Evaluation of the Shear Strength Behavior of TDA Mixed with Fine and Coarse Aggregates for Backﬁlling Around Buried Structures

sustainability

Article Evaluation of the Shear Strength Behavior of TDA Mixed with Fine and Coarse Aggregates for Backﬁlling around Buried Structures

Hany El Naggar * and Ali Iranikhah

Department of Civil and Resource Engineering, Dalhousie University, Halifax, NS B3H 4R2, Canada; [email protected] * Correspondence: [email protected]; Tel.: +1-902-494-3904

Abstract: Although some discarded tires are reused in various applications, a considerable number end up in landfills, where they pose diverse environmental problems. Waste tires that are shredded to produce tire-derived aggregates (TDA) can be reused in geotechnical engineering applications. Many studies have already been conducted to examine the behavior of pure TDA and soil-TDA mixtures. However, few studies have investigated the behavior of larger TDA particles, 20 to 75 mm in size, mixed with various types of soil at percentages ranging from 0% to 100%. In this study, TDA was mixed with gravelly, sandy, and clayey soils to determine the optimum soil-TDA mixtures for each soil type. A large-scale direct shear box (305 mm × 305 mm × 220 mm) was used, and the mixtures were examined with a series of direct shear tests at confining pressures of 50.1, 98.8, and 196.4 kPa. The test results indicated that the addition of TDA to the considered soils significantly reduces the dry unit weight, making the mixtures attractive for applications requiring lightweight fill materials. It was found that adding TDA to gravel decreases the shear resistance for all considered TDA contents. On the contrary, adding up to 10% TDA by weight to the sandy or clayey soils was Citation: El Naggar, H.; Iranikhah, A. Evaluation of the Shear Strength found to increase the shear resistance of the mixtures. Adding up to 10% TDA by weight to the clayey ◦ ◦ Behavior of TDA Mixed with Fine soil also sharply increased the angle of internal friction from 18.8 to 32.3 . Moreover, it was also and Coarse Aggregates for Backfilling found that the addition of 25% TDA by weight to the gravelly or sandy soils can reduce the lateral around Buried Structures. earth pressure on buried structures by up to 20%. In comparison, adding 10% TDA to clay resulted Sustainability 2021, 13, 5087. https:// in a 36% reduction in the lateral earth pressure. doi.org/10.3390/su13095087 Keywords: tire-derived aggregates (TDA); fine aggregates; coarse aggregates; soil-TDA mixtures; Academic Editor: Jorge de Brito large-scale direct shear box

Received: 18 March 2021 Accepted: 30 April 2021 Published: 1 May 2021 1. Introduction Growing populations are generating significant numbers of discarded scrap tires Publisher’s Note: MDPI stays neutral every year. Disposing of waste tires in landfills contributes to serious environmental with regard to jurisdictional claims in published maps and institutional affil- problems, and it is crucial to find an environmentally friendly solution. Since scrap tires iations. are bulky, they occupy considerable space in landfills. In addition, during rainy seasons, stockpiled tires collect rainwater, which provides a breeding ground for many insects, including mosquitoes, that can transfer dangerous diseases such as encephalitis to humans. Moreover, there is a potential risk of fire from stockpiled tires [1]. Over 500 million scrap tires are discarded in the USA annually [2]. Only around 22% of them are recycled and Copyright: © 2021 by the authors. reused in various applications, while the rest end up in landfills and illegal dumps [1]. Licensee MDPI, Basel, Switzerland. According to the Rubber Association of Canada, approximately 30 million scrap tires are This article is an open access article distributed under the terms and generated in Canada each year, which is almost one tire per person [3]. Fortunately, some conditions of the Creative Commons countries such as Canada and the USA are currently implementing an active tire recycling Attribution (CC BY) license (https:// program. To fund the program, an environmental fee is applied to the purchase of new creativecommons.org/licenses/by/ tires, and the money collected is used to divert scrap tires from landfills. 4.0/).

Sustainability 2021, 13, 5087. https://doi.org/10.3390/su13095087 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 5087 2 of 25

Scrap tires are comprised of synthetic rubber, ﬁbers, and steel cords. Steel and ﬁber wires reinforce the rubber, and a steel belt is positioned below the tread. Table1 lists the typical materials used in passenger car and truck tires.

Table 1. Typical composition of tires [4].

Material Passenger Tire Truck Tire Natural rubber 14% 27% Synthetic rubber 27% 14% Carbon black 28% 28% Steel 14–15% 14–15% Fiber 16–17% 16–17% Average weight (new) 11 kg

Tire shreds can be categorized based on their size and the shredding techniques used. According to Strenk et al. [5], tire particles ranging from 50 to 305 mm are referred to as tire shreds, while those ranging from 12 to 50 mm are called tire chips. These two particle types are commonly known as tire-derived aggregate (TDA). Tire particles less than 12 mm in size are referred to as granular or crumb rubber. Because smaller tire particles require greater processing and shredding time, their production cost is higher than that of larger alternatives. Table2 presents the cost of processing and shredding scrap tires based on the particle size. As shown in the table, reducing the particle size leads to higher processing costs.

Table 2. Tire shredding costs [4].

Particle Size Cost per Ton Process Rate (tons/hour) 50 mm $12 10–12 <50 mm $31 7 <12.5 mm $31–$68 2–3

Based on ASTM D6270 [6], tire shreds are classified into two types, A and B. Type A tire shreds have a maximum dimension of 200 mm in any direction. Type B tire shreds have a maximum dimension of 450 mm in any direction, or a maximum dimension of 300 mm for at least 90% of the sample by weight. TDA and soil-TDA mixtures have been used for many years in geotechnical engineering applications. Such mixtures can be used as lightweight embankment fill, retaining wall backfill, backfill above and around buried pipes, culverts and cut-and-cover tunnels, underneath shallow foundations or as landfill drainage layers, and in vibration control applications, among several other civil engineering applications [7–20]. Pure TDA fill up to 3 m in thickness can be used without leading to internal heating problems [6]. Fill layers thicker than 3 m may suffer from self-ignition due to internal heating. Soil-TDA mixtures have great potential as a lightweight fill material that can reduce the problem of TDA self-ignition and decrease compressibility behavior [21]. Several researchers investigated various soil-TDA mixtures. For instance, Kim and Santamarina [22] mixed TDA with small particle sizes (granulated with D50 = 3.5 mm) with Ottawa sand at different mixture compositions. The obtained results revealed that the mixture response was controlled by the behavior of the sand for low TDA content mixtures; however, at higher TDA contents, the response was governed by the behavior of the TDA. Likewise, Mashiri et al. [23] mixed sand with various contents of small TDA particles (8 mm × 20 mm). It was found that adding up to 35% TDA to the mixture improved the shear strength and resulted in a significant reduction in the dilatancy. Soltani et al. [24] examined the potential of mixing TDA with clay with high expansivity. The TDA used in the examined mixtures had a mean particle diameter, D50, ranging between 0.46 mm to 3.34 mm. The study concluded that adding 10% TDA, with gradation equivalent to medium or coarse sands, enhances the Sustainability 2021, 13, 5087 3 of 25

strength of the mixture and reduces the soil’s swelling potential from high to moderate expansivity. Furthermore, Araujo et al. [25] mixed TDA having a mean particle diameter, D50 of 20 mm, with ﬁne lateritic soil from the Central-West region of Brazil. The study showed that the mixture with 5% TDA content was the optimum mixture composition. As shown above and further in the next section, most previous research has been focused on small TDA particles that do not contain steel wires. Hence, the primary objective of this study is to investigate the shear stress-strain behavior of various types of soil mixed with TDA with large particle sizes. Moreover, testing several soil-TDA mixtures and comparing their results side by side will enable a comprehensive insight into the behavior of the different mixtures and facilitate direct comparisons between the performance of each of them. The study also determines the effect of the TDA content on the stiffness behavior of the mixtures. Another objective is to examine how the TDA content affects the shear strength parameters of the mixtures, including the cohesion and the angle of internal friction.

2. State-of-the-Art Review of the TDA Shear Strength Behavior This section ﬁrst presents results of previous studies of the shear strength behavior of TDA and soil-TDA mixtures obtained from direct shear tests. Then, research results concerning the shear strength behavior of TDA and soil-TDA mixtures obtained from the triaxial compression method are described. Finally, a comparison is made between direct shear tests and the triaxial compression method to determine the advantages and disadvantages of each testing method.

2.1. Direct Shear Tests Humphrey and Sandford [26] conducted a series of large-scale direct shear box tests on pure tire shred samples from three different suppliers, with particle sizes ranging from 13 to 76 mm. The tests were conducted under low confining pressures of 17, 34, and 68 kPa. Failure was defined at a peak, and in some cases, in the absence of a peak, at 10% relative lateral displacement. Internal friction angles ranging from 19◦ to 25◦ and cohesion intercepts from 7.7 to 11.5 kPa were reported for the tire shreds. Similarly, Zahran and El Naggar [27] and El Naggar et al. [28] studied the effect of sample size and particle size of pure TDA samples on the shear strength behavior by using a large-scale direct shear apparatus. TDA particles ranging in size from 12 mm to 100 mm were considered. In addition, tests to determine the effect of the sample size were conducted by using five different square shear boxes ranging in size from 60 mm to 305 mm. Zahran and El Naggar [22] found that the angle of internal friction increased slightly as the shear box dimensions decreased. However, they reported that the maximum variation in the angle of internal friction and cohesion results for the different shear boxes was only 1.9◦ and 2.4 kPa, respectively. The study recommended that a shear box with a ratio between the width of the box and the maximum particle size (W/Dmax) greater than or equal to 4 should be used when evaluating the shear strength parameters of TDA to completely eliminate the effect of the size of the shear box. Their studies reported internal friction angles ranging from 22◦ to 25◦ and cohesion intercepts from 14.2 to 16.6 kPa for different particle and specimen sizes. Foose et al. [29] carried out a series of large-scale direct shear tests on sand-tire shred mixtures to determine the effects of the applied normal stress, sand matrix unit weight, shred content, and shred length and orientation on the shear strength behavior of sand- TDA mixtures. Foose et al. [29] used three TDA samples, with particle sizes of <50 mm, 50 to 100 mm, and 100 to 150 mm. They mixed each sample with sand in the amount of 10%, 20%, and 30% TDA. These researchers applied normal stresses ranging from 3 to 120 kPa to each sample upon shearing. Failure was defined at a peak or, in the absence of a peak, at 9% relative horizontal displacement. The tire shred content, sand matrix unit weight, and normal stress were found to be the main factors affecting the shear strength behavior of the mixtures. Adding 30% tire shreds with a particle length of 150 mm to sand was observed to enhance the shear strength, achieving an internal friction angle as large as Sustainability 2021, 13, 5087 4 of 25

67◦. For pure tire shreds, regardless of length, the results showed an internal friction angle of 30◦ and cohesion of 3 kPa, a slightly higher friction angle and lower cohesion than what was reported by [26–28]. Similarly, El Naggar et al. [3] conducted a series of large-scale direct shear box tests to study the effect of TDA gradation on the shear strength properties of sand-TDA mixtures. TDA samples with particle sizes of 0.3, 23.5, and 48.5 mm were selected and mixed with sand in the amounts of 15%, 25%, 50%, and 100% TDA by volume. They found that a composition of 15% TDA by volume (with 25% of the particles 0.3 mm in length, 25% 23.5 mm in length, and 50% of the particles 48.5 mm in length) resulted in a greater shear strength than other sand-TDA mixtures. El Naggar et al. [3] also recorded an increase of 3◦ to 6.5◦ in the angle of internal friction of the mixtures compared to pure sand. They noted that the addition of TDA to the sand resulted in a strain-hardening behavior of the mixture. Likewise, Tatlisoz et al. [30] conducted a series of large-scale direct shear tests on clean sand and sandy silt mixed with tire chips to determine the shear strength properties of the mixtures. Tire shreds with a length of 30 to 110 mm were mixed with the soils in the amount of 10% to 100% by volume. These researchers applied three low normal stresses of less than 50 kPa upon shearing. The addition of 30% tire shreds by volume to the clean sand was found to increase the mixture shear strength significantly, but the further addition of tire shreds beyond 30% then reduced the shear strength. It was observed that the addition of more than 30% tire shreds to the sand contributed to the segregation of the soil and the tire shred particles, resulting in a soil-TDA mixture with weak shear strength properties. It should be noted that according to Tatlisoz et al. [30], no significant increase in shear strength was observed when tire shreds were added to the sandy silt. Furthermore, a study was performed by Akbulut et al. [31] to investigate the shear strength behavior of clayey soil mixed with randomly oriented scrap rubber tire or synthetic fibers ranging from 2 to 15 mm in length (i.e., small particles). These researchers used a small-scale direct shear ring with a ring diameter of only 60 mm and a height of 35 mm. Akbulut et al. [31] found that adding waste tire rubber to clayey soil enhanced the shear strength properties of the soil. The length and percentage content of the rubber tire fibers were found to be the main factors influencing the shear strength of the mixture. Akbulut et al. [31] observed that the shear strength increased when up to 2% rubber tire fibers with a length of 10 mm were added to the soil, but then decreased with the further addition of rubber tire fibres.

