Tire Shreds and Tire Crumbs Inclusion: Contrast Effects on Bearing Capacity of Sand Reza Jamshidi Chenari1, Mehran Karimpour Fard2, Javad Shafie3, Aref Ghorbanpour4 1 Associate Professor, Department of civil engineering, Faculty of engineering, The University of Guilan,
[email protected] 2 Assistant Professor, Department of civil engineering, Faculty of engineering, The University of Guilan,
[email protected] 3 M.Sc. Graduate, Department of Civil Engineering, Faculty of Pardis, The University of Guilan,
[email protected] 4 M.Sc. Graduate, Department of Civil Engineering, Faculty of Pardis, The University of Guilan,
[email protected]
ABSTRACT In recent years various studies have been undertaken on how to use waste materials in civil engineering projects. Among waste materials, waste rubber has been highlighted to be used for different soil reinforcement purposes. The objective of this study is to investigate the feasibility of using tire shreds and tire crumbs for bearing capacity improvement purposes. A series of laboratory large scale model tests were conducted to evaluate the bearing capacity of a circular footing rested on tire-sand mixtures. Tire crumbs and tire shreds content and shreds aspect ratio are the main parameters that affect the bearing capacity. Four shred contents of 5, 10, 15 and 20% and five tire crumbs contents of 5, 10, 15, 20 and 25% by weight were selected. It was found that the addition of tire shreds to sand increases bearing capacity ratio (BCR) and decreases settlement reduction factor (SRF). In contrast, it was observed that BCR decreases with increasing the tire crumbs content due to replacement of rigid sand particles with soft and compressible crumbs. The findings suggest that different waste tire products may render mixed results when considering for soil improvement.
KEYWORDS: Tire shreds; Tire crumbs; Large scale rigid box; Circular footing; Bearing capacity Ratio; Settlement reduction factor.
INTRODUCTION Earth reinforcement is an effective and reliable technique for increasing the strength and stability of soils. This technique is used today in a variety of applications ranging from retaining structures and embankments to subgrade stabilization beneath footings and pavements. In past practice, reinforcements have typically consisted of long, flexible, galvanized steel strips with either a smooth or a ribbed surface. Most field research to date on the mechanics of reinforced earth has tended to focus on high modulus, steel strips (Yetimoglu and Salbas, 2003). Therefore, randomly distributed fiber reinforced soils have recently attracted increasing attention in geotechnical engineering.
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Disposal of scrap recycling tires has become a challenging task in many countries. In the recent decades, hundreds of millions of scrap tires are generated and accumulated in the worldwide, due to the increasing population. In Iran nearly 20 million tons of scrap tires are discarded every year, in India nearly 100 million tones, Korea generates nearly 20 million tons, in U.S., an estimate touches 250 million tons and Canada generates nearly 28 million tons of waste tires every year (Yoon et al., 2008). Increase in the amount of waste tires makes them harder and more expensive to dispose safely without threatening human health and environment. For instance, stockpiled waste tires are flammable, prone to fires with toxic fumes and may then cause a major health hazard for both human beings and animals (Attom, 2006). The application of waste tires in various forms has been recently developed in reinforcing soil for a variety of geotechnical applications ranging from retaining structures and earth embankments, asphalt pavement and paving system, foundation beds and other applications. As a practical point of view, the use of waste rubbers may be offered in geotechnical applications due to four advantages; (1) the re-use of waste materials such as tires and tubes offers reduction in environmental health hazard and saving huge spaces and costs to maintenance of wastes, (2) the reduction in consumption of competent natural soil and its cost saving benefit, (3) soil reinforcement, which can demonstrate a substantial increase in shear strength of mixture compared to soil alone, and (4) the exhibition of a higher capacity to absorb and to dissipate energy than soil alone and tend to decrease the stress and shocks transferred into the ground when subjected by dynamic loads (Tafreshi and Norouzi, 2012). Therefore, using recycled materials, particularly wastes tires when mixed/combined with soil is becoming more popular due to the shortage of natural mineral