Engineering properties of plastic waste reinforced sandscientiairanica.sharif.edu/article_22020_98f37c1d51e... · 2021. 1. 21. · 136 mean particle diameter (D 50) of the sand was

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Engineering properties of plastic waste reinforced sand

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Bahram Ta'negonbadi1, Reza Noorzad

*2, Pardis Shakery

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PhD in Geotechnical Engineering, Faculty of Civil Engineering, Babol Noshirvani University of 5

Technology, Babol, IRAN 1 6

Email: [email protected] 1 7

Mobile: +989112800472 8

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Associate Professor in Geotechnical Engineering, Faculty of Civil Engineering, Babol 10

Noshirvani University of Technology, Babol, IRAN2 11

*Corresponding Author: Email: [email protected], [email protected] 2 12

Tel: +981135278377 mobile: +989121763764 13

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M.sc in Geotechnical Engineering, Faculty of Civil Engineering, Babol Noshirvani University of 15

Technology, Babol, IRAN 3 16

Email: [email protected] 3 17

Tel: +981135278377 18

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Words: 5884; Figures: 11; Tables: 4 20

Submitted to: 21

Scientia Iranica 22

mailto:[email protected]:[email protected]

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Bahram Ta'negonbadi was born in Gonbade Kavoos, Golestan, Iran in 24

1989. He received the B.Sc. degree in Civil engineering from Ferdowsi 25

University of Mashhad in 2011, M.Sc. degree in Geotechnical engineering 26

from Sharif University of Technology in 2013. He obtained his Ph.D. degree 27

in Geotechnical engineering from Babol Noshirvani University of 28

Technology, Bbaol, Iran, in 2018. 29

His main areas of research interest are soil stabilization, soil improvement, 30

underground spaces (tunnels) under dynamic loadings, soil-structure 31

interaction, earth dams and geotechnical earthquake engineering. 32

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Reza Noorzad * was born in Nour, Mazandaran, Iran in 1964. He received 34

both his B.Sc. and M.Sc. degree in Civil engineering and Geotechnical 35

engineering from Tehran University, Tehran, Iran, in 1990 and 1994 36

respectively, He obtained his Ph.D. degree in Geotechnical engineering from 37

Sharif University of Technology, Tehran, Iran, in 2000. 38

He is currently an Associate Professor in the Department of Civil 39

Engineering at Babol Noshirvani University of Technology. From 2004 to 2007 he was the Head 40

of Central Library and Documentation Centre at Mazandaran University. His research interests 41

include soil improvement, reinforced soil, earth dams, marine geotechnics, and geotechnical 42

earthquake engineering. 43

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Pardis shakery was born in Babol, Mazandaran, Iran in 1987. She 45

received the B.Sc. degree in Civil engineering from Tabari Institute of 46

Higher Education in 2010, M.Sc. degree in Geotechnical engineering from 47

Babol Noshirvani University of Technology in 2014. She is interested in soil 48

improvement and soil reinforcement. 49

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* Corresponding author

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Engineering properties of plastic waste reinforced sand

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55 Abstract: A series of triaxial compression tests were performed to evaluate the benefits of plastic 56

wastes and investigate the engineering properties of sand reinforced with these materials. In this 57

research, the effects of plastic waste contents (0, 0.25, 0.5, 0.75, and 1% by dry weight of sand), types of 58

plastic wastes -polyethylene terephthalate (PET) and polypropylene (PP) fibers- and confining pressures 59

(50, 100 and 200 kPa) on the behavior of Babolsar sand were investigated. The values of deformation 60

modulus (up to 84%), peak (up to 7 times of the unreinforced sand) and steady state shear strength 61

increased with reinforcement. Also, axial strain at failure for fiber-reinforced sand increased up to 1.5 62

times of unreinforced one (from 3.36% to 8.53% for 1% PP usage at 50 kPa confining pressure). 63