2.2. Triaxial Compression Tests Wu et al. [32] carried out a series of small-scale triaxial compression tests with a constant stress path method to examine five tire chip products having different sizes and shapes. The tire chips ranged from 2 to 38 mm in length, and the shapes were flat, granular, elongated, and powdered. These researchers conducted tests at low confining pressures ranging from 34.5 to 55 kPa and observed that the shear strength was fully mobilized at an axial strain of more than 5%. Wu et al. [32] recorded tire chip internal friction angles ranging from 44◦ to 56◦ at various axial strains. The largest interparticle friction angle was recorded for flat tire chips with a length of 38 mm. These researchers also noted that the effect on the cohesion intercept was negligible due the low confining pressures, ranging from 34.5 to 55 kPa. El Naggar and Zahran [33] carried out a series of large-scale triaxial compression tests of large size TDA particles at confining pressures ranging from 50 to 200 kPa to investigate the effect of the particle size on the TDA shear strength parameters. Whereas El Naggar et al. [34] conducted consolidated drained triaxial tests on TDA with large particles using the same large-scale triaxial apparatus of [33] to develop an empirical hyperbolic material for TDA. El Naggar and Zahran [33] and El Naggar et al. [34] reported internal friction angles ranging from 21.3◦ to 25.6◦ and cohesion intercepts from 28 to 32 kPa for different particle sizes when the failure was assumed to occur at 15% as per the ASTM standards as no peak was observed. Ashari et al. [35] carried out a series of large-scale triaxial compression tests of sand-TDA mixtures at confining pressures ranging from 50 to 200 kPa. These researchers noted that the friction angle remained relatively constant, but the cohesion increased as the TDA content increased in Sustainability 2021, 13, 5087 5 of 25

the sand-TDA mixtures. Ahmed [36] carried out a series of large-scale triaxial compression tests to examine the shear strength behavior of tire chips and sand-tire chip mixtures. He compacted the tire chips by using various compaction energy methods (modified, standard, and 50% standard) and applied confining pressures ranging from 31.02 to 206.8 kPa to the specimens. Ahmed [36] determined the shear strength of the mixtures at 5%, 10%, 15%, and 20% axial strains and found that adding up to 38% tire chips by weight to the sand reduced the dry unit weight while increasing the shear strength of the mixture. He reported internal friction angles ranging from 25.46◦ to 38.1◦ and cohesion intercepts from 36.4 to 49.99 kPa for TDA mixing ratio of 38% tire chips at axial strains of 5% to 20%. Masad et al. [37] conducted a series of triaxial compression tests utilizing smaller samples (with a specimen diameter of 71.1 mm) for small-size tire shreds, sand, and sand-tire shred mixtures. The maximum particle size of the tire shreds used was 4.75 mm. These researchers found that the addition of tire shreds to sand increased the compressibility and reduced the density of the mixture. They also noted that the modulus of elasticity of the sand-tire shred mixture was significantly lower than that of sand alone. However, at a higher confining pressure, the resilient modulus of the mixture was higher than that of sand alone. They concluded that sand-tire shred mixtures can be used as a lightweight material beneath conventional soils, and can exhibit a high resilient modulus due to confinement. Masad et al. [37] also observed a strain-hardening behavior in pure tire chips similar to the observations of [33,34]. Lee et al. [9] performed a series of large-scale triaxial compression tests to study the shear strength behavior of tire chips mixed with sand. They removed exposed steel from the tire chips and limited the size of the tire chips to 30 mm. The vibration method was used to compact the mixtures. Tests were performed on specimens in a consolidated drained condition, and confining pressures ranging from 28 to 193 kPa were applied to the specimens. The test results showed that the relationship between deviatoric stress and the axial strain was almost linear up to a strain of 25%. A similar observation was made for volumetric change versus strain. Lee et al. [9] also noted that the addition of tire chips to the sand resulted in increased dilatancy behavior. Shear strength properties of tire shreds mixed with sand were also studied by Youwai and Bergado [38], who used a series of triaxial compression tests. They selected mixing ratios of 0:100, 50:50, 60:40, 70:30, 20:80, and 100:0 by weight, and limited the maximum tire shred size to 16 mm. They found that the addition of tire chips to sand contributed to a mixture with both dilation and compression characteristics. The amount of deformation was significant for mixtures containing more than 70% tire shreds. These researchers also developed a constitutive model within a critical state framework to predict the stress versus strain behavior of sand- tire shred mixtures. They noted that due to the high deformability of the mixtures upon axial loading, such a model should be considered as both elastic and plastic. The study also found that adding up to 70% tire shreds by weight to the sand reduced the internal friction angle from 34◦ to 30◦. Likewise, Zornberg et al. [39] performed a series of large-scale triaxial compression tests to examine the shear strength behavior of pure tire shreds and sand-tire shred mixtures. The tire shreds had a maximum length of 102 mm and were mixed with the sand in amounts of 0% to 100% by weight. The tests were performed under confining pressures ranging from 48.3 to 207 kPa upon axial loading. Zornberg et al. [39] found that the percentage of tire shreds and the aspect ratio were the main factors affecting the shear stress versus strain behavior of the mixtures. They noted that an increase in aspect ratio enhanced the shear strength of the mixtures. They also found that adding up to 35% tire shreds to the sand increased the shear strength of the mixtures, but further addition of tire shreds beyond 35% then reduced the shear strength. Zornberg et al. [39] also found that adding tire shreds to the sand resulted in a mixture that did not fail up to 15% axial strain. Their results showed that compaction energy had a negligible effect on the shear strength properties of the mixtures. Finally, Rao and Dutta [40] conducted a series of small-scale triaxial compression tests on sand-tire chip mixtures having s maximum particle size of only 20 mm. The tire chips were mixed with sand in the amounts of 0%, 5%, 10%, 15%, and 20% by weight and were distributed randomly in the sand. Rao and Dutta [40] noted that Sustainability 2021, 13, 5087 6 of 25

increasing the tire chip content from 0% to 20% by weight increased the angle of internal friction slightly from 38◦ to 40.1◦ and increased the cohesion intercept from 0 to 18.4 kPa. They reported that adding 20% tire chips to the sand resulted in a mixture with behavior similar to that of a sand-gravel mixture. They also noted that adding tire chips to the sand resulted in increased compressibility. Most studies confirm that, within limits, the addition of TDA to soil results in enhanced shear strength properties while reducing the dry unit weight. Thus, soil-TDA mixtures can be used as a lightweight alternative in geotechnical engineering applications such as highway embankment fill and retaining wall backfill. The reuse of scrap tires in geotechnical engineering applications is a practice that promises to benefit the environment by diverting stockpiled tires from landfills. Direct shear testing and the triaxial compression method have been widely used for many years to determine the shear strength behavior of soil. The two approaches provide comparable results regarding soil shear strength parameters, such as the cohesion intercept and the angle of internal friction. According to Foose et al. [29], the shear strength parameters of TDA and soil-TDA mixtures obtained from a direct shear test are more accurate than those obtained from a triaxial compression test. These researchers noted that the main factor affecting the triaxial compression test results is the ratio between the maximum particle size and the specimen size. According to ASTM D7181 [41], the largest particle size must be six times smaller than the specimen diameter in a triaxial compression test. Thus, the most commonly available triaxial apparatuses are not large enough to accommodate TDA particle sizes, and hence direct shear tests are the preferred option.

3. Methodology To address the objectives of this research, three types of soil were selected and mixed with various amounts of TDA, ranging from 0% to 100% by weight. The soil types selected were gravelly (coarse grain) soil, sandy (medium grain) soil, and clayey (fine grain) soil. Large-scale direct shear box (305 mm × 305 mm × 220 mm) tests were then performed at confining pressures of 50.1, 98.8, and 196.4 kPa. Previous research has utilized different experimental approaches, including direct shear tests and the triaxial compression method, to evaluate the shear strength properties and compressibility behavior of TDA and soil-TDA mixtures. In a triaxial compression test, there is full control of the confining pressure and saturation, and the failure progresses in a natural plane. However, there is no control over the pore water pressure at the shear surface in direct shear tests, and the failure plane lies in a predetermined horizontal direction, which may not be the weakest plane. On the other hand, the greater simplicity of direct shear tests compared to the triaxial compression method has made direct shear testing a versatile, frequently used tool for geotechnical designers.

4. Laboratory Experiments 4.1. Materials 4.1.1. Tire-Derived Aggregate (TDA) The TDA sample used in this study was type A TDA, which was shredded and processed by Halifax C&D Recycling Ltd., located in Enfield, Nova Scotia, Canada. A photograph illustrating the TDA sample used in this study is shown in Figure1. Because most of the TDA particles were flat and elongated, a histogram analysis was performed to find the particle size distribution of the TDA sample, as recommended by Foose et al. [29] and El Naggar et al. [3]. To conduct the histogram analysis, a TDA sample weighing about 5 kg was randomly selected and a ruler was used to measure the particles in every direction. Sustainability 2021, 13, x FOR PEER REVIEW 7 of 26

According to ASTM D3080 [42], the maximum particle size of aggregates should be at least 10 times smaller than the length of the shear box, to eliminate boundary effects. However, Humphrey and Sandford [26] have suggested that for aggregates with larger particles, such as TDA, a direct shear test can be performed with particles that are four times smaller than the length of the shear box. The test results of these researchers showed that the boundary effect is minimized in this case, and the particles are sheared in the box with minimal external effects. This finding was confirmed by Zahran and El Naggar [27], Sustainability 2021, 13, x FOR PEER REVIEW 7 of 26 who likewise recommended the aspect ratio of particles four times smaller than the length Sustainability 2021, 13, 5087 of the shear box to eliminate the effect of the size of the shear box in the evaluation of7 TDA of 25 shear strength parameters. According to ASTM D3080 [42], the maximum particle size of aggregates should be at least 10 times smaller than the length of the shear box, to eliminate boundary effects. However, Humphrey and Sandford [26] have suggested that for aggregates with larger particles, such as TDA, a direct shear test can be performed with particles that are four times smaller than the length of the shear box. The test results of these researchers showed that the boundary effect is minimized in this case, and the particles are sheared in the box with minimal external effects. This finding was confirmed by Zahran and El Naggar [27], who likewise recommended the aspect ratio of particles four times smaller than the length of the shear box to eliminate the effect of the size of the shear box in the evaluation of TDA shear strength parameters.

FigureFigure 1. 1.Photograph Photograph ofof thethe TDATDA sample sample used used in in this this study. study.

