Soil Reinforcement with Synthetic Fibers and Plastic Waste Materials: A Review

The soil reinforcement technique is used to improve the engineering characteristics of soils. In this technique, various types of reinforcement elements have been used in constructions for a very long time. As mentioned in the previous literature, the use of synthetic fibers and plastic waste materials has shown promising results. when these fibers inclusion in soils, significant enhancements are demonstrated in the overall mechanical properties (especially strength properties) of reinforced soils. Sequentially, the randomly distributing methods of fibers have involved increasing attention in geotechnical applications due to its efficiency in improving soil properties. In this study, the soil-reinforcement mechanism, types of fibers (synthetic and plastic waste material), and applications of these fibers in various types of constructions were reviewed. The advantages and disadvantages of synthetic and plastic waste material fibers were also discussed. As well, some recommendations were also mentioned in this review paper in order to fully understand the behavior of reinforced soils if these recommendations were taken into account


INTRODUCTION
In many countries of the world, engineers use different types of materials in the constructions [1].Among these materials, earth materials have been used in structural projects, and stay to be used in present times [2].Further, fibers have been also used in construction [3,4].Historically, composite materials have been used in different buildings, in which fibers were used in these buildings as reinforcement elements.Fibers were used in earlier civilizations such as Babylon, Egyptian, Chinese and Japanese civilizations [3].Recently, more attention has been done to the use of fibers within the soils, to enhance their engineering properties.However, fiber insertion in soils brings many technical, economic and environmental benefits [5][6][7].
[8] investigated the triaxial compression shear strength of silty sand soil mixed with different percents of palm fibers.The results showed that, the reinforced soil samples exhibited approximately 25% increase in friction angle and 35% in cohesion as compared to those of unreinforced samples.
Literature review that has been accomplished by some previous authors in their relevant research to those of unreinforced samples.[9] reported that the inclusion of 1% fiber in sand stabilized with cement increased the unconfined compressive strength.Moreover, [10] found that the addition of polypropylene fibers enhanced both the unconfined compressive and tensile strengths of reinforced clay soil.[11] studied the CBR of two soils reinforced by kenaf fibers.The results showed that the CBR values improved with fiber addition.The improvement in the strength related to that, during loading, the forces acting on soil grains lead to activating the strength in the fibers (i.e.tensile resistance) [12,13] In general, fibers are found in huge quantities in natural, synthetic and waste forms.All these types of fibers are used in civil engineering constructions [14][15][16][17][18] ]19[.Fig. 1 illustrates different types of fibers which can be used to enhance the geotechnical properties of soils, and some of them are reviewed in this paper.

Objective
The main objective of this paper is to review the mechanism of soil reinforcement, the types and properties of synthetic fibers and plastic waste materials which used in geotechnical engineering applications.Moreover, the advantages and disadvantages of these types of fibers are reviewed and discussed.Test results of prior studies that successfully tried to utilize these types of fibers in geotechnical applications are then reviewed.Finally, some recommendations for further research works are suggested.

SOIL REINFORCEMENT
Soils in nature are strong in compression and can resist most applied loads, but they are weak in tension [10].In most construction applications, soil properties can be enhanced by various techniques [12,[20][21][22].Among these techniques, is the soil reinforcement.Generally, reinforcement means the insertion of certain materials having a good tensile strength within the soils that are absent of this property (i.e., tensile strength).[11].Different kinds and forms of reinforcement elements are used to enhance the engineering characteristics of soils (like strength, compressibility, etc.), leading to a new material, named the reinforced soil [3,4,11,23,24].Soil reinforcement is not a new method in civil engineering applications.The ancient Ziggurat (see Fig. 2) found in Iraq, is an excellent example of soil reinforcement application.Moreover, the soil reinforcement technique can be noticed in nature.It can be observed that the natural fibers of different sizes (roots of plants) stabilize the soil in sloping and level ground [11].With knowledge of the role of roots to reinforce soils, the concept of fiber reinforcement come to be important in civil engineering applications.