Therefore, it can be generally stated that the use of plastic wastes in the sand leads to low cost soil 64

reinforcement and also lessens the disposal problem of these kinds of wastes. 65

Keywords: Sand reinforcement, Shear strength, Plastic waste, Fiber-shaped plastic, Triaxial 66

compression test. 67

1. Introduction 68

Nowadays lack of accessibility to lands with fair bearing capacity for construction is an 69

important problem, so this problem force engineers to use local lands. In such cases, soil 70

improvement techniques such as soil reinforcement and soil stabilization behaved satisfactorily 71

in many conditions. Soil reinforcement has been performed with different methods and materials 72

such as various types of geosynthetics and fibers. 73

There are at least two advantages in using randomly distributed fiber as reinforcement. First, 74

the discrete fibers are simply added and mixed randomly with soil, in much the same way as 75

cement, lime, or other additives. Second, randomly distributed fibers limit potential planes of 76

weakness that can develop parallel to oriented reinforcement [1-4]. 77

Nowadays, the tendency to use alternative materials which can fulfill design specification is 78

promoted because of environmental and economic problems. One possible way to reuse these 79

wastes is to convert them into materials for soil reinforcement and construction applications like 80

highway base material and backfill of retaining walls. The soil which is reinforced by waste 81

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plastic strip can be used in embankment/road construction which leads to significant reduction in 82

cost as well as safe disposal of these waste materials in an environmental friendly manner [5-9]. 83

So, many researchers have focused on finding suitable ways for reusing waste materials. Plastic 84

wastes are usually materials with high strength and less reaction with acids and alkalis. These 85

kinds of wastes are not biodegradable, so they remain unchanged for years and cause 86

environmental pollution [6-7]. 87

Using plastic wastes such as tire shreds to improve the mechanical properties of soil dates 88

back to the 1990. Many researchers studied on the engineering properties of plastic waste 89

reinforced soils [2, 4, 6-18]. The idea of incorporating other plastic wastes in soil was first 90

proposed by Benson and Khire [19]. They used translucent HDPE milk jugs which were cut into 91

strips as reinforcing material. Direct shear tests were conducted on samples, and results showed 92

that adding these wastes into the soil increased friction angle and shear strength of sand. Consoli 93

et al. [20] conducted an experimental study on uncemented and artificially cemented soil 94

reinforced with polyethylene fibers derived from plastic wastes. The results demonstrated that 95

plastic wastes improved the stress-strain response of both uncemented and cemented sand. Kim 96

et al. [21] investigated the behavior of reinforced and unreinforced lightweight soils. The 97

unconfined compression tests as well as those of one-dimensional compression tests showed that 98

the strength of reinforced lightweight soil generally increased after adding waste fishing net (0%, 99

0.25%, 0.5%, 0.75% and 1% of the dry soil weight), however the level of increase in 100

compressive strength was not directly proportional to the percentage of waste fishing net. The 101

results of the tests indicated that the maximum increase in compressive strength was obtained for 102

soil mixed with 0.25% waste fishing net. Babu et al. [22] conducted experiments on sand 103

samples reinforced with waste PET pieces (0.5%, 0.75% and 1% of dry soil weight) obtained 104

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from waste water bottles. They observed that inclusion of these plastics in the soil improved the 105

shear strength, tensile strength and internal friction angle of the soil. Muntohar et al. [23] carried 106

out an experimental study on silty soil stabilized with lime and rice husk ash and reinforced with 107

waste plastic fibers. The results showed that this method was an effective way to improve the 108

engineering properties of the silty soil with regard to compressive strength, tensile strength, and 109

shear strength, which enhanced the stability of the soil; also, the optimum amount of fiber in the 110

soil/lime/rice husk ash/fiber mixtures was ranged from 0.4–0.8% of the dry soil weight. 111