AccordingIn addition, to according ASTM D3080 to F [oose42], theet al. maximum [29], the particle use of sizetire shreds of aggregates with a shouldmaximum be atlength least that 10 times is less smaller than half than the the diameter length of of a the direct shear shear box, ring to eliminatereduces the boundary boundary effects. effect However,during shearing. Humphrey Since and the Sandford length of [26the] haveshear suggestedbox used in that this for study aggregates was limited with largerto 305 particles,mm, TDA such particles as TDA, exceeding a direct 75 shear mm testin length can be were performed removed with from particles the sample that (amount- are four timesing to smaller only 3.9% than of the the length sample). of theThus, shear the box.maximum The test particle results size of these was limited researchers to one-fourth showed that the boundary effect is minimized in this case, and the particles are sheared in the box of the shear box length, to eliminate boundary and size effects [26,27]. Figure 2a presents with minimal external effects. This ﬁnding was conﬁrmed by Zahran and El Naggar [27], the histogram of the initial TDA sample, and Figure 2b shows the histogram of the sample who likewise recommended the aspect ratio of particles four times smaller than the length used in this study, following the removal of the particles exceeding 75 mm in length. As of the shear box to eliminate the effect of the size of the shear box in the evaluation of TDA shown in Figure 2, TDA particle lengths were mainly in the ranges of 30–40 mm, 40–50 Figureshear strength1. Photograph parameters. of the TDA sample used in this study. mm, 50–60 mm, 60–70 mm, and 70–75 mm. In addition, according to Foose et al. [29], the use of tire shreds with a maximum lengthIn thataddition, is less according than half theto F diameteroose et al. of a[29], direct the shear use of ring tire reduces shreds the with boundary a maximum effect lengthduring that shearing. is less Sincethan half the lengththe diameter of the shear of a direct box used shear in ring this studyreduces was the limited boundary to 305 effect mm, duringTDA particles shearing. exceeding Since the 75 length mm in of length the shear were box removed used in from this thestudy sample was (amountinglimited to 305 to mm,only TDA 3.9% particles of the sample). exceeding Thus, 75 themm maximum in length were particle removed size was from limited the sample to one-fourth (amount- of ingthe to shear only box 3.9% length, of the to sample). eliminate Thus, boundary the maximum and size particle effects size [26,27 was]. Figure limited2a to presents one-fourth the ofhistogram the shear of box the length, initial to TDA eliminate sample, boundary and Figure and2 bsize shows effects the [26,27]. histogram Figure of the2a presents sample theused histogram in this study, of the following initial TDA the sample, removal and of Figure the particles 2b shows exceeding the histogram 75 mm of in the length. sample As usedshown in inthis Figure study,2, TDAfollowing particle the lengths removal were of mainlythe particles in the exceeding ranges of 30–40 75 mm mm, in 40–50length. mm, As shown50–60 mm,in Figure 60–70 2, mm, TDA and particle 70–75 lengths mm. were mainly in the ranges of 30–40 mm, 40–50 mm, 50–60 mm, 60–70 mm, and 70–75 mm.

(a) (b) Figure 2. Histograms of (a) the initial TDA sample and (b) the TDA sample used in the shear box tests.

(a) (b)

FigureFigure 2. 2. HistogramsHistograms of of (a) (a) the the initial initial TDA TDA sample sample and and (b (b)) the the TDA TDA sample sample used used in in the the shear shear box box tests. tests.

The TDA particles had an average length/width aspect ratio of 2.8, and an average thickness of 8.9 mm. The dry unit weight (γd) of the TDA sample was determined in Sustainability 2021, 13, 5087 8 of 25

accordance with the standard test method described in ASTM D698 [43]. It should be noted that due to the ﬂexibility of the TDA particles, the compaction energy had a negligible effect on the dry unit weight. Therefore, in accordance with Humphrey and Sandford [26], 60% of the standard Proctor energy was applied to the specimen. Similarly, because the addition of water to the TDA sample had no effect on the dry unit weight, the compaction test was performed on an air-dried sample [1 and 6]. Table3 shows the physical properties of the TDA sample used in the shear box tests.

Table 3. Characteristics of the TDA and soils used in this study.

Soil Type Characteristics TDA Gravelly Sandy Clayey 1 D10 (mm) 19 0.30 0.24 0.007 1 D30 (mm) 28 2.40 0.42 0.021 1 D50 (mm) 45 5.90 0.65 0.048 1 D60 (mm) 49 7.00 0.80 0.075 2 Coefﬁcient of uniformity, Cu 2.58 23.33 3.33 10.71 2 Coefﬁcient of curvature, Cc 0.84 2.74 0.92 0.84 Optimum water content, ω (%) - 7.50 13.0 14.0 3 Maximum dry density, γm kN/m ) 6.82 19.2 16.8 18.4 Plasticity index, PI - - - 9.3 1 D10, D30, D50 and D60 represent the particle diameters where the percentage of particles with diameters below 2 2 D60 D30 these values are 10%, 30%, 50% and 60%, respectively. Cu = , Cc = . D10 D10D60

According to ASTM D6270 [6], to ensure the effective use of TDA particles, rubber- to-rubber contact should be maximized by reducing the amount of exposed steel. In this study, wire cutters were therefore used to remove exposed wires from the edges of the TDA particles.

4.1.2. Soil Samples Relatively uniform gravelly and sandy soils obtained from a local supplier were used in this study. In addition, a clayey soil was obtained from land in Enfield, Nova Scotia. In a natural condition, the clayey soil sample had a clayey till characteristic. Therefore, it was first dried at 110 ◦C for 24 h, and then broken down into fine grains before being used in the study. The particle size distribution of the soil samples was determined by using a sieve analysis in accordance with ASTM D422 [43]. It should be noted that for the clayey soil, first a sieve analysis was performed to find the distribution of particles larger than 75 µm (retained on the no. 200 sieve). Then, a hydrometer analysis was conducted to find the distribution of particles smaller than 75 µm (which passed through the no. 200 sieve). Figure3 shows the particle size distribution of the TDA and soil samples used in this study. To find the dry unit weight and optimum water content (ω) of the soil samples, standard Proctor compaction tests were performed in the laboratory in accordance with ASTM D698 [44]. An Atterberg limit test was also performed on the clayey soil, in accordance with the standard test method described by ASTM D4318 [45], to determine the plasticity index. Table3 presents the physical characteristics of the soil samples used in this study. In accordance with the unified soil classification system procedure (USCS, ASTM D2487 [46]), the gravelly soil was classified as well-graded gravel with sand, the sandy soil was classified as poorly graded sand, and the clayey soil was classified as sandy lean clay. Similarly, the TDA was classified as poorly graded, in accordance with the USCS. Sustainability 2021, 13, x FOR PEER REVIEW 8 of 26

The TDA particles had an average length/width aspect ratio of 2.8, and an average thickness of 8.9 mm. The dry unit weight () of the TDA sample was determined in accordance with the standard test method described in ASTM D698 [43]. It should be noted that due to the flexibility of the TDA particles, the compaction energy had a negligible effect on the dry unit weight. Therefore, in accordance with Humphrey and Sandford [26], 60% of the standard Proctor energy was applied to the specimen. Similarly, because the addition of water to the TDA sample had no effect on the dry unit weight, the compaction test was performed on an air-dried sample [1 and 6]. Table 3 shows the physical properties of the TDA sample used in the shear box tests. According to ASTM D6270 [6], to ensure the effective use of TDA particles, rubber- to-rubber contact should be maximized by reducing the amount of exposed steel. In this study, wire cutters were therefore used to remove exposed wires from the edges of the TDA particles.

4.1.2. Soil Samples Relatively uniform gravelly and sandy soils obtained from a local supplier were used in this study. In addition, a clayey soil was obtained from land in Enfield, Nova Scotia. In a natural condition, the clayey soil sample had a clayey till characteristic. Therefore, it was first dried at 110 °C for 24 hours, and then broken down into fine grains before being used in the study. The particle size distribution of the soil samples was determined by using a sieve analysis in accordance with ASTM D422 [43]. It should be noted that for the clayey soil, first a sieve analysis was performed to find the distribution of particles larger than 75 µm (retained on the no. 200 sieve). Then, a hydrometer analysis was conducted to find the Sustainability 2021, 13, 5087 9 of 25 distribution of particles smaller than 75 µm (which passed through the no. 200 sieve). Fig- ure 3 shows the particle size distribution of the TDA and soil samples used in this study.

FigureFigure 3. 3.Particle Particle sizesize distributiondistribution ofof thethe TDA TDA and and soil soil samples samples used used in in this this study. study.

4.2. SampleTo find Preparation the dry unit weight and optimum water content (ω) of the soil samples, standard ProctorAs described compaction above, tests in this were study performed three, types in the of laboratory soil were selectedin accordance to be mixed with ASTM with TDAD698 particles. [44]. An BeforeAtterberg being limit mixed test withwas thealso TDA, performed the soil on samples the clayey were soil, dried in inaccordance an oven forwith 24 the h atstandard 110 ºC test and method were then described broken by down ASTM into D4318 fine [45], grains to determine if required. the The plasticity TDA sampleindex. was air dried at room temperature for 72 h. Following drying of the materials, the requiredTable percentages 3 presents ofthe the physical soil and characteristics TDA samples of the were soil measured samples carefullyused in this by study. weight, In accordingaccordance to thewith planned the unified mixing ratios.soil classi In addition,fication thesystem optimum procedure water content(USCS, ofASTM each soilD2487[46]), sample wasthe gravelly determined, soil was and classified a corresponding as well-graded amount gravel of water with wassand, added the sandy to each soil soilwas sample. classified The as poorly materials graded were sand, transferred and the to clayey a tray, soil where was classified they were as mixed sandy carefullylean clay. untilSimilarly, a consistent the TDA mixture was classified was obtained. as poorly As mentioned graded, in above,accordance the optimum with the waterUSCS. content for compaction of the TDA particles was found to be zero; hence, no water was added toTable the 3. TDA Characteristics particles. of The the mixed TDA and materials soils used were in this then study. poured gently into the shear box in five layers and compacted by using standard compaction efforts. At each step, the mixture on the tray was mixed thoroughly before being poured into the shear box. Special care was taken, and continuous observations were made, to prevent any inconsistency in the mixtures. This ensured that segregation of the soil and TDA particles did not occur as the sample was prepared and transferred into the shear box. It should be noted that segregation is likely to occur in mixtures with high percentages of TDA. When segregation occurs, the soil settles beneath the TDA particles, and the mixture loses its consistency. Edil and Bosscher [47] conducted a series of triaxial tests on mixtures of sand and tire chips. They found that with a tire chip content greater than 30% by volume, segregation increased between the sand and the tire chip particles. Bosscher et al. [48] likewise conducted a field study, where they observed that segregation occurs in mixtures containing more than 50% TDA by volume. Table4 shows the percentage of TDA by weight for each mixture. TDA percentages of 0%, 10%, 25%, 50%, and 100% were used. The name of each mixture indicates the mixture contents, where G stands for gravel, S for sand, C for clay, and T for TDA. The digits at the end of each mixture name indicate the percentage of TDA by weight. For example, mixture GT25 contains gravel and TDA, with 25% TDA by weight. It should be noted that the percentage of TDA by weight is defined as the ratio of the weight of the TDA to the total weight of the soil-TDA mixture. Sustainability 2021, 13, 5087 10 of 25

Table 4. Properties of the soil-TDA mixtures.

3 ◦ Mixtures TDA % (by Weight) Soil % (by Weight) γbulk (kN/m ) Friction Angle ( ) Cohesion (kPa) Gravel-TDA GT0 0 100 19.84 44.0 24.8 GT10 10 90 18.38 45.4 17.0 GT25 25 75 14.77 43.9 14.7 Sustainability 2021, 13, x FOR PEER REVIEW 10 of 26 GT50 50 50 10.21 42.2 15.4 GT100 100 0 6.81 23.9 18.2 Sand-TDA Table 4. Properties of the soil-TDA mixtures. ST0 0 100 18.73 37.1 4.8 ST10 10 90TDA % 17.72Soil % 38.4Friction An- Cohesion 13.4 Mixtures ST25 25 75(by Weight) (by 15.73 Weight) (kN/m3) 38.3 gle (°) (kPa) 14.3 ST50 50 50Gravel-TDA 12.06 31.8 16.2 ST100 100GT0 00 6.81100 19.84 23.944.0 24.8 18.2 GT10 Clay-TDA10 90 18.38 45.4 17.0 CT0 0GT25 10025 20.40 75 14.77 18.843.9 14.7 21.8 CT10 10GT50 9050 18.93 50 10.21 32.342.2 15.4 29.2 CT25 25GT100 75100 15.33 0 6.81 25.623.9 18.2 29.0 CT50 50 50Sand-TDA 11.00 25.0 19.6 CT100 100ST0 00 6.81100 18.73 23.937.1 4.8 18.2 ST10 10 90 17.72 38.4 13.4 ST25 25 75 15.73 38.3 14.3 TheST50 particle size distributions50 of the 50gravel-TDA, 12.06 sand-TDA, and31.8 clay-TDA16.2 mixtures are presentedST100 in Figure4100. Figure 4a shows 0 that for a TDA6.81 content23.9 of up to 25%18.2 TDA by weight, the gravel mixtures had fairlyClay-TDA similar gradation characteristics, with D50 values rangingCT0 from 9 mm to 13 mm.0 The gradation100 characteristics20.40 of the gravel-TDA18.8 mixture21.8 with 50% TDACT10 varied to a greater10 extent, as the 90 D 50 of this mixture18.93 was32.3 21 mm, approximately29.2 doubleCT25 that of the mixtures25 GT0, GT10, and 75 GT25. As15.33 can be seen25.6 in Figure4b,29.0 the sand- TDA mixturesCT50 exhibited50 a similar trend, 50which was further11.00 accentuated25.0 in the19.6 clay-TDA mixtures,CT100 shown in Figure1004c. 0 6.81 23.9 18.2

(a) (b)

(c)

FigureFigure 4.4. Particle size dist distributionsributions of of the the (a (a) )gravel-TDA, gravel-TDA, (b ()b sand-TDA,) sand-TDA, and and (c) ( cclay-TDA) clay-TDA mix- mixtures. tures.

