State-of-the-Art Review: Fiber-Reinforced Soil as a Proactive Approach for Liquefaction Mitigation and Risk Management

Soil liquefaction is a phenomenon that occurs in which the behavior of soils changes from solid to viscous liquid due to the efect of earthquake intensity or other sudden loadings. Te earthquake results in excess pore water pressure, which leads to saturated loose soil with weaker characteristics and potentially causes large ground deformation and lateral spreading. Soil liquefaction is a dangerous event that can lead to catastrophic outcomes for humans and infrastructures, especially in countries prone to earthquake shaking, where soil liquefaction is considered one of the most prevalent types of ground failure. Hence, precautions to reduce and/or prevent soil liquefaction are essential and required. One of the countermeasures to avoid soil liquefaction is the introduction of fbers in the soil since fbers can act as reinforcement by enhancing the soil’s strength and resistance to liq-uefaction. Te process of including fbers into the soil is known as soil stabilization and is considered one of the ground improvement techniques. Terefore, this paper aims to summarize and review the consequences of adding fber as a reinforcement technique to overcome the issue of soil liquefaction.


Introduction
Liquefaction of soil is a serious phenomenon that carries tremendous risks to human lives as well as properties during earthquakes, such as landslides, ground failure, and damage to infrastructure [1][2][3][4].Tere are diverse types of ground failure caused by liquefaction, including failure of retaining walls as a result of the increase in lateral loads causing them to slide or tilt, lateral spreads as a result of lateral movements, ground settlement, reduction in bearing capacity leading to foundation failure, a buoyant rise of buried structures, and ground oscillation with repetitive displacement.Te solution to constructing structures on liquefable foundations can be either swapping the project location/ structure type or enhancing the foundation situation, but the latter solution is feasible and logical to make [5].Tis event mainly takes place in saturated cohesionless soil, which causes the soil to lose its strength and stifness as well as its ability to withstand the weight of building above it due to an increase in pore water pressure during strong and large amplitude seismic waves resulting in deformation of the soil and thus a reduction in its efective stress as a consequence of dynamic loading [6].Te three types of liquefaction are static, dynamic, and blast, but static, which is caused by rainfall, and dynamic, which is caused by earthquake shaking, are the most common types of liquefaction, followed by blasting, which is considered to be the least likely to happen [7,8].Te characteristics of soils, such as shear strength and stifness, are related to the situation of the environment and the type of structure above the soil as well as external factors such as earthquakes [9].Te most susceptible soil type to both static and dynamic liquefaction is fne silty sand due to its engineering properties since the fne or powder content induces static liquefaction compared to clean sand [10][11][12].Te attention on liquefaction and its hazards were not focused until the occurrence of the Niigata and Alaska earthquakes in 1964 [13], since these disastrous incidents advanced the study and research about the mechanism of soil liquefaction, leading to progress in ground improvement approaches [14][15][16][17].Generally, multiple factors that afect the possibility of liquefaction have been analyzed, including relative density, earthquake magnitude, fne content, saturation degree, vertical stress, and ground motion characteristics [17][18][19].Some studies were conducted to evaluate the settlements created by earthquakes and the dissipation of pore water pressure due to liquefaction [20][21][22].In addition to that, numerous researchers examined the liquefaction resistance of cohesionless soils in diferent structures, including embankments, dams, and slopes [23][24][25].Currently, there are many methods to eliminate the liquefaction of soil, such as draining, densifcation, and reinforcement of soil [26].Nonetheless, soil reinforcement has been the best method to adopt since draining and densifcation are usually afordable and fruitless [27][28][29].One of the ground improvement methods and liquefaction prevention approaches is the inclusion of fbers, for example, glass, polyethylene, steel, and polypropylene, as reinforcement techniques.One of the advanced approaches for mitigating the risk of liquefaction is the utilization of nature-based solutions.Tese solutions include coconut coir, jute, and sisal fbers which can be an adequate replacement for synthetic fbers for sustainability and cost-efectiveness advantages.Another example of nature-based solutions is the use of vegetation to reinforce soil since vegetation can be used in combination with fberreinforced soils to stabilize soil slopes and prevent erosion.Another advanced approach for mitigating liquefaction is the use of nature-inspired solutions which refer to engineering or design approaches that inspired by natural systems and processes.One example of an NIS in FRS is the use of biomimetic geotextiles.Tese geotextiles are designed to mimic the properties of natural materials such as spider silk, which is known for its strength and fexibility.By using a biomimetic approach, geotextiles can be designed to provide similar or even superior reinforcement properties to traditional geotextiles while also being more environmentally friendly.However, polypropylene is considered to be the most used fber for soil reinforcement [30][31][32].Accordingly, the current state of the art is missing a detailed paper regarding the liquefaction potential of soil and the fber efect.Terefore, this paper provides a detailed, comprehensive review study on the impact of utilizing diferent types of fbers as reinforcement techniques on the liquefaction potential of soil.A part of the paper, a review of the available studies concerning the inclusion of natural and synthetic fbers on soil behavior in terms of liquefaction occurrence, was performed.Besides, it discusses and compares the efect of using distinct types of fbers as reinforcement techniques on the soil liquefaction potential.