Abbaspour et al. [7] performed a series of static and cyclic laboratory tests to contribute to 112

manage and prevent the burial of a part of hazardous wastes produced during the recycling 113

process of worn tires. Their results showed that the fiber inclusion enhances all geotechnical 114

properties of the soil under static state. Also, they concluded that under dynamic state, the fibers 115

can increase the energy absorption and dissipation properties of the soil, as well as the resilient 116

modulus and damping ratio with an optimum fiber content of 1–2%. 117

Therefore, based on previous researches, the main advantages of using short fibers over planar 118

reinforcement can be summarized as follows: 119

(1) Improving soil physical properties. 120

(2) Provide more uniformity. 121

(3) Provide considerable flexibility. 122

(4) Provide high levels of stiffness to weight ratio. 123

(5) Increase in toughness. This means more energy absorption ability. So, this property 124

makes it suitable for subgrades of airport pavements, blast resistant structures and etc. 125

In this paper an experimental study of the utilization of two kinds of fiber-shaped plastic 126

wastes (PET and PP) for sand reinforcement and their effects on shear strength, ductility and 127

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stiffness of the sand are described. Also, the effects of plastic waste fiber content, confining 128

pressure and length of PET fibers on sand behavior is examined. 129

2. Materials 130

2.1. Sand 131

The sand used in this research was obtained from shores of Caspian Sea (Babolsar- Iran). The 132

grain size distribution curve of the soil, which was obtained based on ASTM D-422 [24], is 133

shown in Figure 1. Babolsar sand is uniform and clean with subrounded to subgranular particles 134

and classified as SP according to the unified soil classification system, ASTM D-2487 [25]. The 135

mean particle diameter (D50) of the sand was 0.2 mm. The specific gravity of sand particles was 136

determined a value of 2.75 based on ASTM D-854 [26]. Minimum and maximum dry unit 137

weights of the sand were obtained according to ASTM D-4254 [27] and ASTM D-4253 [28] 138

equal to 14.8 and 17.4 kN/m3 respectively. 139

2.2. Reinforcing materials 140

The reinforcing materials used in this study were two kinds of plastic wastes, polyethylene 141

terephthalate (PET) and polypropylene (PP). The PET waste was recycled from plastic water 142

bottles. These plastic water bottles were accumulated and then melted to transform into fiber-143

shaped material. The fibers were cut into 5, 10 and 15 mm pieces to be used as reinforcing 144

elements in sand (Figure 2). The waste PP fibers have been taken from a factory which produced 145

polypropylene bags and then cut into 15 mm length pieces (Figure 3). The characteristics of used 146

fibers are shown in Table 1. 147

3. Sample preparation and testing procedure 148

To prepare samples mixed with PET fibers, an appropriate amount of the sand as well as 149

plastic waste was weighed. To ensure uniform distribution of PET fibers in the mixture, at first 150

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the specified dry soil was mixed with 5% water content and then the specified weight of plastic 151

waste (by dry weight of the soil) was mixed with soil until obtaining a uniform mixture (It is 152

suggested to add an amount of water to the sand to achieve a better mixture of sand and fiber 153

until it does not cause them to float [29]. In this research, to determine the required amount of 154

water for preparation of samples, different water contents including 5, 10, and 15% were 155

considered and the homogeneity of samples was evaluated. Based on the results, 5% water 156

content was determined as the optimum moisture content to prepare the uniform sand-fiber 157

mixtures). Since the tests were conducted on dry specimens; so, the mixtures were put into an 158

oven at 65°C for 48 hours prior to the tests, to get dry. Because it was observed that the PET 159

fibers begin to deform when the heat was over 65°C, so this temperature was used for the 160

purpose. 161

To prepare mixtures with PP fibers, the specified amount of sand and PP fibers were mixed 162

in dry condition. 163

After preparing the sand and plastic waste mixture, specimens were statically compacted in 164

four layers in a cylindrical mold - 38 mm diameter and 76 mm height – (similar to the mold that 165

used by Yadav and Tiwari [18] for the maximum length of 15 mm fiber) to a relative density of 166