5. Laboratory Experiments Following the preparation of the mixtures, large-scale direct shear tests were performed for each mixture. The direct shear tests were carried out according to the standard testing procedure described by ASTM D3080 [42]. Figure 5 shows the large-scale direct shear box apparatus used in this study. The square box, with nominal dimensions 305 mm × 305 mm × 220 mm, was made of steel Sustainability 2021, 13, 5087 11 of 25

5. Laboratory Experiments Following the preparation of the mixtures, large-scale direct shear tests were performed for each mixture. The direct shear tests were carried out according to the standard Sustainability 2021, 13, x FOR PEER REVIEW 11 of 26 testing procedure described by ASTM D3080 [42]. Figure5 shows the large-scale direct shear box apparatus used in this study. The square box, with nominal dimensions 305 mm × 305 mm × 220 mm, was made of steel withwith a a thickness thickness of of 9 9 mm. mm. The The lower lower half half of of the the box box was was 90 90 mm mm high high and and was was seated seated on on a a movablemovable base. base. The The upper upper half half of of the the box box had had a a height height of of 80 80 mm. mm. To To accommodate accommodate the the high high compressibilitycompressibility of of the the TDA TDA sample sample and and soil-TDA soil-TDA mixtures mixtures during during shearing, shearing, the the apparatus apparatus waswas customized, customized, and and an an extension extension with with a a height height of of 50 50 mm mm was was constructed constructed and and mounted mounted onon top top of of the the upper upper half half of of the the shear shear box. box. Thus, Thus, the the total total height height of of the the shear shear box box amounted amounted toto 220 220 mm. mm. To To shear shear the the mixtures mixtures in in the the box, box, the the upper upper half half of of the the box box was was fixed fixed in in place, place, whilewhile the the lower lower half half of of the the box box was was moved moved at at a a controlled controlled rate, rate, powered powered by by an an electric electric motor.motor. The The specimen specimen could could therefore therefore be be sheared sheared near near a single a single shear shear plane plane in the in middlethe middle of theof shearthe shear box. box. As shown As shown in Figure in Figure5, the 5, confining the confining pressures, pressures, which which were applied were applied to each to specimeneach specimen by dead by weights, dead weights, were transferred were transf toerred the top to ofthe the top specimen of the specimen via a lever via loading a lever arm.loading It should arm. beIt should noted that be noted before that positioning before positioning the loading arm,the loading a steel platearm, capa steel was plate placed cap onwas top placed of the specimen,on top of the and specimen, the confining and pressurethe confining was then pressure applied was to then the specimen.applied to As the shownspecimen. in Figure As shown6, a load in Figure cell with 6, a a load linear cell variable with a linear displacement variable transducer displacement (LVDT) transducer was installed(LVDT) to was measure installed the to shear measure force. the In addition, shear force. two In other addition, LVDTs two were other installed LVDTs to monitor were in- thestalled horizontal to monitor displacement the horizontal and vertical displacement displacement and vertical of the displacement mixture during of the shearing. mixture Oneduring LVDT, shearing. which One was installedLVDT, which above was the installed box, contacted above the a steel box, plate contacted over thea steel sample plate toover measure the sample the vertical to measure displacement. the vertical The otherdisplacement. LVDT, which The other was installed LVDT, which horizontally, was in- contactedstalled horizontally, the lower half contacted of the box the to lower measure half the of horizontal the box to displacement. measure the Allhorizontal the LVDTs dis- were connected to a data acquisition system (CAMPBELL SCIENTIFIC CR200 Series), and placement. All the LVDTs were connected to a data acquisition system (CAMPBELL SCI- the data were recorded and monitored with the aid of a computer. The positioning of the ENTIFIC CR200 Series), and the data were recorded and monitored with the aid of a com- LVDTs can be seen in Figure6. puter. The positioning of the LVDTs can be seen in Figure 6.

Figure 5. The large-scale direct shear apparatus with a square box. Figure 5. The large-scale direct shear apparatus with a square box.

In this study, the TDA content and the confining pressures were the primary variables considered. The direct shear tests were conducted on mixtures with 0%, 10%, 25%, 50%, and 100% TDA by weight, at confining pressures of 50.1, 98.8, and 196.4 kPa. In the past, some studies have considered the effect of TDA particle orientation on the shear strength behavior of soil-TDA mixtures. Since under field conditions tire shreds Sustainability 2021, 13, x FOR PEER REVIEW 12 of 26

mixed with soil usually have a random orientation, the effect of TDA orientation was not considered in this study. The mixtures were tested at relatively high confining pressures, ranging from 50.1 to 196.4 kPa. It should be noted that at lower confining pressures, the shear stress cannot mobilize completely through the shear plane, which may result in some slip and pull-out effects during shearing. In other words, lower confining pressures can influence the shear strength behavior of the mixtures and contribute to an apparent friction angle [49]. All tests were performed in strain-controlled conditions with a constant shear rate. While each test was running, the shear force, horizontal displacement, and vertical displacement were recorded, up to a total relative lateral displacement of 14%. Then, at 14% relative lateral displacement, the test was terminated, and all the data were collected. It should be noted that the shear rate of clayey soil needs to be slow enough to prevent the buildup of excess pore water pressure in the specimen, allowing the dissipation of pore water pressure during shearing. Hence, it may take several days to perform a direct shear test on clayey soil. However, according to Bowles [50], a shear rate of 1.2 to 1.3 mm/min gives a close approximation to the drained shear strength of a clayey soil obtained from a direct shear test. Therefore, a slow shear rate of 0.5 mm/min was used in this study. As mentioned above, the shear force was applied by an electric motor and was recorded during shearing by using LVDTs. After each shear test was terminated, the shear stress was calculated by dividing the shear force by the horizontal cross-sectional area of the box (930.25 cm2). Then the shear strength of the mixture was defined at a peak shear stress. If no peak shear stress occurred prior to 14% relative lateral displacement, the shear stress at 10% relative lateral displacement was taken as the shear strength of the mixture (ASTM D3080 [42]). It should be noted that some tests were performed twice to ensure that the test pro- Sustainability 2021, 13, 5087 cedure used in the study was accurate and that the results were indicative. The repeated 12 of 25 tests exhibited similar results, which confirmed the accuracy and repeatability of the tests.

FigureFigure 6. 6.Location Location ofof thethe LVDTs.

In this study, the TDA content and the confining pressures were the primary variables considered. The direct shear tests were conducted on mixtures with 0%, 10%, 25%, 50%, and 100% TDA by weight, at confining pressures of 50.1, 98.8, and 196.4 kPa. In the past, some studies have considered the effect of TDA particle orientation on the shear strength behavior of soil-TDA mixtures. Since under field conditions tire shreds mixed with soil usually have a random orientation, the effect of TDA orientation was not considered in this study. The mixtures were tested at relatively high confining pressures, ranging from 50.1 to 196.4 kPa. It should be noted that at lower confining pressures, the shear stress cannot mobilize completely through the shear plane, which may result in some slip and pull-out effects during shearing. In other words, lower confining pressures can influence the shear strength behavior of the mixtures and contribute to an apparent friction angle [49]. All tests were performed in strain-controlled conditions with a constant shear rate. While each test was running, the shear force, horizontal displacement, and vertical displacement were recorded, up to a total relative lateral displacement of 14%. Then, at 14% relative lateral displacement, the test was terminated, and all the data were collected. It should be noted that the shear rate of clayey soil needs to be slow enough to prevent the buildup of excess pore water pressure in the specimen, allowing the dissipation of pore water pressure during shearing. Hence, it may take several days to perform a direct shear test on clayey soil. However, according to Bowles [50], a shear rate of 1.2 to 1.3 mm/min gives a close approximation to the drained shear strength of a clayey soil obtained from a direct shear test. Therefore, a slow shear rate of 0.5 mm/min was used in this study. As mentioned above, the shear force was applied by an electric motor and was recorded during shearing by using LVDTs. After each shear test was terminated, the shear stress was calculated by dividing the shear force by the horizontal cross-sectional area of the box (930.25 cm2). Then the shear strength of the mixture was defined at a peak shear stress. If no peak shear stress occurred prior to 14% relative lateral displacement, the shear stress at 10% relative lateral displacement was taken as the shear strength of the mixture (ASTM D3080 [42]). It should be noted that some tests were performed twice to ensure that the test procedure used in the study was accurate and that the results were indicative. The repeated tests exhibited similar results, which confirmed the accuracy and repeatability of the tests. Sustainability 2021, 13, x FOR PEER REVIEW 13 of 26

Sustainability 2021, 13, 5087 13 of 25 6. Results and Discussion

6.1. Dry Unit Weight of the Mixtures 6. Results and Discussion 6.1. DryIn Figure Unit Weight 7, bulk of unit the Mixtures weight is plotted against TDA content for the gravel-TDA, sand-TDA, and clay-TDA mixtures used in the large-scale direct shear tests. For comparison purposes,In Figure7 the, bulk TDA unit content weight ranges is plotted from against 0% (corresponding TDA content to for pure the gravel-TDA,soil) to 100% sand- (cor- TDA, and clay-TDA mixtures used in the large-scale direct shear tests. For comparison responding to pure TDA). It can be seen that the dry unit weight of the mixtures decreased purposes, the TDA content ranges from 0% (corresponding to pure soil) to 100% (corre- considerably as the percentage of TDA increased. It should be noted that this decrease in sponding to pure TDA). It can be seen that the dry unit weight of the mixtures decreased dry unit weight is due to the low dry unit weight of the TDA (6.82 kN/m3), which is less considerably as the percentage of TDA increased. It should be noted that this decrease in than half that of the conventional soils used in this study. A reduction in dry unit weight dry unit weight is due to the low dry unit weight of the TDA (6.82 kN/m3), which is less is beneficial for the design of a retaining wall or a box culvert, since in such applications, than half that of the conventional soils used in this study. A reduction in dry unit weight the lateral earth pressure needs to be minimized for an economic design. In addition, if is beneficial for the design of a retaining wall or a box culvert, since in such applications, the soil beneath a fill layer is weak, the use of a light soil mixture helps to overcome this the lateral earth pressure needs to be minimized for an economic design. In addition, if problem. Hence, soil-TDA mixtures can be a useful lightweight alternative in geotechnical the soil beneath a fill layer is weak, the use of a light soil mixture helps to overcome this applications, especially for backfilling over or around buried pipes and culverts. problem. Hence, soil-TDA mixtures can be a useful lightweight alternative in geotechnical applications, especially for backfilling over or around buried pipes and culverts.

FigureFigure 7. 7. BulkBulk unit unit weight weight versus versus TDA TDA content content for for the the soil-TDA soil-TDA mixtures. mixtures.