Soil Liquefaction
2.1.Mechanism of Soil Liquefaction.Indeed, soil liquefaction is a phenomenon that usually happens in loose saturated soil with a low potential of existence in viscous rocky clay soil.On the other side, a study on the Wenchuan earthquake showed that gravelly soils are capable of exhibiting liquefaction under particular circumstances [33].Te process of soil liquefaction starts with the compression of the densely packed and sheared sand particles, followed by expansion during the sliding of the sand particles over each other.Saturated sand consists of two porous layers of soil particles and pore water.Te densely sheared saturated sand particles hinder the pore water drainage due to the dynamic efect causing a rise in volume, shear strength, and efective stress accompanied by a decline in pore water pressure.Te initiation of excess pore water pressure as well as weakening of the soil and the increment of deformation are related to the behavior of dense soil under small cyclic shear strain within undrained pore water circumstances [6,34].As the shear strain increases, the volume increases as well, resulting in a decrease in excess pore water pressure and hence an increase in shear resistance of the soil.Limited liquefaction occurs when a great amount of deformation is impeded after cyclic loadings stop due to the accumulated undrained shear strength resulting in strain hardening [6,35].On the other hand, cyclic mobility can be defned as the gradual weakening of dense saturated sand under static load within limited undrained cyclic shear strain [36].Soil liquefaction is a diferent event from cyclic mobility since liquefaction exerts a negligible rise in shear resistance despite the value of deformation [37].Te soil subjected to cyclic mobility shows softening frst, followed by stifness in case the monotonic loading was applied in the absence of drainage due to a rise in volume and decrease in pore water pressure.Furthermore, the soil subjected to cyclic loading builds up deformation and produces a low magnitude of static shear pressure in comparison to residual shear resistance.However, the term "cyclic liquefaction" was introduced in 1994, which describes the existence of deformation as the static shear pressure surpasses the shear resistance of the soil [38].Te condition at which the initial static shear pressure of expansible soil is not appreciable is called the zero efective stress state [38,39].In general, soil liquefaction can be categorized into fow failure, circulating fuidity, and sand boils [40].Sand liquefaction is the phenomenon where the ratio of pore water pressure reaches one while the sand strength approaches zero and the sand is in the liquid state [41,42].Lastly, the liquefaction of saturated sand during an earthquake must meet two fundamental requirements, which are the existence of adequate vibration intensity capable of ruining the structure of the soil and the development of the progressive rise of excess pore water pressure as the number of stress cycles increases until the value of excess pore water pressure causes the shear strength of the sand to diminish entirely or partially [43].

