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Waste Management 26 (2006) 1033–1044 www.elsevier.com/locate/wasman Construction of a test embankment using a sand–tire shred mixture as fill material Sungmin Yoon a, Monica Prezzi b a,* , Nayyar Zia Siddiki b, Bumjoo Kim c a School of Civil Engineering, 550 Stadium Mall Drive, Purdue University, West Lafayette, IN 47907-1284, USA Division of Materials and Tests, Indiana Department of Transportation, 120 S. Shortridge Rd. in Indianapolis, IN 46219-0389, USA c Dam Safety Research Center, Korean Institute of Water and Environment, Daejon 306-711, Korea Accepted 10 October 2005 Available online 15 December 2005 Abstract Use of tire shreds in construction projects, such as highway embankments, is becoming an accepted way of beneficially recycling scrap tires. However, in the last decade there was a decline in the use of pure tire shreds as fill materials in embankment construction, as they are susceptible to fire hazards due to the development of exothermic reactions. Tire shred–sand mixtures, on the other hand, were found to be effective in inhibiting exothermic reactions. When compared with pure tire shreds, tire shred–sand mixtures are less compressible and have higher shear strength. However, the literature contains limited information on the use of tire shred–soil mixtures as a fill material. The objectives of this paper are to discuss and evaluate the feasibility of using tire shred–sand mixtures as a fill material in embankment construction. A test embankment constructed using a 50/50 mixture, by volume, of tire shreds and sand was instrumented and monitored to: (a) determine total and differential settlements; (b) evaluate the environmental impact of the embankment construction on the groundwater quality due to leaching of fill material; and (c) study the temperature variation inside the embankment. The findings in this research indicate that mixtures of tire shreds and sand are viable materials for embankment construction. 2005 Elsevier Ltd. All rights reserved. 1. Introduction According to Dickson et al. (2001), there are more than 500 million tires (5.5 million metric tons) stockpiled across the United States, and 270 million (3 million metric tons) more are generated each year. About 30% of these tires are disposed of in landfills, and thousands of tires are left in empty yards and even dumped illegally. In the state of Indiana, approximately 5 million waste tires (56,000 metric tons) are generated each year, and there are 40 stockpiles containing millions of tires in different counties (Kaya, 1992). Similar disposal problems also exist worldwide. For example, in England and Wales, there are about 38 stockpiles with nearly 14 million tires (0.16 million metric tons) stored in them * Corresponding author. Tel.: +1 765 494 5030; fax: +1 765 496 1364. E-mail address: [email protected] (M. Prezzi). 0956-053X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2005.10.009 (Hird et al., 2002). Therefore, it is essential to find beneficial ways of recycling and/or reusing tires. In civil engineering applications, tires are often used after they have been shredded to small pieces. Tire shreds have various shapes and sizes, typically varying between 50 and 300 mm (ASTM D 6270-98). The steel belting is sometimes removed during processing of the tire shreds. The market for scrap tires in civil engineering applications has increased steadily in the last decade. Approximately 12 million scrap tires (0.13 million metric tons) were used in civil engineering applications in 1995, and 15 million (0.17 million metric tons), in 1996. These applications include leachate collection systems, landfill cover, artificial reefs, clean fill for road embankments, roadbed support, and similar projects (Liu et al., 2000). There are several advantages of using tire shreds in civil engineering applications. One of the most important characteristics of tire shreds is that they are a lightweight material. When roads are constructed across weak and 1034 S. Yoon et al. / Waste Management 26 (2006) 1033–1044 compressible soils, stability and settlement considerations are critical. Various lightweight materials have been used in the construction of fills in order to reduce the weight of highway structures over these soils. Table 1 shows the unit weight and approximate cost of tires shreds, with the corresponding values of other widely used lightweight materials (Elias et al., 1998; Holtz, 1989; ASTM D 627098). One advantage of using tire shreds either as a lightweight fill material in embankments or as retaining wall backfills is that tire shreds are not biodegradable