6.2. Shear Stress versus Shear Strain Behavior For all of the considered soil-TDA mixtures (i.e., gravel-TDA, sand-TDA, and clay- 6.2.TDA), Shear direct Stress shear versus tests Shear were Strain conducted Behavior at three confining pressures of 50.1, 98.8, and 196.4For kPa. all Figureof the8 considered shows the soil-TDA shear stress mixtures plotted (i.e., against gravel-TDA, shear strain sand-TDA, for all mixtures and clay- at TDA),the considered direct shear confining tests were pressures. conducted As shown at three in Figureconfining8, for pressures 100% gravel of 50.1, (GT0), 98.8, a peak and 196.4shear kPa. stress Figure was observed8 shows the at allshear confining stress plotted pressures, against indicating shear strain the shear for all strength mixtures of the at thesamples. considered Likewise, confining for the pressures. 100% sand As shown (ST0) sample,in Figure a 8, clear for peak100% sheargravel stress (GT0), was a peak also shearobserved stress at was all the observed confining at all pressures, confining indicating pressures, the indicating shear strength the shear of the strength samples of inthe a samples.similar fashion. Likewise, On for the the other 100% hand, sand for (ST0) the clay-TDA sample, a mixtures clear peak CT0 shear and stress CT10, was a peak also shear ob- servedstress wasat all observed the confining at a confining pressures, pressure indicati of 50.1ng the kPa, shear indicating strength the of shear the strengthsamples of in the a similarsamples. fashion. However, On the up other to a shear hand, strain for the of clay-TDA 24%, no peak mixtures shear CT0 stress and was CT10, exhibited a peak by shear any stressof the was samples observed at confining at a confining pressures pressure of 98.8 of and 50.1 196.4 kPa, kPa; indicating in other the words, shear the strength clay-TDA of themixtures samples. exhibited However, strain-hardening up to a shear behavior.strain of 24%, no peak shear stress was exhibited by any ofIt the can samples be seen at in confining Figure8 that pressures the addition of 98.8 of and TDA 196.4 to the kPa; gravel in other decreased words, thethe shearclay- TDAresistance mixtures of the exhibited mixtures strain-hardening upon shearing behavior. at all of the considered confining pressures. It shouldIt can be notedbe seen that in theFigure addition 8 that of the up additi to 10%on TDAof TDA by weightto the gravel to the graveldecreased did the not shear result resistancein a significant of the reduction mixtures in upon shear shearing resistance at at all the of confining the considered pressures confining of 98.8 andpressures. 196.4 kPa. It shouldIt is evident be noted from that the the figure addition that the of up addition to 10% of TDA up to by 25% weight TDA to by the weight gravel to did the not gravel result at confining pressures of 50.1 and 98.8 kPa and the addition of up to 10% TDA by weight at a in a significant reduction in shear resistance at the confining pressures of 98.8 and 196.4 confining pressure of 196.4 kPa decreased the peak shear resistance gradually at a higher kPa. It is evident from the figure that the addition of up to 25% TDA by weight to the shear strain. For mixtures containing more than 25% TDA by weight at confining pressures of 50.1 and 98.8 kPa, and more than 10% TDA by weight at 196.4 kPa, no peak shear stress was exhibited up to a shear strain of 24%. In other words, the addition of TDA to the gravel Sustainability 2021, 13, 5087 14 of 25

Sustainability 2021, 13, x FOR PEER REVIEWresulted in a strain-hardening behavior of the gravel-TDA mixtures, and they did not15 failof 26

upon applied shearing.

Figure 8. Shear stress versus shear strain for the gravel-TDA, sand-TDA, and clay TDA mixtures at conﬁning pressures of 50.1,Figure 98.8, 8. andShear 196.4 stress kPa. versus shear strain for the gravel-TDA, sand-TDA, and clay TDA mixtures at confining pressures of 50.1, 98.8, and 196.4 kPa.

Sustainability 2021, 13, 5087 15 of 25

It can also be seen from Figure8 that the addition of up to 10% TDA by weight to the sand then increased the peak shear stress at a similar shear strain. At a confining pressure of 50.1 kPa, increasing the TDA content from 10% to 25% further increased the peak shear stress at a greater shear strain. However, at confining pressures of 98.8 and 196.4 kPa, increasing the TDA content from 10% to 25% by weight did not significantly change the peak shear resistance of the sand-TDA mixtures. However, the peak shear resistance of the ST25 mixture occurred at a higher shear strain than was the case for the ST10 mixture. Increasing the TDA content to more than 25% then reduced the shear resistance of the mixtures at all the confining pressures considered. It should be noted that mixtures containing more than 50% TDA at a confining pressure of 50.1 kPa and more than 25% TDA at confining pressures of 98.8 and 196.4 kPa did not reach a peak shear stress upon shearing. In other words, the addition of TDA contributed to a strain-hardening behavior of the mixtures in a similar fashion to that observed in the gravel-TDA mixtures. On the other hand, it is evident from Figure8 that adding up to 10% TDA by weight to the clay increased the peak shear stress significantly at all the confining pressures considered. However, the further addition of TDA beyond 10% by weight then decreased the shear resistance of the mixtures, with no failure occurring up to a shear strain of 24%. It should be noted that although increasing the TDA content from 10% to 25% decreased the shear resistance of the mixtures, the shear resistance was still higher than the shear resistance of clay alone (CT0). It should likewise be noted that, if failure is considered to occur at 10% relative lateral displacement for mixtures that do not exhibit a peak shear stress, among all the mixtures, the highest shear resistance was observed for mixture CT10 and the lowest shear resistance was observed for clay alone (CT0) at all the confining pressures considered. Finally, a comparison of the shear stress versus shear strain results at the confining pressures considered showed that increasing the confining pressure from 50.1 to 196.4 kPa enhanced the shear resistance of mixtures containing the same amount of TDA.

6.3. Vertical Displacement versus Shear Strain Behavior Figure9 shows the vertical displacement plotted against shear strain for the gravel- TDA, sand-TDA, and clay TDA mixtures at the three considered confining pressures. It was observed that gravel-TDA mixtures containing up to 25% TDA by weight were initially compressed and then dilated upon shearing. For 100% gravel (GT0), compression was negligible at a confining pressure of 50.1 kPa, and the specimen was mainly dilated upon shearing. However, gravel mixtures containing more than 25% TDA by weight were mainly compressed upon shearing at all the confining pressures considered. It should be noted that in gravel-TDA mixtures containing up to 25% TDA by weight, the dilation upon shearing was greater at a lower TDA content, with 100% gravel (GT0) exhibiting the greatest dilation. In mixtures containing more than 25% TDA by weight, compression increased at a higher TDA content, with pure TDA (GT100) exhibiting the greatest compression. In general, adding TDA to gravel increased the compressibility behavior of the mixtures upon shearing. Figure9 also indicates that the dilation behavior of the mixtures decreased at greater confining pressures. In mixtures containing more than 25% TDA by weight, increasing the confining pressure from 98.8 to 196.4 kPa caused only a slight change in the compressibility behavior of the mixtures upon shearing. For the sand-TDA mixtures, it was observed that mixtures containing up to 25% TDA by weight were initially compressed and then dilated upon shearing, at all the confining pressures considered similar to the same trend observed for gravel mixtures. For 100% sand (ST0), the dilation upon shearing was not significant at any of the confining pressures, and the sample returned to almost its initial height after compression. In contrast, mixtures containing more than 25% TDA by weight were mainly compressed upon shearing. It should be noted that sand-TDA mixtures containing 10% and 25% TDA by weight exhibited a similar dilation upon shearing at confining pressures of 98.8 and 196.4. However, mixtures Sustainability 2021, 13, 5087 16 of 25

Sustainability 2021, 13, x FOR PEER REVIEW 17 of 26 containing 50% and 100% TDA by weight exhibited similar compression behavior upon shearing at all the conﬁning pressures considered.

Figure 9. Vertical displacement versus shear strain for the gravel-TDA, sand-TDA, and clay TDA mixtures at confining Figure 9. Vertical displacement versus shear strain for the gravel-TDA, sand-TDA, and clay TDA mixtures at conﬁning pressures of 50.1, 98.8, and 196.4 kPa. pressures of 50.1, 98.8, and 196.4 kPa. Sustainability 2021, 13, 5087 17 of 25

In general, adding TDA to sand increased the compressibility behavior of the mixtures upon shearing. Figure9 also indicates that the dilation behavior of the sand mixtures decreased at greater confining pressures opposite to the behavior observed in the gravel mixtures. As shown in Figure9, at a confining pressure of 50.1 kPa, pure TDA (CT100) was only compressed upon shearing, but all the other mixtures were initially compressed and then dilated upon shearing. For 100% clay (CT0), the amount of dilation upon shearing was insignificant at a confining pressure of 50.1 kPa, and the specimen returned to its initial height after compression. Thus, at a confining pressure of 50.1 kPa, the addition of TDA increased the compressibility behavior of the mixtures upon shearing, with pure TDA (CT100) exhibiting the greatest compression. It should be noted that clay mixtures containing up to 10% TDA by weight exhibited similar compression upon shearing at a confining pressure of 50.1 kPa. However, the dilation of the mixture containing 10% TDA (CT10) was greater than that of clay alone (CT0). At confining pressures of 98.8 and 196.4 kPa, all the clay mixtures were only compressed upon shearing. It should also be noted that adding up to 10% TDA to the clay decreased the compression behavior of the mixture upon shearing significantly at confining pressures of 98.8 and 196.4 kPa. The further addition of TDA, beyond 10% by weight, then increased the compressibility behavior of the mixtures, however the compression observed was still less than that of clay alone. At confining pressures of 98.8 and 196.4 kPa, the greatest compression upon shearing was exhibited by 100% clay (CT0). Figure9 also indicates that the compression behavior of the mixtures increased at greater confining pressures.

6.4. Mohr–Coulomb Failure Criterion for the Mixtures The Mohr–Coulomb failure criterion was used to find the cohesion and angle of internal friction for the soil-TDA mixtures used in this study. As explained above, each specimen was tested at the three normal stresses: 50.1, 98.8, and 196.4 kPa. Failure was defined at a peak or, in the absence of a peak, at 10% relative lateral displacement. The Mohr–Coulomb failure envelope is defined as a linear relationship between the normal stress and the corresponding shear strength. The slope of the line represents the angle of internal friction and the interception of the line with y-axis shows the cohesion. Equation (1) expresses the Mohr–Coulomb failure criterion [51]:

τ = c’ + σ tan ϕ’ (1)

where τ is the shear stress at failure, c’ is the cohesion intercept, and ϕ’ is the angle of internal friction. The Mohr–Coulomb failure envelopes (Figure 10) for the mixtures were determined by referring to the shear stress versus relative lateral displacement behavior of the mixtures, as obtained from the direct shear tests. The shear strength parameters of the mixtures (the cohesion and the angle of internal friction) were then determined from the failure envelopes. The properties of the mixtures considered in this study are summarized in Table4. A discussion of the shear strength parameters ( ϕ’ and c’) of the gravel-TDA, sand-TDA, and clay-TDA mixtures is presented in the following section.

6.5. Shear Strength Parameters of the Soil-TDA Mixtures 6.5.1. Angle of Internal Friction Figure 11 shows the angle of internal friction plotted against the TDA content for the gravel-TDA, sand-TDA, and clay-TDA mixtures. As shown in Figure 11, adding up to 10% TDA by weight to the gravel increased the angle of internal friction slightly, from 44º to 45.4º. The further addition of TDA up to 25% by weight then reduced the angle of internal friction from 45.4◦ to 42.2◦. In general, the addition of up to 25% TDA by weight to the gravel did not change the angle of internal friction signiﬁcantly. It may be argued that for these mixtures, gravel was the dominant particle in the shear plane and thus controlled the shear strength behavior of the mixtures. Adding more than 25% TDA by weight to the gravel then sharply reduced the angle of internal friction. Sustainability 2021, 13, x FOR PEER REVIEW 18 of 26

6.4. Mohr–Coulomb Failure Criterion for the Mixtures The Mohr–Coulomb failure criterion was used to find the cohesion and angle of internal friction for the soil-TDA mixtures used in this study. As explained above, each specimen was tested at the three normal stresses: 50.1, 98.8, and 196.4 kPa. Failure was defined at a peak or, in the absence of a peak, at 10% relative lateral displacement. The Mohr–Coulomb failure envelope is defined as a linear relationship between the normal stress and the corresponding shear strength. The slope of the line represents the angle of internal friction and the interception of the line with y-axis shows the cohesion. Equation (1) expresses the Mohr–Coulomb failure criterion [51]: τ = c’ + σ tan φ’ (1) where τ is the shear stress at failure, c’ is the cohesion intercept, and φ’ is the angle of internal friction. The Mohr–Coulomb failure envelopes (Figure 10) for the mixtures were determined by referring to the shear stress versus relative lateral displacement behavior of the mixtures, as obtained from the direct shear tests. The shear strength parameters of the mixtures (the cohesion and the angle of internal friction) were then determined from Sustainability 2021, 13, 5087 the failure envelopes. The properties of the mixtures considered in this study are summa- 18 of 25 rized in Table 4. A discussion of the shear strength parameters (φ’ and c’) of the gravel- TDA, sand-TDA, and clay-TDA mixtures is presented in the following section.