Evaluation of Soil
Liquefaction.Diferent approaches were suggested to detect the factors responsible for the evaluation of liquefaction resistance of soil, including stressbased [44], strain-based [45], and energy-based [46,47].Te most widely used approach is based on the shear stress and several cycles as the criteria of assessment regardless of the predicament of measuring efective uniform shear stress or shear strain during tests [44].Another approach is based on the dissipated energy, initial efective stress, and high pore water pressure which has been utilized by numerous 2 Journal of Engineering

Bridging effect of fibers
Figure 3: Bridging efect of fbers across the crack opening (reproduced from [58]).
Journal of Engineering researchers due to its simplicity in evaluating the liquefaction resistance of soil [46,48,49].Te energy-based approach involves using distinct factors to assess soil liquefaction such as the relationship between the generation of pore water pressure and energy release [48], the energy attenuation equation [49], the energy principles [50], and the shear energy as a replacement to shear strain and a number of cycles [51].In general, the most common tests for sand liquefaction are the direct shear test and triaxial test despite the fact that the sample volume and strain range in the traditional direct shear test apparatus and triaxial test apparatus is little which yields some drawbacks during the assessment of sand liquefaction.Te ring shear test apparatus is an adequate device for evaluating sand liquefaction due to its advantages such as big strain range and sustained shear surface [52][53][54].

Fiber as Reinforcement Technique in Soil
Fiber is a material that possesses fexibility, a large length to thickness ratio, and fneness properties.Usually, fbers are randomly mixed with soil creating fber-reinforced soil (FRS)    4 Journal of Engineering with the main purpose of enhancing the strength characteristics and other qualities of the soil [55].Reinforcement of soil can be defned as the procedure of mixing materials with the soil in order to enhance the soil's properties.Generally, fber reinforcement can be divided into natural, synthetic, and waste fbers depending on the source of the material.Natural fbers such as coconut (coir) and palm fbers have advantages such as high strength, low cost, and good impact on the environment as illustrated in Figure 1.Synthetic fbers such as polypropylene (PP), polyester, and glass fbers are manufactured to meet the desired requirements which enable the control and modifcation of soil geometry to improve the characteristics of fbers, especially under the variable environmental factors as presented in Figure 2.
Waste fbers such as old or used tires and waste plastic fbers are incorporated as soil reinforcement which can eliminate the environmental issues related to the disposal of these materials.Fibers can be added to soils either as oriented or random where the oriented approach focuses on systematically arranging the fbers layer by layer in the same orientation and the random approach focuses on mixing the fbers with soil discretely which provides strength isotropy and minimize the potential of forming weak planes as demonstrated in Figure 3. Te mechanism of fber going under tension is shown in Figure 4.
As can be observed in Figure 5, the advantages of using these fbers are the availability of these materials [61][62][63], the development of the strength characteristic [64,65], and the hindrance of the tensile crack propagation [66,67].
Te result of the inclusion of fbers in the soil is the rise in the peak shear strength and minimization of the postpeak reduction in shear resistance as well as a rise in the stifness and cohesion of the mixture.In addition to that, a certain fber content percentage can result in better bonding between the soil particles as well as enhance the structure of soil and reduce displacement.Figure 6 shows the soil reinforcement methods.

A Summary of Various Types of Fiber Material Utilized in Soil
Indeed, this section is intended to illustrate the various types of fbers that were previously used to reinforce soil materials as well as to briefy highlight the advantages and infuence of each type.

Natural Fibers.
Te possibility of utilizing natural fbers in soil reinforcement has been the interest of many studies recently due to the need for eco-friendly materials in the feld of ground improvement.However, the incorporation of natural fber has been implemented for some time in some developing countries in cement mixtures and block applications due to the cost-efectiveness and availability [68][69][70].
In fact, there are some factors afecting the quality of natural fber including the age of the plant, the method of isolating the fber, and the place in the plant where fber is brought from [71].Tere are diferent types of natural fbers and each type will be concisely discussed.