70% based on the procedure proposed by Baldi et al. [30]. Both kinds of plastic wastes were 167

mixed with sand at different percentages (0, 0.25, 0.5, 0.75 and 1% by dry soil weight). 168

The prepared samples were tested in dry condition in a conventional triaxial apparatus. There 169

has been a lot of research on dry samples of granular soils in the past [31-33]. In this research, 170

the specimen's volume change was recorded and measured to monitor this property during the 171

shear, and also for using in area correction. A twin-burette volume change was applied to 172

measure the volume change of specimens on the cell pressure line. A total of 51 triaxial 173

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compression tests were performed on the unreinforced and reinforced specimens at a strain rate 174

of 0.35% per minute. The tests were performed on the samples with three values of confining 175

pressures (50, 100 and 200 kPa). Deviator load was applied till the specimens fail or attain the 176

axial strain of 15%. Corrections including membrane force [34], membrane penetration [35] and 177

cross-sectional area were taken into account and applied. The investigated variables are 178

illustrated in Table 2. 179

4. Results and discussion 180

In this section, the results of performed triaxial compression tests on specimens of 181

unreinforced and reinforced sand with two types of plastic wastes are presented based on peak 182

strength, ductility, failure strain, volumetric strain and secant modulus of deformation. 183

4.1. Effect on peak strength 184

As it can be seen in Figure 4, the peak strength increased with reinforcement of the sand. This 185

increment became more considerable with an increase in percentage and length of plastic wastes. 186

For example, the peak strength of the unreinforced sand under a confining pressure of 50 kPa 187

was increased from 230.5 to 288.8 kPa due to the reinforcement by 0.25% PET with 5 mm 188

length. This value was increased to 961.3 kPa (more than 4 times of the unreinforced one) for a 189

sample reinforced by 1% PET with 15 mm length. Also, for PP waste fiber-reinforced sand at 190

1% usage, the peak strength was reached to 1614 kPa (about 7 times of the unreinforced one) at 191

the same confining pressure. A similar trend was observed in previous works [4, 12-13, 20]. As 192

the plastic waste reinforced sand was subjected to deformation, friction which is appeared 193

between soil and plastic wastes led to development of tensile stress in the plastic wastes, also 194

increase in confinement of the sample, and consequently increase in the shear strength of the 195

samples. As it can be seen in Figure 5, at all PET contents and all confining pressures, the peak 196

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shear stress of sand improved with an increase in the length of waste PET fibers from 5 mm to 197

10 mm but after this value, the improvement of shear stress was negligible. This may be due to 198

the fact that longer fibers have longer embedment length; therefore, they endure greater tension 199

during shear. Actually, the value of increase in peak shear stress for the sand reinforced with 15 200

mm length PET fibers is almost the same as that for the sand reinforced with 10 mm length fibers 201

with a negligible difference. Previous research has also indicated a similar trend between the 202

shear strength improvement of fiber-reinforced soil and the length of the fiber [4]. This 203

phenomenon can be attributed to the following reasons: 204

The decrease of the number of fibers which participate in the failure zone – when the 205

percentage of the fiber is constant, the number of fibers decreases with the increase in fiber 206

length. 207

As the fiber length was increased, it got more difficult to make a uniform mixture with the 208

sand because the long fibers piled up together resulting in slippage; so, less improvement was 209

achieved when the percentage of the fiber was constant in the soil and waste plastic fibers 210

length was increased. 211

Nevertheless, the stress ratio (ratio of peak deviatoric stress of reinforced specimen to the 212

corresponding value of unreinforced one) decreased with the increase of confining pressure as 213

shown in Figure 6. Consider, for instance, in the sample reinforced by 1% PET with 15 mm 214

length, the stress ratio for 50 kPa confining pressure is 3.98, while for 200 kPa confining 215

pressure is about 1.57. A similar trend was observed in previous researches [4, 32, 36-37]. 216

Improvement of shear strength in dilating soils, with the inclusion of fibers is related to the level 217

of interaction between fibers and soil particles as well as the amount of dilation during the shear 218

stage that mobilizes the tensile strength of fibers. Dilatancy is the result of shear zone expansion 219