Fig. 1 Pictures show some types of fibers
The soil reinforcement with roots can be artificially replicated by the insertion of various kinds of fibers within the soil (natural and synthetic fibers).Plastic waste materials can also be used as fibers to reinforce the soil [25][26][27].
In laboratory and field works, fibers are mixed with soil in a random state.Random state means the soil and fibers were mixed thoroughly and Al-Rafidain Engineering Journal (AREJ) Vol.28, No.2, September 2023, pp.33-47 randomly in a dry state.The benefit of using the randomly distributed method of fibers is that this method limits the potential planes of weakness (between soil and fiber) that may create oriented-toparallel reinforcement [12,28,29].Many research papers (especially the papers mentioned in this review paper) have presented that the use of fibers (even waste fibers) causes significant improvement in the engineering characteristics of soils (especially strength characteristics).
Moreover, these papers referred that reinforced samples exhibit lesser loss of post-peak strength and higher extensibility more than the unreinforced samples.This behavior is attributed to the fiber-reinforced soil being more ductile.It is worth noting that, the fiber-reinforced soil is a composite material and its behavior mainly depends on both behaviors of soil, fiber and the interface between them [1, 13, 30-32] [33].The addition of fibers to the soil enhances its stress-strain behavior by mechanically cooperating with the soil grains by interlocking, bonding and friction [34].The interlocking causes a load to transfer from the soil particles to the fiber by activating the tensile stress of the fiber itself and this mechanism is presented in Fig. 3. Also, this figure illustrates that, during loading, the applied stress on the soil grains causes fiber deformation.During fiber deformation, the tensile resistance will be activated in the fiber itself and consequently imparts more strength to the soilfiber composite system.

TYPES OF FIBERS
Fiber is a unit of the material described by fineness, elasticity and a large aspect ratio (the ratio of length to diameter or thickness).Moreover, fiber is a type of material that are continuous filaments or are in separate pieces, like lengths of thread [11].
Human uses for fibers are varied.They can be used in civil construction applications and industry.Fibers are got from natural, synthetic and waste materials.For waste materials, only non-hazardous type is used.
Natural fibers represent those fibers produced by plants and they are characterized by their low cost, acceptable tensile strength, bulk availability and environmentally friendly characteristics.The only practical drawback of natural fibers is biodegradability with time and they have poor durability [32,35].Natural fibers can be used as unconventional low-cost reinforcing elements for enhancing the engineering properties of soils.They can be used in several applications, like the construction of roads for villages, forest areas and other environmentally friendly applications [12,[36][37][38] It is worth noting that, only synthetic and plastic waste materials are presented in this review paper and will be discussed in the next sections.

Synthetic fibers
Synthetic means man-made or artificial, so man-made fibers or artificial fibers are generally named synthetic fibers.These fibers are usually made from petrochemicals, and other specific types are made from natural materials such as cellulose.Thus, the mechanical characteristics of synthetic fibers mainly depend on the properties of the materials made from them.Moreover, the methods of fiber production also affect the properties of those fibers [1,3,11].Synthetic fibers are made for many purposes in civil engineering constructions and can often be produced cheaply and in large quantities as compared to natural fibers [39].Different types of synthetic fibers are used in construction applications, these types are polymeric, glass, steel, plastic, basalt and carbon fibers [40][41][42].Table (1) illustrates some of the papers conducted on synthetics fiber-reinforced soils.
[43] evaluated the polypropylene and nylon fibers with class F fly ash as potential stabilizers in improving volume change characteristics of sulfate rich expansive soils.The test results illustrated enhancing in decreasing shrinkage, swelling and plasticity properties of soils and the most effective treatment was noticed with nylon fibers.The maximum enhancements of fiber treated soils were noted with 0.2% fibers and as fiber percent increase these enhancements were decreased.This behavior was due to poor compaction at higher percentages of fiber and the formation of large voids among fibers and soil grains.[44] reported that, the use of polypropylene fibers in the reinforcing sandy soil improved the strength properties and transformed the behavior of cemented reinforced samples from brittle behavior to ductile ones.
In another study carried out by [45] The unconfined compressive strength and California Vol.28, No.2, September 2023, pp.33-47 bearing ratio increased with polypropylene fibers addition.Moreover, the direct shear strength of reinforced soil samples increased with the presence of fibers due to increasing the cohesion value.While the angle of internal friction did not change significantly with fiber addition as reported by the authors.[13] examined the mechanical interaction behavior between soil grains and polypropylene fibers as illustrated in Fig. 4.They observed that the interfacial shear resistance among fiber and soil grains is mainly related to the fiber surface roughness, re-arrangement resistance of soil grains, the effective contact area between fiber/soil, and soil components.[18] concluded that the addition of 0.5% of polyester fibers to the clay soil leads to increasing both the CBR and indirect tensile (Brazilian) strength.This result was proofed during scanning electron microscopic test which illustrated that the reinforced soil samples were a dense matrix than unreinforced one.For steel fibers, many kinds of research have illustrated that when steel fibers are present in concrete structures, there are many enhancements in the overall strength properties.These fibers enhance the concrete behaviour in terms of resistance to fatigue, ductility, cracking, shrinkage and toughness.Moreover, strength properties, like compressive, flexural and tensile strengths are increased with steel fibers additions]49,50[.
The increasing in strength is attributed to that, steel fibers absorbed the applied energy and control the cracks propagation.Finally, the only disadvantage of steel fibers (compared with to steel reinforcement) is it vulnerable to corrosion which resulting to deterioration of the concrete materials.