and thus are more durable. The cost of tire shreds varies from $5 to $50 per m3 (1 m3 is approximately 0.45 ton), depending on the quality of the tire shreds (personal communication with tire shredding companies in the State of Indiana: C.C.E. Inc., Elk Distributing Inc., and Tire Recyclers of America, LLC., 2002). Tire shreds are relatively inexpensive when compared with the cost of other lightweight fill materials. Since tire shreds are lightweight, smaller horizontal stresses are induced behind a retaining wall constructed with a mixture of soil and tire shreds as the backfill material, compared with a retaining wall having only soil as the backfill material. Also, tire shred–sand mixtures have relatively high shear strength. Due to these properties, the width of a retaining wall can be reduced and savings obtained. In addition, tire shreds are free-draining materials, and thus do not contribute to excessive pore pressure generation that can cause stability problems during loading of the backfill material. Based on the results of a full-scale field test, Humphrey and Eaton (1993) indicated that the use of tire shreds could also reduce the depth of frost penetration and be a cost effective solution when an insulation layer is necessary for a given application. However, there are some concerns regarding the possibility of internal combustion of fills constructed with tire shreds exclusively, as observed in Washington and Colorado (Humphrey, 1996), and of groundwater contamination. Tire shreds have been used as a lightweight fill material in many embankments and retaining structures in various US states, including Colorado, Indiana, Maine, Minnesota, Oregon, Vermont, Washington and Wisconsin, and outside the USA (Bosscher et al., 1992; Humphrey, 1996; Humphrey et al., 2000; Dickson et al., 2001; Moo-Young et al., 2001; Khan and Shalaby, 2002; Zornberg et al., 2004). Seven states (North Carolina, Oregon, Minnesota, Washington, Colorado, Indiana, and New York) have provisions for the use of tire shreds (Moo-Young et al., 2001). These studies show that tire shred–soil mixtures have lower Table 1 Unit weight and cost of various lightweight fill materials Lightweight material Unit weight (kN/m3) Approximate cost ($/m3) Shredded tires Geofoam (EPS) Wood fiber/sawdust Expanded shale and clay Fly ash 5.5–6.4 0.2–1 8–10 3–10 10–14 20–30 35–65 12–20 40–55 15–21 compressibility and higher shear strength and thus perform better than only tire shreds. In addition, they also show that the impact of the construction of embankments with tire shreds or tire shred–sand mixtures on the groundwater and air quality is negligible. 2. Research objectives Purdue researchers and Indiana Department of Transportation (INDOT) engineers have cooperated in an active program to study the use of recyclable materials in the construction of transportation facilities. As a result of this effort, a demonstration tire shred–soil embankment was planned and constructed by INDOT in 2001. The test site is located at State Rd. 31, in Lakeville, IN (USA). The demonstration embankment is part of a reconstruction project with the removal of an old bridge. The tire shred– soil embankment was constructed with an approximate 23/77 weight ratio (50/50 volumetric ratio) of tire shreds and soil as fill material. In this paper, the results of laboratory tests performed to characterize the engineering properties of the tire shred–soil mixture and the data obtained from field instrumentation of the test embankment are presented. The instrumentation includes settlement monitoring using settlement plates, vertical and horizontal inclinometer monitoring, temperature monitoring, and groundwater quality analysis. The embankment was monitored for a period of 1 y after opening of the roads to traffic. The successful construction and performance of tire shred–sand embankments, such as the one described in this paper, will promote the use of tire shreds as a fill material, with large benefits to society. 3. Engineering properties of tire shred–soil mixtures 3.1. Unit weight Tire shreds are a lightweight material. The unit weight of different types of compacted tire shreds, as reported in the literature, ranges from 2.4 to 7.0 kN/m3 (Humphrey and Manion, 1992; Ahmed, 1993; Ahmed and Lovell, 1993; Humphrey et al., 1993). These values are approximately 0.1–0.4 times the unit weight of typical soils. The effect of the compaction energy on the unit weight of tire shred–soil mixtures is only important for tire shred–soil mixtures with tire shred contents less than 20% by weight. The importance of the compaction energy decreases as the tire shred content of the mixture increases (Ahmed, 1993). Even though the maximum dry unit weight of tire shred– soil mixtures increases as the tire shred–soil ratio decreases, tire shred–soil mixtures are still considered lightweight when compared with conventional fill materials. 