Coconut (Coir) Fiber.
It is the outer surface of a matured coconut (coconut husk) with a length ranging between 50 and 350 mm.One of the substances composing coir fber is lignin which is responsible for the slow degradation and long infeld life between 4 and 10 years in comparison to other natural fber types [67].Coir fber degrades based on the climate situations and the nature of implementing soil where the coir fber sustained 80% of its tensile strength beyond 6 months of implementing the fber in clay.Coir fber is stronger and more fexible in nature due to the high friction coefcient compared to synthetic fbers whereas some coir fbers provided 47.50% development in resilient modulus compared to synthetic fbers which exhibited only 40% [72].In addition, coir fber distributed randomly exhibited satisfying performance in minimizing the possibility of swelling of soil [73,74].
Geosynthetic family (geogrid, geotextile, geocomposite, geonet, and geocell) Randomly distributed fibers (natural, manmade, and mineral fibers) Figure 6: Diferent procedures of soil reinforcement (reproduced from [67]).[75].Te inclusion of sisal fber or coir fber with 4% refected a signifcant efect on the ductility and a slight increment in the compressive strength [68].Moreover, the fber percentage and fber length were noted to afect the dry density in an inversely proportional manner where any increase in these two parameters results in a decrease in the dry density of the soil [76].Lastly, as the percentage of sisal fber increases, the shear strength increases up to 0.75% fber content where any further increase leads to a reduction in the shear strength.

Palm Fiber.
Te material obtained from degraded palm trees exhibits low tensile strength, brittleness, low modulus of elasticity, and high-water absorption [77].8 Journal of Engineering However, the currently produced palm fber possesses unique characteristics including durability, costefectiveness, tensile capacity, availability, and relative strength to combat deterioration [78].Te increment in fber percentage between 0% and 1% while the fber length is constant results in an increase in the maximum and residual strength as well as a reduction in the diference between the maximum and residual strength [66].Te integration of palm fber in soil considerably improved the deviator stress at failure and shear strength parameters [79].
4.1.4.Jute.Te material obtained from the bark of jute plants with a length up to 2.5 m generally grows in China, India, Bangladesh, and Tailand.Typically, jute is considered eco-friendly material with diferent applications such as stabilization of soil, drainage, and fltration [67].Te fber utilized in the stabilization of soil in pavement applications is commercially named GEOJUTE which is woven from jute fbers.Lastly, the maximum dry density is minimized with the inclusion of jute fber whereas optimum moisture content increases.

Synthetic Fiber.
It is manufactured fber with a longer life than natural fber.Te length and form of fber are completely controlled by man whether making it into staple or cut.Unlike natural fber, synthetic fber such as polypropylene (PP), polyester (PET), and polyethylene (PE) are harmful to the environment.Synthetic fber has been utilized in the feld of soil reinforcement due to its high tensile strength and good corrosion resistance [80,81].Tere are diferent types of synthetic fbers and each type will be briefy discussed.

Polypropylene (PP) Fiber.
It is the most prevailing man-made fber used in the reinforcement of soil.[30][31][32].PP fber is implemented for numerous advantages, including minimizing the shrinkage characteristics, combating biological and chemical degradation, and improving the strength characteristics [82][83][84].In addition to that, PP fber proved its ability to increase the unconfned compressive strength, decrease swell pressure, and volumetric shrinkage strain for expansive clays [84,85].It was seen that soils reinforced with PP fber provided hardness during the loadsettlement response test compared to unreinforced soils, which displayed almost perfectly plastic behavior making this fber applicable for soil reinforcement in embankments and shallow foundations [86].Lastly, the conclusion of performing conventional triaxial compression and tension test was that including fbers signifcantly increased strength in compression but showed a negligible efect in tension [87].

Polyester (PET).
Te fber content is the crucial factor in enhancing the ultimate and peak strength of the soil [88].Moreover, the fber content and fber length infuenced the unconfned compressive strength, ultimate bearing capacity, and settlement of highly compressible clay, where any increment in the fber content and fber length led to an increment in unconfned compressive strength and ultimate bearing capacity as well as led to a reduction in settlement of the soil [89,90].Short PET fber was proved its ability to enhance the stability of levees against seepage and food since it possesses strong piping resistance [91].