resources and increasing waste disposal costs. However, with increasing the use of waste tires in geotechnical applications, a need for further understanding of the behavior of rubber–soil mixture/combination is required (Tafreshi and Norouzi, 2012). Experimental results reported by various researchers have shown that the synthetic/natural fiber-reinforced soil and shredded rubber-reinforced soil are potential composite materials, which can be advantageously employed in increasing soil strength. Bosscher et al. (1997) used rubber–soil mixture as a replacement for embankment. They reported a better performance when the mixture covered by a soil cap layer compared to using the mixture in the whole of the fill. In addition, this soil cap not only reduces the ongoing settlement but also prevents tire shreds from possible ignition. Yoon et al. (2004) presented the beneficial use of sidewalls of waste car tires as reinforcing material in sand from laboratory plate load tests. The results showed that the reinforcement by a single planar layer in medium dense sand is enough to reduce the settlement more than half and increase bearing capacity more than a factor of two. Hataf and Rahimi (2006) performed a series of laboratory model tests to investigate the use of shredded waste tires as reinforcement to increase the bearing capacity of soil. Shred content and shreds aspect ratio are the main parameters affecting the bearing capacity. Tire shreds with rectangular shape and widths of 2 and 3 cm with aspect ratios 2, 3, 4 and 5 were mixed with sand. Five shred contents of 10, 20, 30, 40 and 50% by volume were selected. The results showed that the addition of tire shreds to sand increases bearing capacity ratio (BCR) from 1.17 to 3.9 depending on the shred content and aspect ratio. The maximum BCR was attained at shred content of 40% and dimensions of 3×12 cm. It was shown that increasing the shred content increases the BCR. However, an optimum value for shred contents was observed after which, a decrease in BCR is expected. For a given shred width, shred content and soil density it seemed that the aspect ratio of 4 gives maximum BCR. Yoon et al. (2008) performed plate load tests on tire cell reinforced sand. The sand samples with different relative densities of 40, 50 and 70% were prepared. They used three layers of tire cell in foundation bed. A BCR of 2.5 for loose sand with tire cell was concluded as an improvement while this improvement gets less highlighted when sand skeletal density increases. Tafreshi and Norouzi (2012) investigated the feasibility of using rubber shreds, randomly distributed into the soil, as soil reinforcement beneath
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the footing. The results show that the efficiency of rubber reinforcement was increased by addition of rubber content, the thickness of rubber-reinforced soil layer and the soil cap thickness up to their optimum values. The main objective of this study is to investigate the feasibility of using shredded waste tires and tire crumbs with different weight contents as reinforcement to improve the bearing capacity of soil. A series of laboratory scale bearing capacity tests in a rigid box have been carried out on sand reinforced with randomly distributed tire shreds and tire crumbs to investigate their contrast effects on bearing capacity of the mixture.
THEORY Ranjan et al. (1996) utilized an original force-equilibrium model of Waldrom (1977) to describe the load-deformation characteristics of soils reinforced with plant roots. Conventional MohrCoulomb’s shear strength equation (τ=c+σtanφ) was invoked in a modified form to describe the effect of different reinforcement parameters on shear strength of reinforced soil as: 𝜏𝜏𝑟𝑟 = 𝑐𝑐 + 𝜎𝜎𝜎𝜎𝜎𝜎𝜎𝜎𝜎𝜎 + ∆𝑆𝑆
(1)
where τr is the shear strength of reinforced soil and ΔS is the portion of shear strength attributed to the reinforcement effect. An oriented fiber reinforced sand with inclined fibers assumed by Gray and Ohashi (1983) and Grey and Al-Refeai (1986) was adopted to explain the theoretical basis for shear failure mechanism of a shear plane crossed by inclined fibers as illustrated in Figure 1. The initial inclined fibers get distorted due to shear deformation along the shear bed and this causes mobilization of tensile force in reinforcement elements. The mobilized tensile strength is decomposed into normal and tangential components. The normal component increases the effective stress normal to the shear plane and the tangential component renders shear strength to the plane directly.
Figure 1: Fiber reinforcement model of fiber oriented at angle “i” to shear surface (Ranjan et al., 1996).