3.2. Hydraulic conductivity The drainage characteristics of fill materials have significant influence on the stability of embankments or S. Yoon et al. / Waste Management 26 (2006) 1033–1044 1035 retaining structures under saturated conditions. Fill materials with low hydraulic conductivity under a saturated condition may induce slope failures due to the generation of excessive pore pressures. Compacted tire shreds and tire shred–sand mixtures have hydraulic conductivity values ranging from 1.8 · 10 3 cm/s (16% shredded tires by weight) to 15.4 cm/s (100% shredded tires); these values are greater than (or equal to) those of typical sand or coarse gravel (Hall, 1990; Humphrey et al., 1992; Ahmed, 1993; Humphrey and Sandford, 1993; Zimmerman, 1997). were conducted on field test embankments with tire shred as fill materials have shown that tire shreds have only a negligible impact on the environment and on the groundwater quality (Minnesota Pollution Control Agency, 1990; Bosscher et al., 1992; Humphrey et al., 2000). However, in severe conditions, leaching of metals can occur due to exposure of the metal reinforcements present in the tire shreds (OÕShaughnessy and Garga, 2000). Under such conditions, zinc is often used as an indicator that leaching has occurred (Collins et al., 1995; Vashisth et al., 1998). 3.3. Shear strength 3.5. Degradation of tire shreds The shear strength of a tire shred–soil mixture is affected mainly by the confining stresses, the tire shred–soil ratio and the density of the mixture. In mixtures of tire shreds and sand, the tire shreds have a reinforcing effect (Edil and Bosscher, 1992; Ahmed, 1993; Foose, 1993; Bernal et al., 1996). Ahmed (1993) conducted triaxial tests on tire shred–soil mixtures (tire shred size = 25 mm) with various mixing ratios. A tire shred–soil mixture ratio of approximately 40:60 by dry weight (65:35 by volume) was reported to produce maximum shear strength values at low to medium confining stresses. Although the mixing ratio that produces the maximum shear strength varies depending on the size of the tire shreds, the mixture ratio mentioned above can be used as a reference in the selection of the tire shred–soil mixture ratio to be used in the construction of embankments. Small size tire chips (12–50 mm) are usually equidimensional and larger size tire shreds (50–305 mm) are long and flat; their shape has an influence in the resulting stress–strain behavior of the mixtures (ASTM D 627098, 1998). Degradation of tire shreds can be due to two types of processes: (1) microbial action and (2) ultra-violet light exposure. Tires are non-reactive materials. The growth of micro-organisms is inhibited due to the presence of zinc oxide, anti-degradants and vulcanization accelerators. Hence, the attack of micro-organisms on tire shreds has been found to be minimal (Crane et al., 1975). Recent work indicates that the effect of micro-organisms on scrap tires is not well understood and further research into the microbial processes involved is required (Wallingford, 2005). Carbon black present in tires blocks the damaging ultra-violet component of sunlight. Surface degradation can be prevented when tires are kept away from sunlight, placed underwater or buried underground (Collins et al., 2002). 3.4. Environmental properties Tires are made of natural and synthetic rubber that contains: (a) organic substances (carbon black, polymers, oil, paraffin, volatile organic compounds); and (b) inorganic substances (pigments, fabrics, bead or belt materials, aluminum, barium, chromium, iron, lead, manganese, sulfur, and zinc) (Moo-Young et al., 2001). Various laboratory leaching tests have been conducted by many researchers on shredded tire samples to evaluate the potential environmental problems that they can cause (Minnesota Pollution Control Agency, 1990; Ealding, 1992; Edil and Bosscher, 1992). It is reported that the pH level can affect the leaching process. Organic components are found to leach more freely under neutral conditions, while metals leach more freely under acidic conditions (Westerberg and Màcsik, 2001). Laboratory leaching tests on shredded tire samples indicate that both the metallic and organic components are well below the US EPAÕs (Environmental Protection Agency) TCLP (toxicity characterization leaching procedure) and EP (extraction procedure) standards and that tire shreds show little