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during the mobilization of the reinforcing elements. Increase in the confining pressure limits the 220

rearrangement of the soil structure resulting in less dilation and this restricts the amount of fiber 221

stretch during the shear [4, 32, 36]. Hence, the efficiency of fiber reinforcement to increase the 222

shear strength of the dilating soil is obviously influenced by an increase in the confining 223

pressure. It is evident that the stress ratio of fiber- reinforced sand was reduced from the value of 224

4 at the confining pressure of 50 kPa to the 1.5 times of unreinforced sand at the confining 225

pressure of 200 kPa and it can be concluded that the efficiency of fibers in increase of the shear 226

strength of medium dense sand reduces at high confining pressures regardless of the fiber 227

content. So, to reach the maximum efficiency, it is appropriate to use these fibers as 228

reinforcement for the soils with low to medium overburden stress range, such as base layer in 229

road construction. 230

The results obtained from the conducted tests indicated that, for all different waste contents 231

and confining pressures, sand reinforced with PP plastic wastes had a higher peak deviatoric 232

stress compared to the sand reinforced with waste PET fibers. For example the tests conducted 233

under a confining pressure of 50 kPa, indicated that the peak shear stress for sand reinforced with 234

15 mm length PET fiber (Figure 4-c) is 4.17 times more than unreinforced sand and for sand 235

reinforced with waste PP fibers (Figure 4-d), it was 7 times more than that of the unreinforced 236

one; these values were achieved for specimens with 1% plastic waste. This may be due to the 237

tensile strength of wastes; as the tensile strength of PP fibers was more than the PET fibers, so 238

the PP fibers were crushed during the shearing (Figure 7) but the PET fibers torn in the failure 239

zone. 240

4.2. Effect on ductility 241

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Based on Figure 4 and Table 3, it can be seen that the post-peak loss of shear stress is 242

decreased for reinforced specimens. Actually, steady state stress of reinforced samples was 243

increased with an increase in plastic waste content and length. The reason for this can be 244

explained by the fact that when the samples are loaded, the fibers act like a bridge and prevent 245

the occurrence of early and large deformations in the soil. As a result, the soil shows significant 246

strength against larger strains and less strength drop. So, the brittleness index, which indicates 247

the fragility and ductility of the reinforced soil, that was calculated based on Bishop's [38] 248

definition (equation 1) and depicted in Table 4, decreased with the inclusion of waste fibers in 249

the soil. 250

IB=q𝑝 −qs

q𝑝 (1) 251

Where IB is brittleness index, qp is peak shear stress and qs is steady state shear stress. 252

According to the Table 4, the brittleness index of the reinforced specimens decreased with an 253

increase in the waste fiber length and percentage. However, the amount of this reduction was 254

very small for the reinforced sand with 5 mm length PET fibers (about 11% for 1% usage). But, 255

the greatest decrease in the brittleness index was observed for the waste PP-reinforced sand 256

(about 75% for 1% usage). Actually, sand reinforced by the waste PP fibers has more ductile 257

behavior than the sand reinforced by the waste PET fibers. The reason for this is explained 258

previous and is related to the tensile strength of wastes. Generally, it can be argued that the 259

ductility of the Babolsar sand improved by reinforcing with plastic wastes. Increase of soil 260

ductility lead to improvement of seismic stability of geotechnical projects such as airport 261

runways and rail embankments [39]. 262

4.3. Effect on failure strain 263

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As demonstrated in the Figures 8-9, sand reinforced with plastic waste had a greater axial 264

strain at failure in comparison to the unreinforced sand and the strain at failure increased with an 265

increase in fiber-shaped waste content and length. As an example, for the sample reinforced with 266