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The shear strength increase due to oriented fiber reinforcement in a sand is estimated from Eq. (2). ∆𝑆𝑆 = 𝜎𝜎𝑡𝑡′ (sin(90 − 𝜓𝜓) + cos(90 − 𝜓𝜓) 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡) 1 𝜓𝜓 = 𝑡𝑡𝑡𝑡𝑡𝑡−1 � 𝑥𝑥 � � � + 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 −1 𝑧𝑧
(2)
(3)
Where σ′t = (Af/A)σt is the mobilized tensile strength in oriented fibers per unit area of shear plane, σ′t is the developed tensile stress in each fiber, Af is the area of fibers crossing the shear plane and A is the area of shear plane itself. Other parameters are illustrated in Figure 1 accordingly. Gray and Ohashi (1983) proposed a formula for the tensile strength of fibers found to be a function of fiber properties and confining stress. Fiber properties of interest are length, diameter, fraction, modulus and skin friction. Fragaszy and Lawton (1984) also analytically and experimentally illustrated that the bearing capacity of reinforced sand varies with the length of the reinforcing metal strips. They also emphasized the importance of the soil-reinforcing strip friction coefficient. The above theoretical discussion reveals that soil reinforcement performance is a function of fiber orientation (inclination), fiber length and material (modulus). This means that different behavior shall be expected when comparing bearing capacity of sand reinforced with different waste tire products. The products of shredded rubber tires are usually referred as “tire chips” when they are between 12 to 50 mm in size, and are called “tire shreds” when they are more than 50 mm in size (Youwai and Bergado, 2004; Mohammad et al., 2013; Marto et al., 2013). Tire crumbs or granulated rubber are also referred to particles having diameters less than 12 mm (El-Sherbiny et al., 2013). This means that tire chips/shreds act totally different from “tire crumbs” as tire crumbs are soft granules without noticeable reinforcing length to render shear strength to the mixture. It might instead soften the mixture due to its low elasticity modulus. For tire chips/shreds, the length and orientation are obviously important and these parameters will definitely affect their performance in bearing capacity of reinforced sand. Maher and Gray (1990) conducted a statistical analysis on sand reinforced by randomly distributed discrete fibers with the aim to predict the average orientation of fibers in any arbitrary plane. They stated that the average orientation of fibers is perpendicular to the shear failure plane in triaxial compression tests. However, tire shreds especially large sizes are initially placed randomly oriented in a fill but tend to rearrange themselves because of the compaction by a bulldozer or high gravity loads (overburden) and align predominantly in the horizontal direction. This is also enhanced by the flattening of usually curved large-size pieces (Heimdahl and Drescher, 1999). It is well established from theory of bearing capacity for shallow foundations that the shear failure slip planes in passive zones around the central rigid wedge are inclined at an angle of 45◦-φ/2 with horizontal. This means that horizontal tire chips/shreds will be probably at an angle of 45◦-φ/2 with the slip surfaces. According to Eq. (2) they will render shear strength to the host soil as far as the reinforcing fibers are long enough to mobilize tensile strength of the crossing fibers through skin friction action. At what follows, an experimental program will run to show the contrast effects of different waste tire products on bearing capacity of sand.
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EXPERIMENTAL STUDY Material The sand material tested in this study was collected from Chamkhaleh Beach adjacent to Chamkhaleh River, located on SW of Caspian Sea. The sand was classified under the Unified Soil Classification System as SP (poorly graded sand) and under the AASHTO Soil Classification System as A-3. The particle size distribution curve of sand is shown in Figure 2. The specific gravity was determined according to ASTM D 854, and maximum and minimum dry unit weights were determined based on ASTM D 4253 and ASTM D 854, respectively. It had a specific gravity of 2.63, a maximum dry unit weight of 16.1 kN/m3 (i.e., minimum void ratio of 0.63) and a minimum dry unit weight of 14.2 kN/m3 (i.e., maximum void ratio of 0.85). The sand had a coefficient of uniformity of 1.54, a coefficient of curvature of 0.95, and the friction angle of the sand was 38° when its relative density Dr was 60%. The index properties of this soil are summarized in Table 1 (Karimpour Fard et al., 2015; Jamshidi Chenari et al., 2016).