or no likelihood of being a material hazardous to the environment. Also, various studies that 3.6. Exothermic reactions in tire shred fills In 1995, exothermic reactions were experienced in three tire shred fill embankments in Washington (Humphrey, 1996). Ignition can occur due to the rise in temperature caused by exothermic reactions. Potential causes of initiation of exothermic reactions are as follows: (1) oxidation of exposed steel wires, (2) oxidation of rubber, and (3) consumption of liquid petroleum products by microbes. Oxidation of exposed steel wires is the primary cause of exothermic reactions (Humphrey, 1996). Mixing of tire shreds with soil impedes the oxidation process due to the reduction in the contact area of the exposed steel wires to oxygen and the formation of a non-combustible insulating layer that increases the critical ignition temperature (Wallingford, 2005). 3.7. Optimum size of tire shreds Tire shred sizes range from 50 to 300 mm. Whole tires or parts of scrap tires are shredded by shredders equipped with knives or blades. Usually, multiple passes through a shredder are required for tire shred sizes less than 300 mm (GeoSyntec Consultants, 1998). It is more expensive to produce and obtain smaller sizes of tire shreds. For this reason, larger-size tire shreds are more economical when the goal is to use them as construction materials, but the engineering properties of the tire shred–soil mixtures must also be considered. 1036 S. Yoon et al. / Waste Management 26 (2006) 1033–1044 3.8. Optimum tire shred–sand mixing ratio Tire shreds have been used either separately or mixed with soil. Mixing of tire shreds with soil reduces both the compressibility and the combustibility of tire shred fills. As mentioned previously, the occurrence of exothermic reactions produced by tire shreds has been a very critical issue in the design and construction of tire shred embankments. The presence of sufficient air in tire shred fills may have played a role in the ignition process that developed in the incidents where combustion was observed (Humphrey, 1996). To prevent self-ignition, isolation of the tire shreds is required. When tire shreds are, on average, floating in a soil matrix, isolation of the tire shreds is practically guaranteed. Lade et al. (1998) proposed a theoretical formulation for the void ratio of mixtures composed of particles of two different sizes. Their findings can be used for mixtures of tire shreds and sand. Fig. 1 shows a schematic diagram of the minimum void ratio of binary packing of particles. In Fig. 1, the minimum void ratio of a material composed entirely of tire shreds or entirely of sand is denoted by emin,ts and emin,s, respectively. When sand particles are added to the voids of an existing volume of tire shreds, the sand particles fill the voids between the tire shreds, and the minimum void ratio of the mixtures (emin,mix) decreases. This will continue until the voids between the tire shreds are completely filled with sand particles. When all voids between the tire shreds are filled with sand particles, any further addition of sand will increase both the overall volume of the mixture and its minimum void ratio, as shown in Fig. 1. From this point on, further addition of sand produces mixtures in which the tire shreds Fig. 1. Schematic diagram illustrating minimum void ratio arrangements of binary packing of particles (after Lade et al., 1998). are on average floating in a sand matrix. This explains why, on a plot of emin,mix vs. sand content, there is a minimum at a certain value of sand content. Ahmed (1993) performed laboratory tests to determine the maximum dry unit weight of mixtures prepared with various tire shred–sand mixing ratios. The mixtures consisted of 25-mm tire shreds and Ottawa sand. Using a vibratory table, the oven-dried sand was densified to obtain the minimum void ratio, and the maximum unit weight of the sand was determined after several trials. The sand was then mixed with tire shreds at different mixing ratios, and the maximum unit weight of each mixture was determined in accordance with ASTM D 4253 (Standard test method for maximum index density and unit weight of soils using a vibratory table). The sand content of the tire shred–soil mixtures (by dry weight) varied from 0% to 100% (i.e., from pure sand to pure tire shreds). As shown in Fig. 2(a), the maximum unit weight of the mixtures of tire shreds and sand increases linearly with increasing percentages of sand (ranging from 4.9 to 18.6 kN/m3) because the specific gravity GS of the sand is larger than that of the tire shreds. The specific gravity of the sand and of the