Plastic waste materials as fibers
Plastic means a material that is used to define extensive kinds of synthetic or semi-synthetic materials.These materials are used in a wide range of applications and huge quantities.Plastic materials can separate according to the chemical structure of the monomers that their polymer is made from.Plastic waste materials cannot be easily recycled due to their difficult chemical composition.Plastic materials are also not biodegradable materials means that they cannot crumble into smaller molecules under environmental conditions rather they can survive for a long time [55].
In many parts of the world, plastic waste materials have been producing environmental and disposal problems.Using these materials in construction can solve disposal problems in an environmentally friendly and cost-effective manner.Plastic waste materials can be used as fibers and fall into two categories: (1) short fibers (i.e., discontinuous fibers) with aspect ratios ranging from 20 to 60, and (2) long fibers (i.e.continuous fibers) having aspect ratio ranged between 200 to 500.The applications of plastic waste materials in different civil constructions are still overgrowing related to their low cost and ease of production.
Table (2) illustrates some of the papers conducted on plastic waste materials reinforced soils.[56] carried out an experimental study to examine the impact of randomly distributed fiber on the engineering characteristics of two different types of fly ash.Two different types of polyester fiber as recycled plastic waste were used.The test results illustrated that fiber addition had a significant influence on the behavior of the reinforced samples.The reinforced samples reached peak axial stresses at higher strain values than unreinforced ones, then the deformation continued under reducing axial stress.Also, these reinforced samples showed a largely ductile behavior as compared with unreinforced ones.The unconfined compressive and shear strength values increased with fiber additions.[57] performed laboratory tests to evaluate the properties of sandy silt soil reinforced with nylon carpet waste fibers.They observed that the triaxial strength increased by 204% with 3% carpet fibers addition, also the elasticity of soil samples increased.Furthermore, in situ trials have shown that fibers of 70 mm in length can be mixed with the soil with traditional equipment.The availability of such cheap fibers could lead to broader use of reinforced soil and more cost-effective construction.But this enhancement is not examined when using other types of fibers.[58] evaluated the effect of waste plastic bottle fibers on consolidation properties of reinforced sandy-silt soil with clay.Three different aspect ratios of bottle fibers (2, 4 and 8) have been used in the experimental program.The test results illustrated that the compression and volume compressibility indices decrease with fiber addition up to a certain limit and then increased with further addition of fiber.[63] Al-Rafidain Engineering Journal (AREJ) Vol.The horizontal deformation rate was considerably lesser for fibrous cemented soil mixtures.Also, there are significant increases in the tensile strength of soil samples.The soil samples reinforced with optimum fiber percentage could resist the tensile stresses in the soilcement mixtures which may be used for road bases.The strength of fibrous cemented-soil increases as the number of fiber inclusion layers increases.When the soil was reinforced throughout five layers of fibers, the strength was twice as strong as natural soil samples and was 1.5times greater than samples with one fiber layer.Different triaxial compression and consolidation tests have been conducted with various percentages of plastic waste.The findings are illustrated in the form of stress-strain response and compression paths.It is observed that the strength of the soil improved with the addition of plastic waste and a significant reduction in compression parameters.[64] performed an experimental study to examine the California bearing ratio (CBR) behavior of red mud reinforced with different waste plastic water bottle fibers percent.The results showed that the soaked and unsoaked CBR values increased with the plastic percent and were found to be optimum at 2% waste plastic fibers percent.[65] examined the engineering behavior of highly plastic silty soil treated with rice husk ash blended with lime, and reinforced with plastic waste fibers.Various CBR, triaxial compression, unconfined compression and indirect tensile tests were conducted on the reinforced treated soil samples.The findings illustrated that the strength properties, durability and stability of reinforced soil samples are enhanced with both chemical agents and reinforced materials.Furthermore, the optimum fiber percent was vary depending on strength properties.[66] performed a comprehensive study on the use of carpet waste fibers with clay soils.Two types of carpet waste fibers with various adding percents were examined and evaluated.The study has shown that the incorporation of these fibers into soil samples can increase the UCS and change the failure behavior from brittle to ductile.The increase in the UCS is mainly related to the both initial water content and dry density of the soil samples.