Figure 2: Grain size distribution curves of tire crumbs and sand used for mixtures. Table 1: Physical properties of tested materials Material
Specific gravity (Gs)
Sand
2.63
Tire Shred Tire Crumb
1.21 0.91
Dry unit weight (kN/m3) 14.2 (min) 16.1 (max) 5.8 4.24
Effective size D10 (mm)
Mean grain size D50 (mm)
Uniformity coefficient Cu
Coefficient of curvature Cc
0.17
0.21
1.54
0.95
0.62
1.86
3.56
1.03
Shredded tire rubbers used in this study, as an alternative reinforcement material was clean and free of any steel and cord. For better performance, tire shreds are cut from waste tires with approximately the same size and thickness. Tires are cut with a special cutter by hand into rectangular shape and different sizes (Figure 3). The nominal size of the tire shreds of 20 mm in width and about 20, 40 and 60 mm in length was selected (aspect ratio between 1 and 3). Properties of the rubber used,
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are presented in Table 1. Tire crumbs used as an alternative light weight material. Tire crumbs were prepared with special machinery where scrap tires were crushed into pieces and powdered after removing steel belting. The processed tire crumbs obtained from local industry were sized predominantly in the range of 0.1 to 10 mm (Figure 3). The specific gravity of the tire crumbs was determined as per ASTM-D854 (2010) and found to be 0.91. The grain size distribution of the rubber particles are presented in Figure 2.
(a)
(b)
Figure 3: Non-conventional materials; a) Tire Shreds; b) Tire Crumbs.
Sample Preparation Tire shreds or tire crumb-sand specimens were formed by mixing tire shreds or tire crumbs with sands at a dry mass ratio η of tire shreds or tire crumbs over the mixture, which was thought as the most significant factor controlling the unit weight and mechanical behavior of the mixtures. Investigated ratios were 5, 10, 15 and 20% for tire shreds and 5, 10, 15, 20 and 25% for tire crumbs by weight. For each designated mixing ratio, the mass-based proportions of sand, tire shreds and tire crumb were determined beforehand. The proportioned materials were mixed thoroughly until the mixtures were homogenous enough. The tank was, then, filled up to specified thickness (20 cm) with the mixed material. Each layer was tamped and compacted to achieve desired dry density of 15.48kN/m3. A list of specimens and corresponding weight and volume ratios is provided in Table 2 for tire shreds and tire crumbs mixed with sand. A total of 18 series (i.e., tire shreds and tire crumbs content η=0 (pure sand) and different tire shreds and tire crumbs contents) of plate load tests were performed. The volumetric ratio of tire shreds and tire crumbs over the combination of tire shreds or tire crumbs and sand in the mixture, χ, was calculated based on the mixing ratios and specific gravities of particles for each mixture, as presented in Eq. (4). η GsT χ= η 100 + Gss GsT
(4)
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where χ denotes the tire shreds or tire crumbs volumetric percentage in the mixture and GsT and Gss denote the specific gravities of tire shreds or tire crumbs and sand, respectively.
Table 2: Summary of testing program on tire shreds and tire crumb –sand mixtures Designation Sand TS-Sand-1* TS-Sand -2 TS-Sand -3 TS-Sand -4 TC-Sand-1** TC-Sand-2 TC-Sand-3 TC-Sand-4 TC-Sand-5
Content by Content by Dry density weight η (%) volume χ (%) (kN/m3) 0 0 15.48 5 9.80 15.48 10 17.85 15.48 15 24.59 15.48 20 30.30 15.48 5 12.6 15.48 10 22.4 15.48 15 30 15.48 20 36.6 15.48 25 41.3 15.48 *Tire Shreds **Tire Crumbs
Aspect ratio 1,2,3 1,2,3 1,2,3 1,2,3 -
The circular model footing was placed in the center of the soil bed. After preparation of sample, the pressure was applied on the sand with the help of a mechanical arrangement. The data acquisition system was developed to automatically read and record both the load and the settlement. Two linear variable differential transducers (LVDTs) with accuracies of 0.01% over their full range (100 mm) were placed on the two sides of the footing model to measure the average settlement of the footing during loading. To ensure accurate readings, all of the devices were calibrated prior to each series of tests. The static load was increased at a rate of 0.1 kN per second. The total of 18 tests were carried out on circular footing on soil mixed with different tire crumbs and tire shreds contents and aspect ratios. A test on fully unreinforced soil was performed to provide a reference load bearing capacity. The results of all tests were compared to the results of tests carried out on unreinforced soil to indicate the effect of reinforcement on bearing capacity of the soil.