Polyethylene (PE)
. PE fber is fundamentally used in geotechnical engineering due to its environmentally friendly properties.Te usage of high-density PE fber led to an increment in the fracture energy, tensile strength, toughness, and strain capacity of the soil [92,93].Another application of high-density PE fber is the implementation in pavement engineering as a subgrade material in order to decrease the thickness of the base course [92].

Glass Fiber.
Te integration of glass fber in soil showed a notable efect on peak strength [94,95].A comparative study regarding the infuence of PP, PET, and glass fber on the mechanical behavior of reinforced soils was conducted where it was found that using PP fber considerably enhanced the brittleness and decreased the deviator stress at failure, while PET and glass fbers improved the deviator stress at failure and decreased the brittleness [96].Te addition of glass fber to the soil exhibited a signifcant efect on the unconfned compressive strength, where the addition of 1% of glass fber to 4% cemented sand resulted in 1.5 times increase in comparison to unreinforced sand [97].

Influence of Fibers on the Liquefaction Potential of Soils
Tis section is devoted to highlighting and reviewing the remarkable studies on the utilization of fbers as reinforcement techniques to mitigate the potential of liquefaction in soil.Generally, Krishnaswamy and Isaac [26,98] performed stress-controlled triaxial tests to examine the infuence of adding coir fber and geotextile fbers on the liquefaction resistance of the sand.It was found that the addition of fber, stress ratio, efective confning pressure, and interface friction all improved the liquefaction resistance of the sand [26,98].Another study conducted by Maheshwari et al. [99] investigated the efect of including diferent types of fber on the liquefaction resistance of the sand.It was found that coir fber exhibited the best performance among fbers where 0.75% coir fber content at 0.1 g acceleration increased the liquefaction resistance by 91%.Furthermore, as the fber content increases, the  Te integration of fber heavily infuenced the stifness and ultimate strength of the soil due to parameters including aspect ratio and weight fraction of fber as well as gradation and particle geometry.Te addition of fber, as well as fber modulus, is linearly proportional to the increase in shear strength.Te optimum glass fber content was 3% (by weight) with lowest fber aspect ratio (L/d) of 60 Krishnaswamy and Isaac 1994 [26] Stress-controlled cyclic triaxial test Woven (polypropylene), nonwoven geotextile, and coir fbers with sand Coir fber provided the best performance in terms of liquefaction resistance due to the high soil-fber interface followed by nonwoven geotextiles due to high stifness and lastly woven geotextiles.Hence, the liquefaction resistance increases as the stifness, stress ratio, and interface friction increase.Moreover, the efect of fber on liquefaction resistance increases as the spacing of reinforcement and relative density decrease.No percentage of fber content was presented in the study Ranjan et al. 1994 [108] Triaxial compression test Sand with plastic fber Te presence of fbers in the sand increases the peak shear strength and decreases the loss of postpeak stress.Moreover, the increase in shear strength is directly attributed to the increase in fber content by up to 2%.Te shear strength rises with the increase in fber aspect ratio while critical confning stress is reduced.Te optimum fber content was 2% (by weight) with fber aspect ratio ranging between 60 and 90