tire shreds is equal to 2.65 and 0.95, respectively. Using the maximum unit weight results of Ahmed (1993), the minimum void ratio of the tire shred–sand mixtures was calculated and plotted as a function of the tire shred content of the mixtures (see Fig. 2(b)). The material consisting only of tire shreds and the material consisting only of sand have minimum void ratios of 0.92 (emin,ts) and 0.39 (emin,s), respectively. As the percentage of sand increases, the minimum void ratio of the mixture decreases up to a point where its value is the smallest, as discussed previously. This happens at a tire shred content of 39% (by weight), for which the minimum void ratio reaches the minimum of 0.1. Based on these results, the minimum sand–tire shred mixing ratio that ensures complete isolation of the tire shreds was determined to be equal to 61/ 39 by weight (36/64 by volume) for a nominal tire shred size of 25 mm. The optimum mixing ratio, which depends on the shape and size of the tire shreds, can be determined experimentally. As the shape and size of tire shreds can vary significantly depending on the shredding method and equipment used, testing of the tire shreds that will be used in the project is required. In order to estimate the optimum mixing ratio for projects in which mixtures of tire shreds and sandy soils will be used as fill material, the following procedure can be followed: (1) determine the size of the tire shreds and the shredding method; (2) determine the minimum void ratio of mixtures of tire shreds and sand prepared with various mixing ratios; (3) select the mixing ratio producing the smallest minimum void ratio, that is, the minimum mixing ratio that ensures floating of the tire shreds in the soil matrix. S. Yoon et al. / Waste Management 26 (2006) 1033–1044 1037 20 Dry Unit Weight (kN/m 3 ) 18 16 14 12 10 8 6 4 2 0 0 20 40 60 80 100 Tire Shreds (% Dry Weight) Fig. 2(a). Maximum unit weight of mixtures of 25-mm tire shreds and sand prepared with different tire shred contents (adapted from Ahmed, 1993). 1.0 0.9 Minimum Void Ratio 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 20 40 60 80 100 Tire Shreds (%) Fig. 2(b). Minimum void ratio of mixtures of 25-mm tire shreds and sand as a function of the tire shred content of the mixtures by weight (adapted from Ahmed, 1993). 4. Fill material used in the test embankment 4.1. Tire shreds Tire shreds meeting the requirements established by the Indiana Administrative Code, 329 IAC 2-9-3 (1996), which are described below, were used in the construction of the test embankment. These tire shreds were substantially free of loose metal fragments and were reasonably clean and free from contaminants, such as oil, grease, etc., that could affect the quality of the groundwater. The attached residual metal pieces extending beyond the cut edges of the tire shreds were not considered loose metal fragments and were kept to a minimum. The nominal size of the tire shreds, which is the size that comprises 50% or more of the throughput in a scrap tire shredding operation (ASTM D 6270), was 38 mm. The tire shred lengths varied between 38 and 76 mm. The steel belts in the tires were partially removed. The total unit weight of the tire shreds delivered at the site before compaction was equal to 4.9 kN/m3. The tire shreds used in this project were produced and supplied by Dillion Tire Recycling Co. located in North Liberty, IN. The selection of the size of the tire shreds was based on technical and economic considerations, since the smaller the size of the tire shreds, the more expensive they become. 4.2. Soil As required by INDOT specifications (INDOT, 1999), a B-borrow fill material was used as the soil in the tire shred– soil mixture. A B-borrow fill material is a uniformly-graded sand with less than 6% of the particles passing the No. 200 sieve. Fig. 3 shows the grain size distribution of the sand used in the embankment construction. It is classified as a poorly-graded sand (SP) according to the Unified Soil 1038 S. Yoon et al. / Waste Management 26 (2006) 1033–1044 100 Percent finer (%) 80 60 40 20 0 10 1 0.1 0.01 Grain size, D (mm) Fig. 3. Grain Size distribution of sand used in the test embankment construction. Classification System (USCS). The total unit weight of this sand before compaction is equal to 17.5 kN/m3. 