Large Scale Plate Load Test A physical model test was conducted in a test bed-loading frame consisting of the testing tank, the loading system and the data acquisition system. The plate load tests for this research were carried out in a test chamber of 1.5 m width, 1.5 m length, and 1.3 m height. The chamber was placed under a loading frame and the foundation bed with thickness of 120 cm includes 40 cm natural ground and 80cm rubber–soil mixture was compacted in layers of 20 cm in thickness until the soil reached the footing level. A hydraulic jack welded against a reaction frame was used to push the footing slightly into the bed for proper contact between the soil and the footing. A schematic diagram of the test setup which contains the unreinforced soil as natural ground, the rubber-reinforced soil (rubber–soil mixture), and the footing is shown in Figure 4. A photograph showing the complete test set-up is shown in Figure 5.
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Figure 4: Schematic representation of the testing apparatus.
Figure 5: Photograph showing complete test set-up. The chamber has rigid boundaries, and the size was determined through the finite difference analyses in order to assure boundary effect elimination. Model footing was made of steel with a cylindrical shape 20cm diameter and 4cm thickness. The base of the model footing was roughened by fixing a thin layer of sand to it with epoxy glue. According to some preliminary test results (not further reported here), under a maximum applied loading stress of 1000 kPa on the model foundation, the measured deflection of sides of the tank were very small demonstrating that they would be negligible at the stress levels applied in the main tests program. Loading system includes the loading frame, the hydraulic cylinder, and the controlling unit. The loading frame consists of four stiff and heavy steel columns and a horizontal beam that supports the pneumatic actuators. The hydraulic actuator may produce monotonic loads with maximum capacity of 100 kN depending on the pressure of the pressurized oil. The data acquisition system was developed to automatically read and record both the load and the settlement. An S-shaped load cell with an accuracy of ±0.01% and a full-scale
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capacity of 50 kN was placed between the loading shaft and the footing to precisely measure the pattern of the applied load. Two linear variable differential transducers (LVDTs) with accuracies of 0.01% were used to measure the displacements.
RESULTS AND DISCUSSION In this section, the results of the laboratory large scale model tests are presented with a discussion highlighting the effects of the different parameters. Tire crumbs, tire shred contents and tire shred aspect ratio are the main parameters that affect the bearing capacity. Since the effect of these parameters cannot be considered separately, different graphs are developed for which, the change in the bearing capacity are presented with the variation of one parameter while others are held constant.
Tire Shred Inclusion Effect Figures 6–8 show typical load–settlement response for unreinforced and reinforced sands obtained of various tire shred contents of 5, 10, 15 and 20%. For both the unreinforced and tirereinforced sand, it is apparent that no clear failure point is evident in load-settlement behavior. Beyond a settlement of 5% there is a reduction in the slope of the pressure-settlement curves indicated by the softening of the load-settlement response curves (Figures 6-8). Beyond this stage, the slope of the curve remains almost constant with the footing bearing pressure continuously increasing. In current experiments, 20% vertical displacement represents the ultimate state where tests are terminated at the allowed maximum displacement, and is considered as absolute upper limit. The performance improvement due to the provision of reinforcement in the soil bed was evaluated for different ranges of footing settlement all less than 20% of footing widths (S/B=2.5%, 5%, 7.5%, 10%, 15% and 20%) as adopted by Tafreshi and Norouzi (2012).
Figure 6: Load–settlement curves for 2×2 cm shreds with different shred contents.