Krishnaswamy and Tomas
Isaac 1995 [98] Cyclic triaxial test Woven (polypropylene), nonwoven geotextile, and coir fbers with sand Woven, nonwoven, and natural coir fbers were added in one and two layers to the soil.Te soil-fber interface, bending stifness, and compressibility of fber have a signifcant impact on the shear mobilization.Moreover, the stifness of fber is proportional to the shear mobilization where any increase in the stifness increases shear mobilization.On the other hand, the compressibility of fber is inversely proportional to the shear mobilization where any increase in compressibility results in a decrease in the shear mobilization.Te best result was seen in the case of coir fber with one layer where it produced the highest angle of interface friction and stifness Cyclic triaxial test Hostun RF sand reinforced with nonwoven geotextiles (polyester) When the stress ratio is lower than the cyclic resistance, the liquefaction resistance rises due to the friction between soil and fber.In case, the stress ratio is higher than cyclic resistance, the liquefaction resistance increases due to the deformable nature of the fber.During testing, the sand is subjected to repetitive expansion cycles but the fber causes a delay in the liquefaction by means of minimizing the interstitial pressure in the sand.No specifc optimum fber content was presented in the study Boominathan and Hari 2002 [105] Stress-controlled cyclic triaxial test Fly ash with fber and mesh (nonwoven polypropylene geogrid sheets) reinforcement Te incorporation of geosynthetic fber and mesh reinforcement is inversely proportional to the confning stress and relative density where any decrease in confning stress and relative stress results in a rise in liquefaction resistance.However, mesh reinforcement showed better performance compared to fber reinforcement because mesh reinforcement produces better interlocking and easier pore water pressure dissipation.Te optimal fber/mesh content to increase liquefaction resistance is approximately 2% Altun et al. 2008 [109] Cyclic torsional shear test Toyoura sand reinforced with (woven, nonwoven) geosynthetics Te addition of fbers to sand resulted in an increase in the liquefaction resistance up to 266% and 258% for one and four layers of woven and nonwoven geotextiles respectively.Furthermore, the type (woven and nonwoven) and arrangement (from one to four layers) of fber play an important role in the liquefaction resistance of the soil.Woven and nonwoven geotextiles enhanced the liquefaction resistance of reinforced soil because the sand layers are isolated by geotextiles.
Soil mixed with nonwoven geotextiles exhibited superior performance in terms of liquefaction resistance due to its dense structure and low elastic modulus in comparison to woven geotextiles.Te best performance was exhibited by four layered nonwoven geotextiles at stress ratio of 0.125 Hostun RF sand with Loksand fexible polypropylene crimped fbers During the drained test, the increase of strength due to the addition of fbers is directly dependent on the content and direction of the arrangement.Te volume of the mixture is highly afected by fbers for compression and tension loading since fbers fll the voids in the sand despite the fact that the stress-strain relationship is slightly afected.During undrained, the increase of strength due to the addition of fbers for both compression and tension is clearly noticeable as well as transforming softening of strain into hardening.Finally, the inclusion of fber resulted in minimization or elimination of static liquefaction for both compression and extension regardless of the fact that liquefaction in extension requires a higher number of fbers.Te optimum fber content was 0.9% (by weight)   [111] Undrained ring shear test Silica sand with polypropylene fber Te addition of polypropylene fber showed little to no signifcant efect on the loose reinforced sand regardless the reinforced specimen maintained structural stability whereas the unreinforced one completely failed.Hence, fbers possess the ability to reduce or eliminate the lateral spreading of sand.On the other hand, the addition of fber showed a noticeable efect on moderately dense and dense reinforced specimens in terms of displaying fuctuations after shear failure, unlike unreinforced samples.In addition to that, moderately dense and dense reinforced specimens as well as dense unreinforced specimens maintained structural stability in comparison to the moderately dense unreinforced specimen which partly failed.Lastly, fber reinforcement provided partial decrease or entire prevention of the lateral spreading caused by static liquefaction.Te optimum fber content was 0.8% (by weight) Maheshwari et al. 2012 [99] Vibration (shake) table Solani sand reinforced with a geogrid sheet, geosynthetic fber, and natural coir fber Te inclusion of coir fber refected the best result in terms of liquefaction resistance compared to other reinforcement types.Coir fber at 0.75% and 0.1 g acceleration provided improvement of liquefaction resistance up to 91% while synthetic fber at the same fber percentage and acceleration magnitude provided 88% improvement and geogrid sheets (fve layers) at the same acceleration magnitude provided 31% improvement.Te addition of fbers is inversely proportional to the acceleration magnitude whereas any reduction in acceleration magnitude (from 0.4 g to 0.1 g) leads to an increase in liquefaction resistance.Lastly, the reinforced sand exhibited a good efect by means of minimizing the settlement.Te optimum fber content was 0.75% (by weight) for coir fber and geogrid sheets (fve layers) 14 Journal of Engineering Wang and Brennan 2015 [113] Two centrifuge tests HST95 congleton sand redhill 100 sand with fexible crimped polypropylene fber Te inclusion of fber in the backfll signifcantly limited the lateral displacement of the quay wall and backfll settlement.Te usage of fber as reinforcement in soil prevented quay wall movement caused by excess pore water pressure.
Te optimum fber content was 0.6% (by weight) Huang and Wang 2016 [114] Dynamic triaxial test Liquefable silt and silty sand with laponite (synthetic layered silicate nanoparticle) Te integration of laponite in the soil prevents liquefaction by means of soil grain cementation, and pore fuid solidifcation and limits the initiation of pore pressure.Te transition in laponite enhanced the liquefaction resistance.Te addition of laponite to the soil slows the formation of pore pressure and reduces the deformation in comparison to unreinforced specimens.Despite the fact that the increase in laponite content or curing period increased the liquefaction resistance, during the frst few cycles, the infuence of laponite is higher while the infuence of the curing period is higher after a few cycles.Te optimum laponite content was 3.5% (by weight) Journal of Engineering  Strain-controlled cyclic simple shear test Clean beach sand from the Gallipoli beach with monoflament polypropylene fber Te combination of fbers in soil notably improved the number of cycles to reach liquefaction and hence, minimized the lateral movement of the soil leading to higher liquefaction resistance.Additionally, the presence of fber in soil and the increment of fber percentage, as well as the increment of fber length all, limited the generation of excess pore water pressure which is attributed to the increment of energy absorption capacity.Moreover, the increment of relative density is linearly proportional to the increment in liquefaction resistance.Te infuence of fber percentage on moderately dense sand (50%) was stronger than that on loose sand (30%).Lastly, the increment of fber percentage showed the most signifcant efect on sand compared to the increment of fber length.Te optimum fber content was 1% (by weight) and the optimum fber length was 19 mm