1% PP at 50 kPa confining pressure, the axial strain at failure increased up to 1.5 times of 267

unreinforced one (from 3.36 to 8.53%). Actually, at all confining pressures, axial strain at failure 268

increased with an increase in plastic waste percentage. As stated earlier, this can be attributed to 269

the appeared friction between the soil and plastic waste fibers (when the load applied to the 270

specimen) which led to development of tensile stress in the plastic wastes. In addition, this 271

tensile stress cause confining pressure in the sample, which result in increase of axial strain at 272

failure in reinforced samples [22]. Due to the greater tensile strength of PP waste fiber (as 273

mentioned earlier), the amount of increase in the axial strain at failure for the PP-fiber reinforced 274

sand is higher than the PET-fiber reinforced one. 275

4.4. Effect on volumetric strain 276

The changes of volumetric strain against axial strain for unreinforced and reinforced sand 277

with plastic waste (PET) were presented in Figure 10. A closer look at this figure shows that: 278

1- As anticipated during the primary part, both unreinforced and reinforced samples 279

exhibited small contraction in their volumes. With the advance of shear stress, the trend is 280

reverted and the samples showed an increase in their volumes. However, the increase in 281

confining pressure restricted the volumetric dilation of them. 282

2- The dilation of specimens is decreased by the plastic wastes (PET) inclusion. Many 283

researchers [40-41] reported that the dilation occurs principally in the center of samples. 284

Dilation and lateral deformation of the top and bottom of the specimen is restrained by the 285

cap and the base. Also, it is believed that plastic fibers reduce lateral deformation. As 286

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stated previously, inclusion of fiber in the sand due to the appeared friction between soil 287

and plastic waste increase the confinement of the sample, which leads to decrease of the 288

lateral deformation. Therefore, it is obvious that plastic waste fibers efficiently reduce the 289

dilation of the specimens. A similar behavior was reported in previous researches [37, 40-290

41]. This phenomenon may be attributed to the decrease of maximum dry unit weight of 291

the sand due to the inclusion of fiber in the specimen and also to the small size of fiber 292

holes compared to the D50 of the sand. This outcome becomes more obvious when the 293

PET content and length increases. 294

4.5. Effect on secant modulus of deformation 295

The secant deformation modulus (E50), which was shown in Figure 11, increased with the 296

increase in waste content for two types of wastes (PET and PP) and for all lengths. The value of 297

this increment is highest for the sample reinforced by 1% PET with 5 mm length. So that, E50 298

increased from 18.6 MPa for the unreinforced sample to 34.2 MPa (about 84% increment) for the 299

mentioned sample at 50 kPa confining pressure. As mentioned earlier, by increasing plastic 300

waste length, the peak strength and axial strain at failure increases. Now, it can be stated that for 301

reinforced sample with shorter fiber lengths, axial strain has a more limited increase than peak 302

strength. As a result, the value of E50 has increased further in this case. Also, the figure shows the 303

amount of increase of E50 has decreased with increasing PET length, however for reinforced 304

samples with 10 and 15 mm lengths of PET is almost the same (similar to the results obtained for 305

peak strength of these two samples). Moreover, due to the better strength feature of PP than PET, 306

the sample reinforced by PP with 15 mm length has a higher E50 than the sample reinforced by 307

PET with 15 mm length. 308

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The results of this study generally showed that the use of PET and PP waste fibers in soil 309

reinforcement improves soil behavior. The achieved improvement of reinforcement with PP 310

waste fiber is more than PET one, but both of them significantly improved the soil properties. 311

Depending on the expected conditions and the type of available plastic waste in the area, each of 312

them can be used. By use of them, both the environmental effects of existence of waste are 313

reduced (as stated by Hejazi et al. [3], the main reason of using plastic wastes in geotechnical 314

engineering is the environmental purposes) and the behavior of the soil is improved according to 315

the results of this research. 316

5. Conclusions 317

In this research an idea to reuse the plastic wastes in geotechnical engineering applications is 318

presented. A series of triaxial compression tests were conducted on the reinforced sand with 319

these materials. The content of these fiber-shaped plastic wastes varied from 0 to 1%. The effects 320