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Figure 7: Load–settlement curves for 2×4 cm shreds with different shred contents.
Figure 8: Load–settlement curves for 2×6 cm shreds with different shreds contents. Figures 9-11 show the bearing pressure of the footing defined at various settlement ratios for different rubber contents and aspect ratios of tire shreds. The graphs indicate that for all settlement ratios (serviceability states) the bearing capacity increases with increasing the tire shred contents, implying that the tire shred inclusion increases shear strength of the mixture by providing tensile strength to the composite mobilized along shred length embedded in the mixture randomly.
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Hataf and Rahimi (2006) carried out a series of laboratory model tests to investigate the bearing capacity of shallow footing directly rested on sand reinforced with randomly distributed tire shreds (i.e. there was no soil cap over the rubber–soil mixture). Although random distribution of tire shreds induces uncertainty and variability in results is thus expected, very good conformity between their results and finding of current study is observed for tire shreds of similar aspect ratios. Their results indicated maximum bearing capacity for rubber-reinforced bed, without considering the settlement limit criterion, obtained at shred content of 40% by volume. Similar qualitative findings were reported by Tafreshi and Norouzi (2012). They obtained an optimum rubber content to achieve a maximum improvement in ultimate bearing capacity, after that increasing shredded rubber led to decrease in bearing capacity. However, qualitatively the optimum contents reported by different researchers in different conditions are not unique and sometimes they are far apart. The reason behind the wide range is that the behavior of the rubber-soil mixture changes from competent-composite material-like to rubber-like depending on the skeletal relative density of sand, material properties of the rubber, the rubber aspect ratio and the packing condition.
Figure 9: Bearing capacity vs. tire shreds contents at various settlement levels with AR=1.
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Figure 10: Bearing capacity vs. tire shreds contents at various settlement levels with AR=2.
Figure 11: Bearing capacity vs. tire shreds contents at various settlement levels with AR=3. Bearing capacity ratio (BCR) and settlement reduction factor (SRF) are used for comparison purposes and defined according to Guido and Christou (1988):
BCR=qur/quu
(5)
SRF= Sur/Suu
(6)
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where qur and quu are ultimate pressure on reinforced and unreinforced soil read at ultimate settlement ratio (S/B=20%) and Sur and Suu are settlement of reinforced and unreinforced soil at ultimate state. For comparison, SRF has been calculated at a pressure level corresponding to ultimate bearing capacity of unreinforced sand. Figures 12 and 13 show bearing capacity ratio (BCR) and settlement reduction factor (SRF) respectively at various contents and aspect ratios of tire shreds. The graphs indicate that the BCR increases with increasing the tire shred contents implying that the tire shred inclusion not only increases shear strength but also decreases compressibility of mixture by stiffening the composite.
Figure 12: Bearing capacity ratio (BCR) vs. rubber content for different aspect ratios.
Figure 13: Settlement reduction factor at various content and aspect ratio of tire shreds. Aspect ratio of the tire shreds is affecting the performance of the tire-sand mixture in a systematic manner. The longer the shreds, the more effectiveness is expected. Figure 14 shows the variation of BCR with aspect ratio. It is clearly indicated that increasing the aspect ratio will lead to an
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increase in BCR in different tire contents. The reason is that increasing the aspect ratio induces larger pull out resistance of the reinforcement, counteracting the tensile force due to shearing. Another observation from Figure 14 is that the rate at which the BCR increases with aspect ratio, gets noticeable in higher tire contents. Finding of current study and those of Hataf and Rahimi (2006) give rise to the conclusion that the aspect ratio of 3 could be the optimum value at least for the material under study.
Figure 14: Bearing capacity ratio (BCR) vs. aspect ratio of shreds.
Tire Crumbs Inclusion Effect Tire crumbs on the other side of the spectrum, affects the bearing capacity behavior of the mixture in a different way. Figure 15 shows the pressure–settlement response for tire crumbs-sand mixtures obtained by varying the tire crumbs content from 5, 10, 15, 20 and 25%. This figure indicates that the addition of tire crumbs to sand decreases the bearing capacity.