ADVANTAGES AND DISADVAN-TAGES OF SYNTHETICS AND WASTE MATERIALS FIBERS
Synthetics and waste materials fibers exhibit many advantages when used in geotechnical engineering applications.Mainly, adding fiber with soil is simply and easy, especially when using a randomly distributed method.In this method, discrete fibers are added to the soil, like chemical additives (i.e.cement, lime, etc.) [65] .Further, this method shows strength isotropy and reduces the weakness planes that can present in soil-fiber mixtures.[66] Another benefit of fiber reinforcement is the restraint of crack propagation in soil samples after initial formation.[67] suggested that before cracking, the fibers seemed to have no obvious influence on the soil behavior.Since the presence of fibers alters the mechanism of failure by preventing the crack propagation [44] [68] [69] [70].Synthetic fibers have good properties and represent one of the most commonly used fibers.They are noncorrosive materials and have excellent resistance to alkalis and chlorides.Therefore, these fibers represent suitable materials for a varied range of applications [3].Also, some types of synthetic fibers like Polyethylene terephthalate have the highest value of density and best mechanical properties [39].Most synthetic fibers have good elasticity and acceptable durability.also, they are less expensive and are more readily available.Further, most synthetic fibers can resist the highest loads as compared with natural fibers.[71] [72] showed that, a significant enhancement in shear strength parameters of glass fiber reinforced-soil happened.Also, they point out that, the availability and nonbiodegradable properties of glass fiber is advantage and proof that this type of fiber can be used for longterm soil enhancement.Plastic waste materials as fibers could be utilized in geotechnical applications as a cost-effective, sustainable choice, and environmentally friendly [73].The main benefit of using plastic waste materials in construction is that the energy and the resources of natural materials can be preserved.Further, the energy used for the burning of landfill materials can be reduced.[74] suggested that, the plastic waste materials should be recycled to reduce their negative environmental effects.The advantages of recycling these materials are reusing them and the decrease of using natural materials like soil in geotechnical applications.The production of industrial waste has increased in recent years due to the presence of industrial factories that produce large quantities of lime and fly ash [75], using one of these compounds along with fibers and soil can improve the soil's durability properties during repeated drying and wetting cycles.[76] mentioned that plastic waste fibers have two advantages in geotechnical engineering.Firstly, improving the engineering characteristics of soils and secondly, the use of these non-degradable waste materials in soil reinforcement also decreases environmental damage.
Mechanical compaction in the field increases the unit weight of soil by (14-32) %, However, the presence of fibers in the soil is not affected by the type of compaction [77], as the unit weight of the soil decreases in the presence of fibers.
A disadvantage of synthetic fibers is that their low-density value causes floating problems in some soil-fibers mixtures.Further, synthetic fibers have small hydrophilic properties as they are hydrophobic.This behavior affects their bonding characteristics with certain mixtures [39].

Direct Shear and CBR Tests
There is an important increment in shear strength parameters and CBR value of fiber-reinforced soil samples.The quantum of enhancement in the soil strengths is governed by plastic waste percent and the dimension of this waste.Vol.28, No.2, September 2023, pp.33-47 Moreover, some types of synthetic fibers (like PET fibers) take large energy to produce due to the manufacturing method, therefore the cost of this type of fibers could be more costly.There are some limitations of synthetic fibers (especially polymeric fibers) including their recycling and reusing.If the previous history of these fibers is unknown and they are collected from an uncontrolled environment, the risks happen.This resulting unstable and inconsistent characteristic of these fibers [78].The disadvantages of using plastic waste fibers in geotechnical applications be governed by what application they are being used for.Also, the cost of recycling and reusing these materials could often be greater than the cost of raw materials, which bounds their efficiency for use [74].Another thing, there are still no scientific standard specifications for using plastic waste materials in soil reinforcement applications, mainly for field applications.