of four factors on the behavior of plastic waste reinforced sand were investigated: plastic waste 321

type, plastic waste length, plastic waste content and confining pressure. The experimental results 322

showed that: 323

(1) Shear strength of sand increased with the inclusion of both kinds of plastic wastes. As the 324

plastic waste reinforced sand was subjected to deformation, friction which is appeared 325

between soil and plastic wastes led to development of tensile stress in the plastic wastes, 326

also increase in confinement of the sample, and consequently increase in the shear strength 327

of the samples. 328

(2) The amount of increase in peak stress increased with an increase in PET fiber length from 329

5 to 10 mm, and due to the fact that longer fibers have longer embedment length, after this 330

value the improvement of peak stress was negligible. 331

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(3) The peak shear strength for the PP fiber reinforced sand (for 1% fiber addition, about 7 332

times of the unreinforced sand) was greater than the PET fiber reinforced sand (for 1% 333

fiber addition and 15 mm length, about 4.17 times of unreinforced sand) due to the more 334

tensile strength of this kind of fiber. 335

(4) The stress ratio (ratio of peak deviatoric stress of reinforced specimen to the corresponding 336

value of unreinforced one) decreased from 3.98 to about 1.57 (for the sample reinforced by 337

1% PET with 15 mm length) with the increase of confining pressure from 50 to 200 kPa. 338

Increase in the confining pressure limits the rearrangement of the soil structure resulting in 339

less dilation and this restricts the amount of fiber stretch during the shear. 340

(5) The inclusion of fiber-shaped plastic wastes in the sand, made its behavior more ductile, 341

and reduced its brittleness index (up to 75%). In the reinforced sample, the fibers act like a 342

bridge and prevent the occurrence of early and large deformations in the soil. As a result, 343

the soil shows significant strength against larger strains and less strength drop. 344

(6) Due to the fiber inclusion in the sand, strain of the fiber reinforced sand at failure 345

increased up to 1.5 times of unreinforced one (from 3.36 to 8.53% for 1% PP usage at 50 346

kPa confining pressure) because of the appeared friction between the soil and plastic waste 347

fibers. 348

(7) The dilation of the plastic waste reinforced sand decreased with an increase in plastic waste 349

content and length, up to about 30%. This phenomenon may be attributed to the decrease 350

of maximum dry unit weight of the sand due to the inclusion of fiber in the specimen. 351

(8) The secant deformation modulus (E50), increased with the increase in fiber (waste) content. 352

The maximum increment is observed for the sample reinforced by 1% PET with 5 mm 353

length (about 84%). 354

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Conflict of interest 355

The authors confirm that there are no known conflicts of interest associated with this 356

publication. 357

Acknowledgements: 358

The authors acknowledge the funding support of Babol Noshirvani University of 359

Technology through Grant program NO. BNUT/370342/98. 360

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468

469

470

471

472

473

474

20

Table Captions: 475

Table 1. Characteristics of used fibers 476

Table 2. Variable factors in the testing program 477

Table 3. Strength properties of reinforced and unreinforced sand 478

Table 4. Brittleness index for reinforced sand with plastic waste at confining pressure of 100 kPa 479

21

Figure Captions:

Figure 1. Grain size distribution curve for Babolsar sand.

Figure 2. Recycled PET fibers.

Figure 3. Waste PP fibers.

Figure 4. Stress-Strain curves at 50 kPa confining pressure: a) for different PET fiber contents

with a length of 5 mm, b) different PET fiber contents with a length of 10 mm, c) different PET

fiber contents with a length of 15 mm and d) different PP fiber contents with a length of 15 mm.

Figure 5. Effect of PET fiber length on peak deviatoric stress: a) for 100 kPa confining pressure

and different PET contents, b) for 1% PET and different confining pressures.

Figure 6. Stress ratio versus confining pressure for sand reinforced by 1% PET and different

length.

Figure 7. Waste PP fibers in the failure zone.