Figure 15: Load–settlement curves for different tire crumbs contents.
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Figure 16 shows the variation of the bearing capacity of the tire crumbs-sand mixture with tire crumbs content at different settlement levels. It is observed that increasing the tire crumbs content decreases the bearing capacity of the mixture. This behavior can be explained by resorting to the fact that small size tire powder is not long enough to render shear strength within the tire-sand mixture. Tire crumbs are not long enough to mobilize the shear strength of the mixture. The inclusion of soft particles separates soil particles and forms a soft rubber fabric and consequently decreases the bearing capacity of footing due to significant compressible foundation bed. BCR values for tire crumbs-sand mixtures are also depicted in Figure 12 showing that it decreases with the powder contents. 45% bearing capacity loss is expected when 25% tire crumbs is added to the sand matrix. Explanation on the bearing capacity loss lies behind the fact that tire crumbs do not have enough reinforcing length to mobilize their tensile strength and potential induced shear strength of the mixture as discussed theoretically before. It has instead softened the mixture due to soft material inclusion within sand matrix. These results however cannot be extended to tire crumbs of all sizes as the average size of the granules is indeed important. Anvari and Shooshpasha (2014) reported contrast results on bearing capacity of fine sand mixed with tire crumbs. However their crumb size is quite larger than what utilized in current study. They mixed tire crumbs with sizes in the ranges 4 to 9 mm with D50 of 6.4 mm. In current study D50 for the tire crumb granules is 1.86 mm and it means that particles are quite smaller in average size.
Figure 16: Bearing Capacity vs. tire crumbs content at various settlement levels.
PREDICTIVE MODELS The bearing capacity ratio, BCR and the settlement reduction factor, SRF are not only a function of η but also depend on the aspect ratio of the tire shreds mixed with sand and the settlement level adopted. In addition, test results indicate that the above mentioned parameters linearly vary with increasing η, aspect ratio and the settlement level. By using the linear behavior obtained from the test results, multiple linear regression (MLR) models were run to investigate the predictive performance and reasonableness of the developed regression equations. Ultimately, the independent variables chosen for the final MLR models to predict BCR and SRF were: η, AR and the settlement. All
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regression analyses shown in Table 3 were performed using MATLAB MathWorks. These models have the general form: 𝜉𝜉
𝜉𝜉
y=𝜉𝜉0 𝑥𝑥1 1 𝑥𝑥22
(7)
logy=𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙0 + 𝜉𝜉1 𝑙𝑙𝑙𝑙𝑙𝑙𝑥𝑥1 + 𝜉𝜉2 𝑙𝑙𝑙𝑙𝑙𝑙𝑥𝑥2 w
(8)
This can be expressed in a linear form for multiple regression using:
A comparison of predicted parameters in Table 3 and laboratory measured values from large rigid box bearing capacity tests can be seen in Figure 17. As shown in Figure 17 BCR and SRF models from Table 3 provide reasonably close prediction of the laboratory results for TS/TC-sand mixture. The negative exponent of η for BCR (TC) in Table 3 is indicating and confirming the contrast behavior of tire crumbs inclusion in fine sand bearing capacity improvement.
Table 3: Data variables sets and MLR models for bearing capacity parameters Data Independent R2 (%) Equationa set variables BCR (TS)
(
BCR(TC)
SRF (TS)
𝜂𝜂
100
(
), (𝐴𝐴𝑅𝑅 )
𝜂𝜂 ( ) 100
𝜂𝜂
100
), (𝐴𝐴𝑅𝑅 )
0.9617
0.9385
0.9838
𝜂𝜂 0.3182 𝐵𝐵𝐵𝐵𝐵𝐵 = 0.4916( ) (𝐴𝐴𝑅𝑅 )0.4325 100 𝜂𝜂 𝐵𝐵𝐵𝐵𝐵𝐵 = 1.6312( ) −0.3035 100 𝑆𝑆𝑆𝑆𝑆𝑆 = 1.6824(
𝜂𝜂 −0.4674 ) (𝐴𝐴𝑅𝑅 )−0.7587 100
a From the MLR model given in Equation 4, and regression output by using Matlab Mathworks.