(b) (a)

Sustainability 2021, 13, x FOR PEER REVIEW 19 of 26

to 45.4º. The further addition of TDA up to 25% by weight then reduced the angle of internal friction from 45.4° to 42.2°. In general, the addition of up to 25% TDA by weight to the gravel(c) did not change the angle of internal friction significantly. It may be argued that Figure for10. Mohr–Coulombthese mixtures, failure gravel envelopes was the for thedominant (a) gravel-TDA, particle (b )in sand-TDA, the shear and plane (c) clay-TDA and thus controlled Figure 10. Mohr–Coulomb failure envelopes for the (a) gravel-TDA, (b) sand-TDA, and (c) clay- the shear strength behavior of themixtures. mixtures. Adding more than 25% TDA by weight to the TDA mixtures. gravel then sharply reduced the angle of internal friction. 6.5. Shear Strength Parameters of the Soil-TDA Mixtures 6.5.1. Angle of internal friction Figure 11 shows the angle of internal friction plotted against the TDA content for the gravel-TDA, sand-TDA, and clay-TDA mixtures. As shown in Figure 11, adding up to 10% TDA by weight to the gravel increased the angle of internal friction slightly, from 44º

FigureFigure 11.11. The angle of internal internal friction friction versus versus TD TDAA content content for for the the gravel-TDA, gravel-TDA, sand-TDA, sand-TDA, and and clay-TDAclay-TDA mixtures.mixtures.

FigureFigure 11 11 also also showsshows thatthat addingadding upup toto 10%10% TDATDA byby weightweight toto thethe sandsand increasedincreased thethe angleangle ofof internalinternal frictionfriction slightly,slightly, fromfrom 37.137.1ºº to to 38.438.4ºº (an (an increaseincrease of of approximately approximately 4%). 4%). InIn general,general, thethe addition of up to to 25% 25% TDA TDA by by we weightight to to the the sand sand did did not not change change the the angle angle of ofinternal internal friction friction significantly. signiﬁcantly. It Itmay may be be argued argued that that for for mixtures mixtures containing up toto 25%25% TDATDA byby weight, weight, sand sand was was the the dominant dominant particle particle in thein the shear shear plane plane and and thus thus controlled controlled the shearthe shear strength strength behavior behavior of the of mixtures. the mixtures. Increasing Increasing the TDA the TDA content content from from 25% to 25% 50% to then 50% ◦ ◦ sharplythen sharply reduced reduced the angle the angle of internal of internal friction fricti fromon 38.3 fromto 38.3° 31.8 to(a 31.8° reduction (a reduction of about of about 20%). The20%). angle The of angle internal of internal friction thenfriction continued then continued to decrease to asdecrease the TDA as contentthe TDA increased, content upin- tocreased, 100% TDA. up to 100% TDA. TheThe additionaddition ofof upup toto 10%10% TDATDA byby weightweight toto thethe clayclay increasedincreased thethe angleangle ofof internalinternal ◦ ◦ frictionfriction considerably,considerably, fromfrom 18.818.8° toto 32.332.3° (an(an increaseincrease ofof approximatelyapproximately 72%).72%). ItIt maymay bebe arguedargued thatthat thethe adhesionadhesion betweenbetween clayclay particlesparticles resultedresulted inin aa bondbond withwith thethe TDATDA particlesparticles andand contributedcontributed to to the the reinforcement reinforcement of of the the soil so uponil upon shearing shearing at all at the all conﬁningthe confining pressures pressures considered. Thus, the angle of internal friction increased up to a TDA content of 10%. However, further increasing the TDA content to more than 10% then significantly reduced the angle of internal friction. It may be argued that for clay-TDA mixtures containing more than 10% TDA by weight, the clay particles were not able to create a bond with the TDA particles, and therefore the angle of internal friction decreased. In general, adding up to 10% TDA by weight to the clay significantly enhanced the angle of internal friction. However, adding up to 10% TDA by weight to the gravel or sand only slightly increased the angle of internal friction. Furthermore, adding more than 10% TDA to the clay then sharply reduced the angle of internal friction; however, adding up to 25% TDA to the gravel or sand changed the angle of internal friction only slightly. It should be noted that the further addition of TDA to the gravel or sand, beyond 25% TDA by weight, then reduced the angle of internal friction significantly. However, adding more than 25% TDA to the clay did not significantly change the angle of internal friction.

6.5.2. Cohesion Figure 12 shows the cohesion intercept plotted against the TDA content for the gravel-TDA, sand-TDA, and clay-TDA mixtures. The cohesion intercept obtained for the gravelly and sandy soils mixed with TDA is referred to as apparent cohesion. It may be argued that the application of high confining pressures ranging from 50.1 to 196.4 kPa to the specimens upon shearing resulted in apparent cohesion in these mixtures. At lower Sustainability 2021, 13, x FOR PEER REVIEW 20 of 26

confining pressures, the shear stress cannot mobilize completely through the shear plane, which can result in some slip and pull-out effects during shearing. In other words, confining pressures lower than 50 kPa can influence the shear strength properties of the mixtures and contribute to an apparent friction angle (Gray and Ohashi [49]). Therefore, to avoid an apparent friction angle, the confining pressures of 50.1, 98.8, and 196.4 kPa were considered in this study. As seen in Figure 12, the addition of up to 25% TDA by weight to the gravel decreased the cohesion intercept from 24.8 to 15.4 kPa. An explanation may be that due to the high flexibility of the TDA particles, adding up to 25% TDA by weight to the gravel increased the mobilization of shear stress in the shear plane, and thus decreased the apparent cohesion. Increasing the TDA content from 25% to 50% then increased the apparent cohesion from 15.4 to 20.6 kPa. It may be argued that the gravel-TDA mixtures containing more than 25% TDA by weight had a strain-hardening behavior, and their shear strength pa- Sustainability 2021, 13, 5087 19 of 25 rameters were thus obtained at 10% relative horizontal displacement. Therefore, a larger displacement at failure increased the apparent cohesion. considered.As shown Thus, in Figure the angle 12, the of addition internal frictionof TDA increasedto the sand up increased to a TDA the content apparent of cohe- 10%. However,sion. An explanation further increasing may be the that TDA because content the to TDA more particles than 10% were then coarser significantly than the reduced sand thegrains, angle the of TDA internal decreased friction. the It maymobilization be argued of that shear for stress clay-TDA in the mixtures shear plane containing upon shear- more thaning at 10% all the TDA confining by weight, pressures the clay considered, particles were thus not increasing able to create the apparent a bond withcohesion. the TDA Fur- particles,thermore, and the thereforesand-TDA the mixtures angle of containing internal friction more than decreased. 25% TDA by weight had a strain- hardeningIn general, behavior, adding and up their to 10% shear TDA strength by weight parameters to the claywere significantly thus obtained enhanced at 10% rela- the angletive horizontal of internal displacement. friction. However, Therefore, adding a larger up to 10%displacement TDA by weight at failure to the increased gravel or the sand ap- onlyparent slightly cohesion. increased the angle of internal friction. Furthermore, adding more than 10% TDAIt to should the clay be then noted sharply that the reduced cohesion the result angles ofobtained internal for friction; the clay however, were not adding solely upat- totributable 25% TDA to tothe the confining gravel or pressures. sand changed There the is also angle adhesion of internal between friction clay only particles slightly. in It a shouldnatural becondition, noted that which the furthercontributed addition to the of cohesion. TDA to the It gravelwas observed or sand, that beyond the addition 25% TDA of byup weight,to 25% thenTDA reducedby weight the increased angle of internal the cohesion friction from significantly. 21.8 to 29 However, kPa. It may adding be argued more thanthat the 25% addition TDA to theof up clay to did25% not TDA significantly by weight change to the theclay angle decreased of internal the mobilization friction. of shear stress upon shearing at the confining pressures considered, thus increasing the co- 6.5.2.hesion Cohesion intercept. For clay-TDA mixtures containing more than 25% TDA by weight, the cohesionFigure intercept 12 shows then the cohesiondecreased. intercept This may plotted be attributable against the TDAto the content lower forpercentage the gravel- of TDA,clay in sand-TDA, the mixtures, and clay-TDAcausing the mixtures. adhesion The between cohesion clay intercept particles obtained to become for theless gravelly signifi- andcant. sandy Hence, soils the mixedobserved with cohesion TDA is for referred these mixtures to as apparent would cohesion.be due mainly It may to bethe arguedconfin- thating pressure, the application causing of the high cohesion confining intercept pressures to decrease. ranging from 50.1 to 196.4 kPa to the specimensIn general, upon adding shearing up resultedto 25% TDA in apparent by weight cohesion to the gravel in these caused mixtures. a sharp At decrease lower confiningin the cohesion pressures, intercept. the shear However, stress the cannot additi mobilizeon of up completely to 25% TDA through by weight the shear to the plane, sand whichor clay can resulted result inin someincreased slip and cohesion. pull-out It effects should during be noted shearing. that for In otherthe sand, words, the confining cohesion pressuresintercept continued lower than to 50 increase kPa can as influence the TDA thecontent shear increased. strength propertiesWhen the TDA of the content mixtures in- andcreased contribute from 25% to an to apparent 50%, the friction cohesion angle of (Graythe gravel-TDA and Ohashi mixtures [49]). Therefore, increased, to avoid and the an apparentcohesion frictionof the clay-TDA angle, the mixtures confining decreased pressures. ofThe 50.1, further 98.8, andaddition 196.4 of kPa TDA, were beyond considered 50% inby this weight, study. did not significantly change the cohesion intercept of the clay or gravel mixtures.

Figure 12. Cohesion intercept versus TDA content for the gravel-TDA, sand-TDA, and clay-TDA mixtures.

As seen in Figure 12, the addition of up to 25% TDA by weight to the gravel decreased the cohesion intercept from 24.8 to 15.4 kPa. An explanation may be that due to the high ﬂexibility of the TDA particles, adding up to 25% TDA by weight to the gravel increased the mobilization of shear stress in the shear plane, and thus decreased the apparent cohesion. Increasing the TDA content from 25% to 50% then increased the apparent cohesion from 15.4 to 20.6 kPa. It may be argued that the gravel-TDA mixtures containing more than 25% TDA by weight had a strain-hardening behavior, and their shear strength parameters were thus obtained at 10% relative horizontal displacement. Therefore, a larger displacement at failure increased the apparent cohesion. As shown in Figure 12, the addition of TDA to the sand increased the apparent cohesion. An explanation may be that because the TDA particles were coarser than the sand grains, the TDA decreased the mobilization of shear stress in the shear plane upon shearing at all the conﬁning pressures considered, thus increasing the apparent cohesion. Sustainability 2021, 13, 5087 20 of 25

Furthermore, the sand-TDA mixtures containing more than 25% TDA by weight had a strain-hardening behavior, and their shear strength parameters were thus obtained at 10% relative horizontal displacement. Therefore, a larger displacement at failure increased the apparent cohesion. It should be noted that the cohesion results obtained for the clay were not solely attributable to the confining pressures. There is also adhesion between clay particles in a natural condition, which contributed to the cohesion. It was observed that the addition of up to 25% TDA by weight increased the cohesion from 21.8 to 29 kPa. It may be argued that the addition of up to 25% TDA by weight to the clay decreased the mobilization of shear stress upon shearing at the confining pressures considered, thus increasing the cohesion intercept. For clay-TDA mixtures containing more than 25% TDA by weight, the cohesion intercept then decreased. This may be attributable to the lower percentage of clay in the mixtures, causing the adhesion between clay particles to become less significant. Hence, the observed cohesion for these mixtures would be due mainly to the confining pressure, causing the cohesion intercept to decrease. In general, adding up to 25% TDA by weight to the gravel caused a sharp decrease in the cohesion intercept. However, the addition of up to 25% TDA by weight to the sand or clay resulted in increased cohesion. It should be noted that for the sand, the cohesion intercept continued to increase as the TDA content increased. When the TDA content increased from 25% to 50%, the cohesion of the gravel-TDA mixtures increased, and the cohesion of the clay-TDA mixtures decreased. The further addition of TDA, beyond 50% by weight, did not significantly change the cohesion intercept of the clay or gravel mixtures.