Figure 8. Effect of PET: a) fiber content on strain at failure for 50 kPa confining pressure, b)

fiber length on strain at failure for 100 kPa confining pressure.

Figure 9. Effect of PP fiber content on strain at failure for different confining pressures.

Figure 10. Volumetric change curves for PET fibers with different lengths and 50 kPa confining

pressure: a) fiber content of 0.25%, b) fiber content of 0. 5%, c) fiber content of 0.75% and d)

fiber content of 1%.

Figure 11. Deformation modulus of reinforced sand by varying contents of plastic wastes at 50

kPa confining pressure.

22

Tables:

Table 1. Characteristics of used fibers

Fiber

type

Length

(mm)

Width

(mm)

Diameter

(mm)

Specific

gravity

(g/cm3)

Elastic

modulus

(GPa)

UTS*

(MPa)

PET× 5,10,15 - 0.4-0.8 0.92 0.65 200

PP●

15 2-2.5 - 0.92 3.2 300

* Ultimate tensile strength

× Polyethylene terephthalate

● Polypropylene

23

Table 2. Variable factors in the testing program

Variable Range

Confining pressure 50, 100, and 200 kPa

Plastic waste content 0, 0.25, 0.5, 0.75, and 1%

Waste PET length 5, 10, and 15 mm

Waste PP length 15 mm

24

Table 3. Strength properties of reinforced and unreinforced sand

Material

Confining pressure

(kPa)

Peak stress at failure

(kPa)

Steady state stress

(kPa)

Sand

50

100

200

230.5

563

1063.2

130.1

343

747.6

Sand + 1% waste PET

(5mm)

50

100

200

500.2

943.5

1546.1

282.2

602.7

1132.2

Sand + 1% waste PET

(10mm)

50

100

200

917.3

1242.2

1665.4

563.7

880.3

1437.3

Sand + 1% waste PET

(15mm)

50

100

200

961.3

1246.6

1698

730

939.7

1472.3

Sand + 1% waste PP 50

100

200

1614

2198.6

2691.9

1289

1889.8

2399.7

25

Table 4. Brittleness index for reinforced sand with plastic waste at confining pressure of 100

kPa

Material Plastic waste content

0% 0.25% 0.5% 0.75% 1%

Sand + waste PET (5mm) 0.64 0.61 0.61 0.59 0.57

Sand + waste PET (10mm) 0.64 0.53 0.44 0.43 0.41

Sand + waste PET (15mm) 0.64 0.51 0.43 0.38 0.32

Sand + waste PP 0.64 0.56 0.31 0.25 0.16

26

Figures:

Figure 1. Grain size distribution curve for Babolsar sand.

27

Figure 2. Recycled PET fibers.

28

Figure 3. Waste PP fibers.

29

Figure 4. Stress-Strain curves at 50 kPa confining pressure: a) for different PET fiber contents

with a length of 5 mm, b) different PET fiber contents with a length of 10 mm, c) different PET

fiber contents with a length of 15 mm and d) different PP fiber contents with a length of 15 mm.

30

Figure 5. Effect of PET fiber length on peak deviatoric stress: a) for 100 kPa confining pressure

and different PET contents, b) for 1% PET and different confining pressures.

31

Figure 6. Stress ratio versus confining pressure for sand reinforced by 1% PET and different

length.

32

Figure 7. Waste PP fibers in the failure zone.

33

Figure 8. Effect of PET: a) fiber content on strain at failure for 50 kPa confining pressure, b)

fiber length on strain at failure for 100 kPa confining pressure.

34

Figure 9. Effect of PP fiber content on strain at failure for different confining pressures.

35

Figure 10. Volumetric change curves for PET fibers with different lengths and 50 kPa confining

pressure: a) fiber content of 0.25%, b) fiber content of 0. 5%, c) fiber content of 0.75% and d)

fiber content of 1%.

36

Figure 11. Deformation modulus of reinforced sand by varying contents of plastic wastes at 50

kPa confining pressure.