Ghadr et al. 2020 [119]
A pneumatic controlled cyclic triaxial test under undrained conditions Two types of sand, sand A F161 (Firoozkuh 161) and sand B F141 (Firoozkuh 141) mixed with thermoplastic polymeric microsynthetic fber Reinforced and unreinforced soils experience the transition from initial random loose packing (RLP) to random close packing (RCP) with the increase in silt content.In general, the increment of fber percentage is accompanied by the increment of average contact number per particle, higher excess pore water pressure dissipation, and enhanced liquefaction resistance.Te optimum fber content was 1.5% (by weight) Zhang and Russell 2020 [120] Drained and undrained triaxial compression tests Sydney sand mixed with crimped polypropylene Loksand fbers During the drained test, the incorporation of fbers in loose soil exhibited considerable development in the strength of the soil based on the fber percentage where any increase in fber percentage produced higher strength.Te combination of fbers with soil eliminated the static liquefaction except when the fber content is low (0.25%) in comparison to unreinforced sand where static liquefaction is presented despite the initial confning pressure.Te optimum fber content for the consolidated drained was 0.5% and the optimum fber content for the consolidated undrained was 0.75% Journal of Engineering acceleration magnitude decreases.Rao et al. [100] observed that the addition of coir fber to the soil enhanced shear strength and deviator stress at failure, as well as reduced the volumetric expansion.In addition, it was found that the use of randomly distributed coir fber showed superior strength in comparison to layered coir fber.Lastly, the reinforced and unreinforced sand increased the initial tangent, secant modulus, and confning pressure.A study performed by Sivakumar Babu and Vasudevan [62] stated that the usage of fber content between 1% and 2% increased the shear and stifness of the soil.Te integration of coir fber increased the stress-strain behavior where the maximum increase was achieved with fber length between 15 and 25 mm.Indeed, a summary of the previous investigations about the efects of utilizing natural fber in soil on liquefaction potential is arranged chronologically as shown in Table 1.
On the other hand, Vercueil et al. [27] conducted a cyclic triaxial test to investigate the infuence of nonwoven geotextiles on Hostun RF sand and found that when the stress ratio is lower than the cyclic resistance, the liquefaction resistance increases due to the friction between the soil and fber.In case, the stress ratio is greater than the cyclic resistance, the liquefaction resistance increases due to the deformability of the fber.Another study by Boominathan and Hari [105] found that the inclusion of geosynthetic fber and mesh reinforcement is inversely proportional to the confning pressure and relative density where any decrease in confning stress and relative stress results in a rise in liquefaction resistance.However, mesh reinforcement showed better performance compared to fber reinforcement because mesh reinforcement produces better interlocking and easier pore water pressure dissipation as shown in Figure 7. Te optimal fber/mesh content to increase liquefaction resistance is approximately 2%.
Ibraim et al. [106] performed conventionally drained and undrained triaxial compression and extension test and found that during the drained test, the increase of strength due to the addition of fbers is directly dependent on the content and direction of the arrangement, as seen in Figures 8 and 9.
In addition to that, the volume of the mixture is highly afected by fbers for compression and tension loading since fbers fll the voids in the sand despite the fact that the stressstrain relationship is slightly afected.
On the other hand, the increase of strength due to the addition of fbers for both compression and tension is noticeable as well as transforming softening of strain into hardening during the undrained test.Lastly, the inclusion of fber resulted in minimization or elimination of static liquefaction for both compression and extension regardless of the fact that liquefaction in extension requires a higher number of fbers.Noorzad and Amini [5] investigated the efect of randomly distributed monoflament polypropylene on Babolsar sand and found that both fber content and length showed a crucial positive efect on the required cycle numbers to reach liquefaction.1% fber content resulted in the highest liquefaction resistance of 280%.Shear modulus improves with the increase of fber content as presented in Figure 10 and Table 2.
Lastly, fbers can be useful in reducing or eliminating the lateral movement of soil due to liquefaction.In fact, a summary of the previous works carried out to evaluate the infuence of adding synthetic fbers to soil on liquefaction potential is provided in Table 3.

