclusion of plastic fibre is generally due to the soil and fibre interactions as it provides resistance to the penetration plunger; and consequently, increases in the CBR value (Neopaney et al. 2012; Yetimoglu and Salbas 2003) are also confirmed through this behaviour. Choudhary et al. (2010) found that the stabilisation of soils with polyethylene fibres improved CBR values three times than the non-stabilised soils. Fletcher and Humphries (1991) stabilised soil samples with polypropylene fibres and found that CBR of the stabilised soil was highly improved especially at longer fibres. Also, Madavi and Patel (2017) concluded that 4% is the optimum content of plastics for obtaining the highest CBR value. Kumar et al. (2018) investigated that 2.0-cm fibre length is the optimum fibre plastic length to obtain the maximum CBR value. This increase in CBR values of subgrade soils can have significant impact on required foundation thicknesses, especially for those pavement design methods such as Design Manual for Roads and Bridges (DMRB) of Highway England, in which the thickness of the pavement foundation is depending on the CBR and modulus of elasticity of subgrades. The increase in subgrade CBR and modulus of elasticity reduces the required sub-base thickness considerably and results in the reduction in road pavement construction costs.

3.4 Resilient Modulus (Mr) Test

Tables 5, 6, 7, and 8 show the resilient modulus test results for the native soil and stabilised soils with two different types of fibres at four ratios of fibre content. In general, the increase in deviatoric stress for coarse-grained soils results in the increase in resilient modulus, while fine-grained soils behave the opposite (Huang 1993). As can be seen from the results, the increase in deviatoric stress in most cases is followed by a decrease in resilient modulus, and a decrease in confining stress results in a decrease in resilient modulus. In all cases, the increase in fibre content resulted in the increase in resilient modulus, this shows the improvement in the behaviour of the stabilised soils, and this can also be explained in a way that the increase in fibre content increases the interconnection between soil particles and subsequent decrease in strains to the applied stresses (Yaghoubi et al. 2016). Figures 17, 18, 19, and 20 show clearly how the change in confining and deviatoric stress affects the change in resilient modulus; this is shown for different stress conditions. Other researches such as Perera et al. (2019), Arulrajah et al. (2017), and Yaghoubi et al. (2016) achieved similar trends. Hafez et al. (2018) investigated the effect of polyethylene terephthalate (PET) on the Mr of clayey soil. The percentages of PET were 0%, 0.2%, 0.6%, and 1.0%. The test results revealed that the Mr of 0.6% PET-reinforced specimens increases by 58% compared to the Mr of the control specimen (0% PET). Kaushik and Sharma (2019) studied the influence of waste polypropylene fibres on the Mr of clay soil. The polypropylene fibres were added in various contents of 0.3%, 0.4%, 0.5%, and 0.6% by the soil weight. Their studies showed that 0.4% fibre content was considered the optimum fibre content to obtain the maximum Mr value.

Table 5 Resilient modulus values of PE at different ratios for 1-cm length
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Table 6 Resilient modulus values of PE at different ratios for 2-cm length
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Table 7 Resilient modulus values of PP at different ratios for 1-cm length
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Table 8 Resilient modulus values of PP at different ratios for 2-cm length
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Fig. 17
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Resilient modulus of PE at different ratios for 1-cm length

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Fig. 18
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Resilient modulus of PE at different ratios for 2-cm length

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Fig. 19
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Resilient modulus of PP at different ratios for 1-cm length

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Fig. 20
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Resilient modulus of PP at different ratios for 2-cm length

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Generally, the results showed that the soils stabilised with PE obtained higher Mr values than the soils stabilised with PP. In addition, for both PE fibre lengths by increasing the fibre content, Mr values increased. The results revealed that 2.0-cm PE fibre length has higher Mr than 1.0-cm PE fibre length. For the stabilisation of soils with PP, Mr values increased to some extent; then, these values were decreased. The optimum fibre content was 2% for both PP fibre lengths. Studies by Ahmadinia et al. (2012) and Perera et al. (2019) revealed that by adding PE materials, Mr values increased. Perera et al. (2019) found that the addition of PE and the increasing deviatoric stress at the same confining pressure result in higher Mr values. Research by Phillip and Cameron (1995), Onyelowe et al. (2019), Hafez et al. (2018), and Ramesh et al. (2019) found that the Mr value improved with the addition of fibre plastics.

To calculate the improvement in the mechanical behaviour of the soils stabilised with fibres, a pavement section was analysed using the KENLAYER programme; for simplicity, the linear elastic method was followed. The load of the tyre was simulated to have a circular shape with a diameter of 152 mm and a pressure of 860 kPa. The proposed pavement section has dimensions and properties as presented in Table 9. In Table 9, the values of the modulus of elasticity of the asphalt and granular layers were 4000 MPa and 200 MPa, respectively, and the stabilised subgrade layer’s modulus was taken to be variable according to the different scenarios of the soil stabilisation.

Table 9 Pavement section dimensions
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The results from the analyses are presented in Table 10. As can be seen, the compressive strain at the top of the subgrade soil is improved with the increase in fibre content from 0 to 4% and with the increase in fibre length from 1.0 to 2.0 cm. The same trend can be noticed for the surface deflection, although it is not a road pavement design criterion. Since the asphalt concrete layer was taken to be constant for all scenarios, the tensile strain at the bottom of the layer is not changed. Research by Rasul et al. (2016) and Rasul et al. (2018) showed that the stabilisation with cement and lime and combinations of both resulted in a significant increase in UCS and Mr values with the increase in the stabiliser content. While in this research, the stabilisation with fibre showed an increase in UCS to a specified ratio of fibre content beyond which the UCS decreases, but there is an increase in Mr values. This can be a major difference between using chemical agents and fibres for stabilisation, in which it can be decided to choose the most suitable stabiliser for the corresponding required properties. For example, for subgrade soils in which the resilient modulus is an essential property to improve, fibre can be used, and for other purposes in which UCS is an important property, a chemical agent would be more favourable.

Table 10 Results of the analyses of the pavement section
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4 Concluding Discussion

Stabilisation considerably improves the engineering properties of soils, and this includes physical, strength, and mechanical properties. This paper has investigated these properties through an experimental laboratory test programme on two different waste materials that can be found in disposal bins, namely, polyethylene and polypropylene. The main conclusions from the study carried out herein are summarised as follows:

  1. (1)

    Soil stabilisation with fibre has different trends for UCS and Mr, as the increase in fibre content is not resulting in the increase in UCS ascendingly, while for Mr, the increase in fibre content resulted in the increase in Mr values. Therefore, optimum fibre content shall be sought for stabilisation with fibres for the highest values of UCS and Mr.

  2. (2)

    PE and PP can efficiently be used to improve the physical and strength properties of soil materials as a foundation for engineering projects.

  3. (3)

    The length of the fibre showed to have an effect on the strength properties of the stabilised soil, as the increase in length resulted in higher strength properties. This may require more research to find the optimum fibre length that gives the highest strength properties.

  4. (4)

    For those road pavement design codes of practice that use the CBR and Mr as design parameters, the fibre stabilisation is cost-effective and it can be used successfully for a sustainable road construction if compared with chemically stabilised soils. The stabilisation with chemical agents is accompanied by carbon dioxide emission, while fibre stabilisation is not; this is one of the advantages of fibre stabilisation over chemical stabilisation.

  5. (5)

    While for CBR and Mr, the increase in fibre content resulted in the increase in value, for UCS, the optimum fibre content could be found between 1% and 2% fibre content for both PE and PP.