4.3. Tire shred–sand mixture The tire shred–sand weight ratio of the mixture used in the construction of the demonstration embankment was equal to 23/77 (50/50 by volume). This mixing ratio was selected based on material availability at the time of construction and on the recommendations of Bosscher et al. (1992). Before compaction of the fill material, mixing was done using a front-end wheel loader with a straight-blade edge. Special care was taken to mix thoroughly the tire shreds and the sand in order to produce a reasonably uni- form mixture. Figs. 4(a) and 4(b) show the mixing of the tire shreds and the sand, and the placement of the tire shred–sand mixture, respectively. Construction of the embankment was inspected quite attentively, lift by lift (lifts were only 300-mm thick). Mixing using this equipment was effective in the field; no segregation of the two materials was observed. However, for embankment construction involving large volumes of a tire shred–sand mixture, mixing using two front-end wheel loaders is recommended (Zornberg et al., 2004). Sieve analysis of samples collected at different locations and depths in the field may be performed to evaluate the occurrence of segregation, provided the sampling process generates representative samples. Fig. 4(a). Mixing of tire shreds and sand. S. Yoon et al. / Waste Management 26 (2006) 1033–1044 1039 Fig. 4(b). Placement of tire shred–sand mixture. 5. Construction of the tire shred–sand embankment The tire shred–sand demonstration embankment was constructed during the months of July and August of 2001 (see Figs. 5(a) and 5(b)). Before construction of the embankment, a test pad was constructed to determine the lift thickness and the number of roller passes required to achieve the desired unit weight in the field. Based on the laboratory test results, the optimum moisture content wopt of the mixture was determined to be equal to 11.5%. The moisture content of the mixture in the field was maintained a few percentage points above or below wopt. The degree of compaction of the test pad was estimated from the weight of the material delivered and the volume of the compacted materials. Whenever tire shred–sand mixtures are used, measures must be taken to prevent segregation of the two materials as they are compacted in the field. Based on the evaluation of the test pad, a 300-mm-thick lift compacted with six vibratory roller passes was found to be adequate since almost no deformation or segregation was observed on the compacted layers of the mixture tested. According to Bosscher et al. (1992), the use of a vibratory compactor is not recommended for tire shred–sand mixtures with a high tire shred–sand ratio. The subgrade was prepared, and a layer of aggregate base was placed and compacted above it. The aggregate base was 150-mm thick. A layer of geotextile was laid on top of the compacted aggregate base as shown in Fig. 5(a). The geotextile was placed transversely with an overlap between rolls of 900 mm. The transverse splices of the geotextile were pinned with hog ring clips. The geotextile layers may provide an additional reinforcing effect on the sides of the slope and help prevent slope failure at very shallow depths. However, since the geotextile overlap between rolls was minimal ( 0.9 m), this reinforcing effect was not considered in the design of the embankment slopes. Fig. 5(a). Schematic cross-section of the test embankment constructed with a mixture of tire shreds and sand. 1040 S. Yoon et al. / Waste Management 26 (2006) 1033–1044 Fig. 5(b). Location of the settlement plates installed in the embankment (Northeast, Center and Southwest sections). A 300-mm-thick lift of pre-mixed tire shreds and sand was then placed on top of the filter fabric. Each layer of the tire shred–sand mixture was uniformly placed across the full width of the roadway cross-section. Compaction of the tire shred–sand mixture was achieved using a D6 10-ton bulldozer with a vibrating steel drum roller. A rubber tire roller was used to spread out the fill material and to provide additional compaction. Seven layers of the premixed material (tire shreds and sand) were constructed. The total unit weight of the tire shred–sand mixture achieved by compaction was monitored by computing the total weight of the mixture (from a record of the weight of the tire shreds and the sand used for construction of the embankment) and the embankment dimensions. The total unit weight of the tire shred–soil mixture was equal to 11.5 kN/m3, and the volume of the test embankment after compaction of the tire shred–sand mixture was equal to 813 m3. As shown in Fig. 5(a), the test embankment was covered with a 0.9-m-thick clay encasement material. The encasement material was placed and compacted at the same time as the tire shred–soil mixture was placed. Seeding of the encasement material was then completed. Based on large-scale direct shear test results, Tatlisoz et al. (1998) indicated that the friction angle of tire shreds alone or of mixtures of tire shreds and soil varies from 30 to 54. The slope of the demonstration embankment was not deemed to be a critical problem due to the large friction angle estimated for the tire shred–sand mixture used. The demonstration embankment was constructed with a slope of 4:1 (H:V). Fig. 6 shows in detail how the geotextile layers were placed in order to form the slope. 