APPLICATION
The applications of synthetic and plastic waste materials fibers are still increasing rapidly due to their low cost and ease of production.A wide-ranging of literature review demonstrates that using synthetic and plastic waste materials fibers in geotechnical applications is feasible in different fields.These fields include concrete works, road construction, bearing soil layers for foundations, retaining walls and railway embankments.The following paragraphs present some applications of synthetic and plastic waste materials fibers in geotechnical engineering.[14] observed that, the adding of plastic waste fibers as aggregate in concrete (up to a certain level) can enhance the abrasion resistance of concrete.This property is important in many practical applications such as hydraulic structures and concrete pavement blocks.[79] found that, the plastic waste fibers can be used to replace a part of the aggregates in concrete, resulting in reducing the total unit weight of the concrete.This type of mixture (i.e., non-bearing lightweight concrete) is suitable for some applications such as concrete panels used in facades.[80] concluded that the sand fiber-reinforced soils were viable alternative materials in road construction for low traffic or temporary roads.From full-scale field tests, found that the presence of synthetic fibers in road [81] layers can improve pavement resistance to rutting, unlike to unreinforced soil pavement when constructed on a weak subgrade.[82] reported that the fiber-reinforced cemented sand increases the bearing capacity of spread footings and can be used in field applications.[83] illustrated that the use of synthetic fibers reinforced sand behind retaining walls enhances the stability of the wall and reduces the lateral earth pressure values.They also documented that these effects are more significant when short fibers are used together with a geo-grid.[20] mentioned that the soil reinforcement can be used as reinforced soil retaining wall, which is an alternative to a conventional concrete retaining wall.In such structures, the reinforcement acts as a tensile element shared to the soil by friction and interlocking, resulting to enhances the stability of the soil mass.By analyzing their finding result.Another study by [84] presented that the enhancement of the local soil by the inclusion of randomly distributed fibers could be used for shallow footings, especially when deep footings represent unsuited solutions due to their higher costs.Finally, throughout numerous types of research, it has been highlighted that the fiber content used to reinforce soil is very significant.In general, not always higher fiber content is better for soil reinforcement.Sometimes the mechanical properties of soils improved with fibers addition up to a certain limit, then decrease.Thus, it is important to determine the optimal fiber content when used to reinforce soils.Moreover, the physical properties of fibers (i.e., length, shape, etc.) also affect the efficiency of fiber-reinforced soil.

CONCLUSION
From this review paper the following conclusions could be drawn: • The hand-mixing method of randomly distributed fibers allows fibers to combine with the soil mass in proper form.• The easy way of recycling plastic waste materials as reinforcement fibers meet a great interest in geotechnical engineering applications.
• The low-cost, availability, ease of work and possibility of use in all-weather circumstances are the overall advantages of synthetic and plastic waste fibers composite soils.• Chemical treatment (using cement, lime, fly ash, etc.) can also be used to enhance the mechanical properties of fiber-reinforced soils, resulting in the development high performing matrices.• The degree of adhesion, bonding and interaction between soil and fiber are very important factors that affect the engineering behavior of the soil-fiber mixtures.• The practical benefits of soil reinforcement consist of increasing soil strength, reducing tensile cracks and decreasing soil brittleness.
Al-Rafidain Engineering Journal (AREJ) Vol.28, No.2, September 2023, pp.33-47 A successful application might help to decrease the quantity of plastic waste materials disposed of in landfills.Also, this application contributes to sustainable development by giving low-cost materials to geotechnical engineering applications.

RECOMMENDATIONS
From the authors' point of view, the research papers on the use of synthetic and plastic waste fibers with cohesive soils have been limited.Thus, further studies need to evaluate the mechanisms of load transformation on the interfaces between clays and fibers.In field applications, much attention is necessary to achieve a reasonably uniform distribution of the fibers within the soil mass.Scale properties affect the behavior of fiber-reinforced soils (like stressstrain behavior).Thus, additional large-scale tests are recommended so that fiber-reinforced soil behavior will be well understood.Lastly, the longterm durability of fiber-reinforced soils and the long-term behavior of plastic waste fibers are recommended.Declaration of interes the authors declare that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Fig. 2
Fig.2 Ziggurat of Ur in Iraq

Fig. 3
Fig.3Mechanism of grain rotation that induces interlock and friction in fiber[29]

Fig. 4
Fig.4Sketch drawing of interfacial mechanical interactions between soil particles and fiber[45] fibers within natural and cement-treated soil samples leads to an increase in the unconfined strength, shear strength and strain at failure.The increasing fiber content in the soil mixture could increase the axial stress and reduces the stiffness and the loss of peak strength

Table 2 .
Some of research papers conducted on plastic waste fiber-reinforced soils