Conclusion
Tis paper intends to review the available studies concerning the inclusion of natural and synthetic fbers on soil behavior in terms of liquefaction occurrence.On the bases of the abovementioned statement, the following points are drawn: (i) Liquefaction of soil is considered one of the most dangerous and widespread types of ground failures (ii) Multiple techniques and methods were implemented to evaluate the parameters related to the liquefaction resistance of soil, such as stress-based, strain-based, and energy-based (iii) Introducing fbers into soil provides many advantages, including enhancing strength characteristics, delay of the tensile crack propagation, and increment in peak shear strength (iv) Te presence of fber has proved its efciency in increasing the liquefaction resistance of soil and limiting the generation of excess pore water pressure (v) Te fber properties, including the type of fber (natural, synthetic, or waste) fber content, fber length, and type of arrangement (randomly or oriented distributed), crucially infuence the liquefaction resistance of soils Finally, the investigation into the inclusion of fbers in soil and its infuence on liquefaction resistance has significant real-world applications in geotechnical engineering and construction.Te ability to mitigate the efects of liquefaction can lead to improved safety and stability of infrastructure, particularly in earthquake-prone regions.Te identifcation of efective techniques for enhancing soil strength and reducing the risks associated with liquefaction will contribute to the development of more resilient and sustainable construction practices.Furthermore, the utilization of waste or natural fbers as reinforcements can ofer additional environmental and economic benefts, promoting more sustainable and cost-efective solutions for soil reinforcement in industry.
Fibers crossing cracks and subjected to tension because of deformation

Figure 7 :Figure 8 :
Figure 7: Variation of cyclic stress ratio with the number of stress cycles for liquefaction (reproduced from [105]).

Table 1 :
Efects of using natural fber in soil on its liquefaction potential.

Table 3 :
Impact of synthetic fber on soil liquefaction.