6.6. Shear Modulus The shear modulus is a mechanical parameter used to analyze the behavior of material during shearing. Equation (2) was used to calculate the shear modulus (Das [51]):

G = τ/ε (2)

where G is shear modulus, τ is shear stress, and ε is shear strain. The shear modulus can be determined by using this equation and referring to the shear stress versus shear strain behavior of the soil-TDA mixtures. To compare the shear modulus of the mixtures used in this study, the secant shear modulus (G50) was defined as 50% of the shear strength divided by the corresponding shear strain. Figure 13 shows the secant shear modulus plotted against the TDA content for the soil-TDA mixtures at confining pressures of 50.1, 98.8, and 196.4 kPa. It can be seen that the addition of TDA to the gravel decreased the secant shear modulus at all the confining pressures considered. However, it should be noted that at a confining pressure of 196.4 kPa, the decrease in the shear modulus was not significant for a TDA content of up to 10% by weight. It can also be noticed from Figure 13 that adding up to 10% TDA by weight to the sand did not affect the secant shear modulus significantly. However, the further addition of TDA to the sand, beyond 10% by weight, then sharply decreased the secant shear modulus at all the confining pressures considered. It can be concluded that for both gravel-TDA and sand-TDA mixtures, the stiffness behavior of the mixtures is governed by the parent solid particles at low TDA contents. As the TDA content increases, the behavior of the TDA particles starts controlling the stiffness behavior of the mixture and results in the sharp stiffness decline. On the other hand, for the clay mixtures, as shown in Figure 13, adding up to 10% TDA by weight to the clay significantly increased the secant shear modulus at all the confining pressures considered. However, the further addition of TDA, beyond 10% by weight, then sharply reduced the secant shear modulus at all the confining pressures. This behavior is different than that observed in the other two mixture groups. The initial increase in the stiffness of the clay-TDA mixture with low TDA content is different than the behavior observed in the gravel-TDA and sand-TDA mixtures. A possible cause of this behavior change may be related to the fact that the pure solid gravel and sand particles in the mixture matrix Sustainability 2021, 13, x FOR PEER REVIEW 22 of 26

= 1sin′) (3) where is the normalized lateral earth pressure at rest, z is the depth, φ’ is the angle of internal friction, and is the dry unit weight of the soil. Figure 14 shows the normalized lateral earth pressure at rest plotted against the TDA content for the gravel-TDA, sand- TDA, and clay-TDA mixtures. It can be seen that adding up to 10% TDA by weight to the soils decreased the normalized lateral earth pressure at rest. For the clay-TDA mixture, adding up to 10% TDA Sustainability 2021, 13, 5087 by weight significantly reduced the normalized lateral earth pressure at rest; however,21 of 25 increasing the TDA content from 10% to 25% then stabilized the normalized lateral earth pressure at rest. In contrast, for the gravel-TDA and sand-TDA mixtures, the normalized had a higher stiffness than that of the rubber TDA particles. Moreover, there were fewer lateral earth pressure at rest continued to decrease as the TDA content increased from 10% available voids in the gravel and sand in their pure stateer, and therefore introducing the to 25%. With the further addition of TDA, beyond 25% by weight, the normalized lateral TDA to them increased the voids and reduced the collective stiffness of the mixture. On earth pressure at rest continued to decrease for the clay-TDA mixtures but did not change the other hand, the used clay sample was softer and had more voids than the used gravel significantlyand sand for samples. the gravel-TDA Accordingly, and thesand-TDA stiffness mixtures. of the pure clay was almost 50% of that of the Hence,gravel andit can the be sand. concluded However, that thethe mixingaddition effort of 25% introduced TDA by weight during to mixing gravel the results clay with in a 20%theTDA reduction particles in the (at lowlateral TDA earth contents) pressure. compacted Likewise, the the voids addition and enhanced of 25% TDA the stiffnessby weightof to the sand mixture. results When in a 19.5% the TDA reduction content in increased the lateral to earth 50% andpressure. more, However, however, adding segregation only started10% TDA to occurby weight in the to mixture clay can and reduce consequently, the lateral the earth stiffness pressure decreased. by 36%, which can result in huge savings.

(a) (b)

(c)

FigureFigure 13. Secant 13. Secant shear shearmodulus modulus versus versus TDA TDAcontent content for the for (a the) gravel-TDA, (a) gravel-TDA, (b) sand-TDA, (b) sand-TDA, and and (c) (c) clay-TDAclay-TDA mixtures. mixtures.

Finally, it is evident from Figure 13 that for mixtures containing the same amount of TDA, increasing the conﬁning pressure from 50.1 to 196.4 kPa considerably enhanced the secant shear modulus.

6.7. Normalized Lateral Earth Pressure at Rest The normalized lateral earth pressure at rest is an important factor used in the design of a geotechnical application such as a retaining wall. The angle of internal friction and the dry unit weight of the soil are two variables that affect the normalized lateral earth pressure. Equation (3) was used to determine the normalized lateral earth pressure at rest [51]: p o = 1 − sin ϕ0 γ (3) z d

po where z is the normalized lateral earth pressure at rest, z is the depth, ϕ’ is the angle of internal friction, and γd is the dry unit weight of the soil. Figure 14 shows the normalized lateral earth pressure at rest plotted against the TDA content for the gravel-TDA, sand-TDA, and clay-TDA mixtures. SustainabilitySustainability2021 2021, 13, 13, 5087, x FOR PEER REVIEW 2223 ofof 2526

Figure 14. Normalized lateral earth pressure at rest versus TDA content for the gravel-TDA, sand- Figure 14. Normalized lateral earth pressure at rest versus TDA content for the gravel-TDA, sand- TDA, and clay-TDA mixtures. TDA, and clay-TDA mixtures. 7. Conclusions It can be seen that adding up to 10% TDA by weight to the soils decreased the normalizedThe main lateral objective earth pressureof this study at rest. was For to theinvestigate clay-TDA the mixture, shear strength adding behavior up to 10% of TDAcoarse-grained by weight significantlyto fine-grained reduced soils the containing normalized 0% lateral to 100% earth TDA pressure by weight, at rest; with however, TDA increasingparticles less the than TDA 75 content mm in from length. 10% In to 25%addition, then stabilizedthe compressibility the normalized behavior lateral upon earth the pressureshearing at of rest. TDA In and contrast, soil-TDA for themixtures gravel-TDA with various and sand-TDA TDA contents mixtures, was evaluated the normalized at dif- lateralferent earthconfining pressure pressures. at rest continuedThe shear tostrength decrease parameters as the TDA of contentmixtures increased with various from 10%TDA tocontents 25%. With were the determined further addition and compared of TDA, beyondaccording 25% to by the weight, Mohr–Coulomb the normalized failure lateral crite- earthrion. pressureThe effect at of rest the continued TDA content to decrease on the no forrmalized the clay-TDA lateral mixtures earth pressure but did was not changethen in- significantlyvestigated. The for thefollowing gravel-TDA conclusions and sand-TDA can be drawn mixtures. from this study: 1. Hence,The dry it unit can weight be concluded of the gravel-TDA, that the addition sand-TDA, of 25% and TDA clay-TDA by weight mixtures to gravel decreased results in a 20%almost reduction linearly inas thethe lateralTDA content earth pressure. increased; Likewise, the addition of 25% TDA by weight2. The to addition sand results of TDA in a to 19.5% the gravel reduction decreased in the lateralthe shear earth resistance pressure. upon However, shearing adding at all onlythe 10% confining TDA by weightpressures to clayconsidered. can reduce However, the lateral the addition earth pressure of TDA by to 36%, the sand which or canclay resultinitially in huge increased savings. and then decreased the shear resistance upon shearing at all the confining pressures. It was also found that increasing the confining pressure en- 7. Conclusions hanced the shear resistance of the mixtures; 3. The maingravel-TDA objective and of sand-TDA this study mixtures was to investigatecontaining theup to shear 25% strengthTDA by behaviorweight were of coarse-grainedinitially compressed, to fine-grained and then soils dilated containing upon 0% shearing to 100% at TDA all the by confining weight, with pressures TDA particlesconsidered. less than The 75 addition mm in length. of TDA In to addition, the gravel the or compressibility sand increased behaviorthe compressibility upon the shearing of TDA and soil-TDA mixtures with various TDA contents was evaluated at behavior of the mixtures upon shearing at all the confining pressures. A similar ob- different confining pressures. The shear strength parameters of mixtures with various servation was made for the clay-TDA mixtures at a confining pressure of 50.1 kPa. TDA contents were determined and compared according to the Mohr–Coulomb failure However, at confining pressures of 98.8 and 196.4 kPa, the compressibility behavior criterion. The effect of the TDA content on the normalized lateral earth pressure was then of the clay-TDA mixture initially decreased with the addition of up to 10% TDA by investigated. The following conclusions can be drawn from this study: weight, and then increased with further addition of TDA; 1.4. AddingThe dry up unit to weight 10% TDA of the by gravel-TDA,weight to the sand-TDA, gravel or sand and clay-TDAincreased mixturesthe angle decreasedof internal frictionalmost linearlyslightly asby the about TDA 3%. content In general, increased; adding up to 25% TDA by weight to the 2. gravelThe addition or sand of did TDA not tosignificantly the gravel change decreased the theangle shear of internal resistance friction. upon However, shearing the at additionall the confining of further pressures TDA, be considered.yond 25%, to However, the gravel-TDA the addition and ofsand-TDA TDA to themixtures sand thenor clay sharply initially decreased increased the and angle then of decreasedinternal friction. the shear In contrast, resistance the upon addition shearing of up at to 10%all the TDA confining by weight pressures. to the clay It was sharply also foundincreased that the increasing angle of theinternal confining friction, pressure which enhanced the shear resistance of the mixtures; then decreased with further addition of TDA; 3. The gravel-TDA and sand-TDA mixtures containing up to 25% TDA by weight were 5. The addition of up to 25% TDA by weight to the gravel decreased the apparent cohe- initially compressed, and then dilated upon shearing at all the confining pressures sion. In contrast, the addition of up to 25% TDA by weight to the sand or clay caused considered. The addition of TDA to the gravel or sand increased the compressibility the cohesion intercept to increase. The cohesion intercept for the sand-TDA mixtures behavior of the mixtures upon shearing at all the confining pressures. A similar then continued to increase as the TDA content increased. However, increasing the observation was made for the clay-TDA mixtures at a confining pressure of 50.1 kPa. TDA content from 25% to 50% by weight enhanced the cohesion of the gravel-TDA However, at confining pressures of 98.8 and 196.4 kPa, the compressibility behavior mixtures and reduced the cohesion of the clay-TDA mixtures; of the clay-TDA mixture initially decreased with the addition of up to 10% TDA by 6. The addition of TDA to the gravel or sand decreased the secant shear modulus at all weight, and then increased with further addition of TDA; the confining pressures considered. However, for the clay, the secant shear modulus increased at all the confining pressures with the addition of up to 10% TDA by weight, Sustainability 2021, 13, 5087 23 of 25

4. Adding up to 10% TDA by weight to the gravel or sand increased the angle of internal friction slightly by about 3%. In general, adding up to 25% TDA by weight to the gravel or sand did not significantly change the angle of internal friction. However, the addition of further TDA, beyond 25%, to the gravel-TDA and sand-TDA mixtures then sharply decreased the angle of internal friction. In contrast, the addition of up to 10% TDA by weight to the clay sharply increased the angle of internal friction, which then decreased with further addition of TDA; 5. The addition of up to 25% TDA by weight to the gravel decreased the apparent cohesion. In contrast, the addition of up to 25% TDA by weight to the sand or clay caused the cohesion intercept to increase. The cohesion intercept for the sand- TDA mixtures then continued to increase as the TDA content increased. However, increasing the TDA content from 25% to 50% by weight enhanced the cohesion of the gravel-TDA mixtures and reduced the cohesion of the clay-TDA mixtures; 6. The addition of TDA to the gravel or sand decreased the secant shear modulus at all the confining pressures considered. However, for the clay, the secant shear modulus increased at all the confining pressures with the addition of up to 10% TDA by weight, and then declined as the TDA content increased further. For mixtures with the same TDA content, increasing the confining pressure from 50.1 to 196.4 kPa significantly enhanced the secant shear modulus; 7. Adding up to 10% TDA by weight to the gravel, sand, or clay reduced the normalized lateral earth pressure at rest. The reduction was the sharpest for the clay-TDA mixture. For the gravel and sand, the normalized lateral earth pressure at rest decreased as the TDA content increased from 10% to 25% by weight, and then did not change significantly as the TDA content increased further. However, for the clay, the normalized lateral earth pressure at rest stabilized as the TDA content increased from 10% to 25% and then decreased sharply with further addition of TDA.