Figure 17: Comparison of measured and predicted performance for different parameters from bearing capacity tests
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CONCLUSION An experimental testing program involving pure sand, tire shred-sand and tire crumbs-sand specimens was undertaken to evaluate the effect of tire shred and tire crumbs content and aspect ratio on bearing capacity of shallow footing resting on reinforced sand. The test results have been used to assess and understand the potential benefits of reinforced soil in terms of the bearing capacity of footing compared with footing on plain beds. Investigations showed that the change in the bearing capacity is a function of both tire shreds and tire crumbs content, and its aspect ratio. The results in this study proved the usefulness in recycling of tires waste in geotechnical aspects of waste management in world. These lead to overall saving in competent soil material costs and re-use of tires waste. Evaluation of the experimental results obtained in this study led to the following conclusions: 1. Addition of tire shred in the sand increases its ultimate bearing capacity. The results show that the efficiency of tire shred was increased by increasing the tire shred content. A maximum of 125% improvement in sand bearing capacity was observed for the longest tire shred under study. 2. It was found that concentration of tire shreds in the soil at various contents can sufficiently reinforce the soil to reduce settlement in comparison to unreinforced model. 3. In the issue of soil reinforcement with tire shred, ultimate bearing capacity was found to increase with the aspect ratio. The longer the shreds are, the more bearing capacity is attained at least within the ranges under study. 4. Bearing capacity ratio (BCR) varies almost linearly with the aspect ratio and shred content at least within the ranges under study. MLR technique proved an efficient tool for prediction and simulation of such linearity. 5. It was shown that increasing the shred content and the aspect ratio lead to more settlement reduction or lower SRF values indeed. 6. In contrary to the finding of bearing capacity improvement of tire shreds, addition of tire crumbs to the sand proved to decrease its ultimate bearing capacity. The results show that the bearing capacity reduction rises by increasing the tire crumbs content. The final remark is that the effectiveness of different waste tire products in improving soil strength and deformation characteristics is strongly dependent on their size. As the distribution is assumed random, statistically the orientation and sizes should be stated in average sense. Depending on the average size of the reinforcement elements, they can either increase or decrease the bearing capacity of their mixture with fine sand. It is generally conceived that tire chips and shreds which are larger than 12 mm are expected to increase the bearing capacity of sand but tire crumb has mixed results depending on its average size.
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19. Tafreshi, Moghaddas, and A. H. Norouzi: "Bearing Capacity of a Square Model Footing on Sand Reinforced with Shredded Tire–An Experimental Investigation" Construction and Building Materials, 2012 (35) pp. 547-556. 20. Yetimoglu, Temel, and Omer Salbas: "A Study on Shear Strength of Sands Reinforced with Randomly Distributed Discrete Fibers" Geotextiles and Geomembranes, 2003 (21) pp. 103-110. 21. Yoon, Yeo Won, Seung Beom Heo, and Keun Soo Kim: "Geotechnical Performance of Waste Tires for Soil Reinforcement from Chamber Tests" Geotextiles and Geomembranes, 2008 (26) pp. 100-107. 22. Yoon, Yeo Won, Sung Han Cheon, and Dae Seong Kang: "Bearing Capacity and Settlement of Tire-Reinforced Sands." Geotextiles and Geomembranes, 2004 (22) pp. 439-453. 23. Youwai, Sompote, and Dennes T. Bergado "Numerical Analysis of Reinforced Wall using Rubber Tire Chips–Sand Mixtures as Backfill Material" Computers and Geotechnics, 2004 (31) pp. 103-114.
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Editor’s note. This paper may be referred to, in other articles, as: Reza Jamshidi Chenari, Mehran Karimpour Fard, Javad Shafie, and Aref Ghorbanpour: “Tire Shreds and Tire Crumbs Inclusion: Contrast Effects on Bearing Capacity of Sand” Electronic Journal of Geotechnical Engineering, 2017 (22.09), pp 3649-3667. Available at ejge.com.