Author Contributions: Conceptualization, H.E.N.; methodology, H.E.N.; data curation, A.I.; investigation, H.E.N.; supervision, H.E.N.; writing—original draft, H.E.N. and A.I.; writing—review & editing, H.E.N.; project administration, H.E.N.; funding acquisition, H.E.N. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Natural Sciences and Engineering Council of Canada (NSERC), grant number 2016-PNG-57311 and by a research grant from Divert NS. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All data, models, and code generated or used during the study appear in the submitted article. Acknowledgments: This study was conducted at Dalhousie University, Halifax, Nova Scotia. The authors would like to acknowledge the financial support provided by the Natural Sciences and Engineering Council of Canada (NSERC) and Divert NS. In addition, the TDA sample used in this study was provided by research collaborator Halifax C&D Recycling Ltd. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Cecich, V.; Gonzales, L.; Hoisaeter, A.; Williams, J.; Reddy, K. Use of Shredded Tires as Lightweight Backﬁll Material for Retaining Structures. Waste Manag. Res. 2016, 14, 433–451. [CrossRef] 2. Edinçliler, A.; Gökhan, B.; Altug, S. Inﬂuence of Different Processing Techniques on the Mechanical Properties of Used Tires in Embankment Construction. Waste Manag. 2010, 30, 1073–1080. [CrossRef] 3. El Naggar, H.; Pendar, S.; Amirnezam, F. Strength and Stiffness Properties of Green Lightweight Fill Mixtures. Geotech. Geol. Eng. 2016, 34, 867–876. [CrossRef] 4. Pehlken, A.; Elhachmi, E. Scrap Tire Recycling in Canada; 08. REPORT MTL 2005-8(CF) Natural Resources Canada; CANMET Materials Technology Laboratory: Ottawa, ON, Canada, 2005. Sustainability 2021, 13, 5087 24 of 25

5. Strenk, P.M.; Joseph, W.; Dennis, G.G.; Dana, N.H.; Mark, F.N. Variability and Scale-Dependency of Tire-Derived Aggregate. J. Mater. Civ. Eng. 2007, 19, 233–241. [CrossRef] 6. ASTM D6270. Standard Practice for Use of Scrap Tires in Civil Engineering Applications; Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, USA, 2017; pp. 1–21. [CrossRef] 7. Humphrey, D.N.; Manion, W.P. Properties of tire chips for lightweight fill. In Grouting, Soil Improvement and Geosynthetics; ASCE: Reston, VA, USA, 1992; pp. 1344–1355. 8. Humphrey, D.N.; Eaton, R.A. Field Performance of Tire Chips as Subgrade Insulation for Rural Roads. In Proceedings of the Transportation Research Board Conference Proceedings, Minneapolis, MN, USA, 25–29 June 1995. 9. Lee, J.H.; Salgado, R.; Bernal, A.; Lovell, C.W. Shredded tires and rubber-sand as lightweight backfill. J. Geotech. Geoenvironmental Eng. 1999, 125, 132–141. [CrossRef] 10. Mills, B.; El Naggar, H.; Valsangkar, A. Use of Tire Derived Aggregate in Highway Embankment Construction—North American Overview and a Canadian perspective. In Ground Improvement: Case Histories and New Directions; Elsevier: Oxford, UK, 2015; pp. 635–655. 11. Sparkes, J.; El Naggar, H.; Valsangkar, A. Compressibility and Shear Strength Properties of Tire Derived Aggregate Mixed with Lightweight Aggregate. J. Pipeline Syst. 2019, 10, 04018031. [CrossRef] 12. Mahgoub, A.; El Naggar, H. Using TDA Underneath Shallow Foundations: Simplified Design Procedure. Int. J. Geotech. Eng. 2019.[CrossRef] 13. Mahgoub, A.; El Naggar, H. Using TDA as an Engineered Stress Reduction Fill over Pre-existing Buried Pipes. J. Pipeline Syst. 2019, 10, 04018034. [CrossRef] 14. Mahgoub, A.; El Naggar, H. Innovative Application of TDA around Corrugated Steel Plate (CSP) Culverts. J. Pipeline Syst. 2020, 11, 04020025. [CrossRef] 15. Mahgoub, A.; El Naggar, H. Coupled TDA-Geocell Stress Bridging System for Metal Pipes. J. Geotech. Geoenvironmental Eng. 2020, 146, 04020052. [CrossRef] 16. Mahgoub, A.; El Naggar, H. Using TDA Underneath Shallow Foundations: Field Tests and Numerical Modelling. J. Comput. Geotech. 2020, 126, 103761. [CrossRef] 17. Moussa, A.; El Naggar, H. Numerical Evaluation of Buried Wave Barriers Performance. Int. J. Geosynth. Ground Eng. 2020, 6, 56. [CrossRef] 18. Moussa, A.; El Naggar, H. Dynamic Characterization of Tire Derived Aggregates. J. Mater. Civil Eng. 2021, 33, 04020471. [CrossRef] 19. Fernández-Ruiz, J.; Medina Rodríguez, L.E.; Costa, P.A. Use of Tyre-Derived Aggregate as Backfill Material for Wave Barriers to Mitigate Railway-Induced Ground Vibrations. Int. J. Environ. Res. Public Health 2020, 17, 9191. [CrossRef][PubMed] 20. Rodríguez, L.M.; Arroyo, M.; Cano, M.M.; Medina, L.; Martín, M. Use of tire-derived aggregate in tunnel cut-and-cover. Can. Geotech. J. 2018, 55, 968–978. [CrossRef] 21. Jamshidi, C.R.; Behzad, F.; Mohammad, A.A.M.; Reza, A. An Experimental and Numerical Investigation into the Compressibility and Settlement of Sand Mixed with TDA. Geotech. Geol. Eng. 2017, 35, 2401–2420. [CrossRef] 22. Kim, H.-K.; Santamarina, J.C. Sand–rubber mixtures (large rubber chips). Can. Geotech. J. 2008, 45, 1457–1466. [CrossRef] 23. Mashiri, M.S.; Vinod, J.S.; Sheikh, M.N.; Tsang, H.-H. Shear strength and dilatancy behaviour of sand–tyre chip mixtures. Soils Found. 2015, 55, 517–528. [CrossRef] 24. Soltani, A.; Taheri, A.; Deng, A.; O’Kelly, B.C. Improved Geotechnical Behavior of an Expansive Soil Amended with Tire-Derived Aggregates Having Different Gradations. Minerals 2020, 10, 923. [CrossRef] 25. Araujo, G.L.S.; Moreno, J.A.S.; Zornberg, J.G. Shear behavior of mixtures involving tropical soils and tire shreds. Constr. Build. Mater. 2021, 276, 122061. [CrossRef] 26. Humphrey, D.N.; Thomas, C.S. Tire Chips as Lightweight Subgrade Fill and Retaining Wall Backfill. In Proceedings of the Symposium on Recovery and Effective Reuse of Discarded Materials and By-Products for Construction of Highway Facilities, Denver, CO, USA, 19–22 October 1993. 27. Zahran, K.; El Naggar, H. Effect of Sample Size on TDA Shear Strength Parameters in Direct Shear Tests. Transp. Res. Rec. 2020, 2674, 1110–1119. [CrossRef] 28. El Naggar, H.; Zahran, K.; Moussa, A. Effect of Particle Size on TDA Shear Strength Parameters in Large-Scale Direct Shear Tests. Geotechnics 2021, 1, 1–17. [CrossRef] 29. Foose, G.J.; Craig, H.B.; Peter, J.B. Sand Reinforced with Shredded Waste Tires. J. Geotech. Eng. 1996, 122, 760–767. [CrossRef] 30. Tatlisoz, N.; Tuncer, B.E.; Benson, C.H. Interaction between Reinforcing Geosynthetics and Soil-Tire Chip Mixtures. J. Geotech. Geoenviron. Eng. 1998, 124, 1109–1119. 31. Akbulut, S.; Seracettin, A.; Ekrem, K. Modification of Clayey Soils Using Scrap Tire Rubber and Synthetic Fibers. Appl. Clay Sci. 2007, 38, 23–32. [CrossRef] 32. Wu, W.Y.; Christopher, C.B.; Robert, F.C. Triaxial Determination of Shear Strength of Tire Chips. J. Geotech. Geoenvironmental Eng. 1997, 123, 479–482. [CrossRef] 33. El Naggar, H.; Zahran, K. Effect of the Particle Size on TDA Shear Strength Parameters in Triaxial Tests. Buildings 2021, 11, 76. [CrossRef] 34. El Naggar, H.; Ashari, M.; Mahgoub, A. Development of an Empirical Hyperbolic Material Model for TDA Using Large Scale Triaxial Testing. Int. J. Geotech. Eng. 2021.[CrossRef] Sustainability 2021, 13, 5087 25 of 25

35. Ashari, M.; El Naggar, H.; Martins, Y. Evaluation of the Physical Properties of TDA-Sand Mixtures. In Proceedings of the GeoOttawa 2017, the 70th Canadian Geotechnical Conference, Ottawa, ON, Canada, 1–4 October 2017. 36. Ahmed, I. Laboratory Study on Properties of Rubber-Soils. Available online: https://docs.lib.purdue.edu/jtrp/1069/ (accessed on 30 April 2021). 37. Masad, E.; Ramzi, T.; Carlton, H.; Thomas, P. Engineering Properties of Tire/Soil Mixtures as a Lightweight Fill Material. Geotech. Test. J. 1996, 19, 297–304. 38. Youwai, S.; Dennes, T.B. Strength and Deformation Characteristics of Shredded Rubber Tire-Sand Mixtures. Can. Geotech. J. 2003, 40, 254–264. [CrossRef] 39. Zornberg, J.G.; Alexandre, R.C.; Chardphoom, V. Behaviour of Tire Shred-Sand Mixtures. Can. Geotech. J. 2004, 41, 227–241. [CrossRef] 40. Rao, G.V.; Dutta, R.K. Compressibility and Strength Behaviour of Sand-Tyre Chip Mixtures. Geotech. Geol. Eng. 2006, 24, 711–724. [CrossRef] 41. ASTM D7181. Standard Test Method for Consolidated Drained Triaxial Compression Test for Soils; Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, USA, 2011; pp. 1–11. [CrossRef] 42. ASTM D3080. Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions; Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, USA, 2012; pp. 1–9. [CrossRef] 43. ASTM D698. Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12,400 Ft-Lbf/Ft3 (600 KN-m/M3)); Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, USA, 2012; pp. 1–13. [CrossRef] 44. ASTM D422. Standard Test Method for Particle-Size Analysis of Soils. Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, USA, 2007; pp. 1–8. [CrossRef] 45. ASTM D4318. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils; Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, USA, 2017; pp. 1–20. [CrossRef] 46. ASTM D2487. Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System); Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, USA, 2011; pp. 1–12. [CrossRef] 47. Edil, T.B.; Peter, J.B. Engineering Properties of Tire Chips and Soil Mixtures. Geotech. Test. J. 1994, 17, 453–464. 48. Bosscher, P.J.; Tuncer, B.E.; Neil, N.E. Construction and Performance of a Shredded Waste Tire Test Embankment. Transp. Res. Rec. 1992, 1345, 44–52. 49. Gray, D.H.; Harukazu, O. Mechanics of Fiber Reinforcement in Sand. J. Geotech. Eng. 1983, 109, 335–353. [CrossRef] 50. Bowles, J.E. Foundation Design and Analysis, 5th ed.; The McGraw-Hill Companies, Inc.: New York, NY, USA, 1982. 51. Braja, M. Advanced Soil Mechanics, 5th ed.; CRC Press, Taylor & Francis: Boca Raton, FL, USA, 2014.