1.2 m 0.3 m 1.2 m 1.2 m 0.9 m 1.2 m 0.9 m 1.2 m 0.9 m 1.2 m 0.9 m 0.9 m 0.9 m 0.3 m 0.3 m 0.3 m 0.3 m 0.3 m 0.3 m Fig. 6. Details of the installation of the geotextile layers on the side of the embankment slope. S. Yoon et al. / Waste Management 26 (2006) 1033–1044 6. Field instrumentation Embankment Width (m) 6.1. Settlement plates Nine settlement plates were installed at three different depths and in three different sections (Northeast, Center, and Southwest sections) of the test embankment, as shown in Figs. 5(a) and 5(b). After the road was opened to traffic, field monitoring of the settlement plates was conducted for 1 y. The embankment is part of US 31, which is a four-lane highway. The traffic volume is estimated to be around 22,400 vehicles per day, with 16% of the total volume of vehicles being trucks. The traffic volume calculated in equivalent single-axle load (ESAL) is 6,141,497 (assuming a service life of 20 y and a slab thickness of 225 mm) (AASHTO, 1993). The tire shred–sand fill settled as a result of the embankment self-weight and the traffic loads. The data obtained from monitoring of the upper settlement plates, which were installed at a depth of 1.2 m from the bottom of the embankment, are shown in Fig. 7(a). As shown in the graph, the settlement of the tire shred–sand test embankment was very small. The maximum settlement of the test embankment after the road had been opened for traffic for more than 300 days was less than 12 mm. The relative settlement with respect to the embankment width is equal to 0.07%. These results show that the settlement was more or less the same for the three test embankment sections (Northeast, Center and Southwest) and that it stabilized after about 200 days of continued traffic. The differential settlement from surveying observations 1 y after the end of construction was equal to 4 mm (Fig. 7(a)). This value matches the data obtained from the horizontal inclinometer measurements fairly well (Fig. 7(b)). The test embankment was visually inspected periodically for over a year. During this period, the test embankment performed well and did not show any visible signs of settlement, lateral movement or erosion. No cracks on the road or ground surface movements were observed. The embankment slopes retained their original shape. The instrumenta- Number of Traffic Days 100 200 300 400 Vertical Settlement (mm) 20 0 1041 10 0 -10 -20 0 2 4 6 8 10 77 days 191 days 247 days 12 Fig. 7(b). Horizontal inclinometer measurements. tion results confirmed that the test embankment performed well. 6.2. Vertical and horizontal inclinometer The inclinometer monitoring system consists of an inclinometer casing, a portable digital inclinometer probe and control cable, and an inclinometer readout unit. As shown in Fig. 5(a), the vertical inclinometer casing was installed on the east side of the embankment. Using the vertical inclinometer measurements, the lateral movement of the test embankment was monitored after the road was opened to traffic. The maximum relative lateral movement of the embankment with reference to the bottom of the embankment was equal to about 2 mm after 42 days of traffic. No evidence of slope instability or sliding was observed at any moment. The horizontal inclinometer was installed in order to monitor differential settlement in the test embankment. Monitoring of the horizontal inclinometer was conducted after 77, 191, and 247 days of road operation. The horizontal inclinometer data is shown in Fig. 7(b). The maximum vertical settlement observed was equal to approximately 11 mm. This is in agreement with the vertical settlement data obtained from monitoring of the settlement plates. These results also indicate that the differential settlement in the test embankment (about 4 mm) was minimal. This value matches the data obtained from the horizontal inclinometer measurements fairly well (Fig. 7(b)). Vertical Settlement (mm) 0 Northeast Side 2 4 6.3. Heat generation Center Southwest Side 6 8 10 12 14 Fig. 7(a). Vertical settlement observed as a function of the number of traffic days. Heat generation is a critical issue in the design of embankments made of tire shreds, since the embankments constructed in Washington and Colorado experienced selfheating (Humphrey, 1996). In the test embankment described in this paper, however, the fill material was composed of 23% by weight (50% by volume) of tire shreds and 77% by weight (50% by volume) of a sandy soil, such that complete encasing of the tire shreds with sand was guaranteed. There are concerns that uniformity of the mixtures cannot be achieved in the field and there might be some 1042 S. Yoon et al. / Waste Management 26 (2006) 1033–1044 localized heat generation within the embankment, which could potentially cause fires. Fires have only been observed for embankments constructed entirely of tire shreds (Humphrey, 1996). To our knowledge, there are no reports of fire occurrences in embankments constructed with tire shred– soil mixtures. The temperature inside the test embankment was monitored at the center of one specific section of the embankment for 1 y using a thermometer equipped with an ANSI (American National Standards Institution) type K thermocouple probe (see Fig. 5(a)). Fig. 7(c) shows the temperature variations observed in the test embankment. As shown in Fig. 7(c), only in the four months around winter time, higher temperatures inside than outside the embankment (outside air temperature) were observed. The higher embankment temperature was due to the insulation effect of the fill material. The difference between the temperature inside the embankment and the outside temperature was, however, small. The highest temperature observed within the embankment during the monitoring period was 20 C. This clearly shows that there is no evidence of exothermic reactions. This data confirms the idea that if tire shreds are combined with soil, internal exothermic reactions do not seem to be a concern. To evaluate spe- Fig. 7(c). Variation of the temperature with time. cific exothermic reactions that can cause heating within embankments, more laboratory studies should be conducted in the future. 6.4. Groundwater monitoring well To evaluate the impact of the tire shred–sand embankment construction on the groundwater quality, a monitoring well was installed to a depth of 13.7 m on the southwest side of the test embankment. The direction of the groundwater flow was established from borings performed at the embankment site before its construction. The monitoring well was installed in the direction of groundwater flow in the downgradient to capture the effects of any leaching. During the well installation, the groundwater level was 11.3 m below the ground surface. The groundwater quality was monitored after the road was opened for traffic. The groundwater samples were analyzed for metals using an inductively-coupled plasma spectrophotometer-atomic emission spectrometry (ICP-AES) in accordance with standard methods (American Public Health Association, 1995). Table 2a shows the results of the water quality analyses performed on several samples taken from the monitoring well. Table 2b presents the 2001 average rainfall data for Lakeville, IN (National Climatic Data Center, 2004). The groundwater samples were tested for some selected metals based on the elemental mineral analysis of tire shreds. The results of the groundwater analyses were compared with the secondary drinking water standard (EPA, 2002) and the maximum contaminant level standard for drinking water of the Indiana Department of Environmental Management (IDEM) (2000). The public water supply quality was evaluated at Lakeville, IN, by the Indiana Department of Environmental Management (IDEM) before the demonstration embankment was constructed. The analysis was performed on May 11, 2000, and its results can be taken as reference. The results of the groundwater quality analyses performed on September 5, October 17, and November 21, 2001 show that all metal levels, except for manganese, are well below the secondary and maximum contaminant standard levels Table 2a Groundwater quality analyses Contaminant Unit Maximum contaminant level for drinking water (IDEM) 5/11/2000 (IDEM) September 5, 2001 October 17, 2001 November 21, 2001 Arsenic Barium Cadmium Chromium Selenium mg/L mg/L mg/L mg/L mg/L 0.05 2 0.005 0.1 0.05 0.0057 0.0565 Limit of detection ion 0.0012 Limit of detection ion 0.0166 0.0943 0.0008 0.0363 Limit of detection ion 0.0148 0.0962 0.0011 0.0551 0.0175 0.0186 0.1132 0.0003 0.0497 0.023 0 9/05/2001 10/17/2001 11/21/2001 0.0478 0.0342 0.0696 Limit of detection ion 0.1891 0.0928 0.0826 0.0175 0.1171 0.0734 0.1649 0.0230 Secondary drinking water Aluminum Iron Manganese Zinc mg/L mg/L mg/L mg/L 0.05–0.2 0.3 0.05 5 – – – – S. Yoon et al. / Waste Management 26 (2006) 1033–1044 1043 Table 2b Average rainfall (in mm) in Lakeville, IN (USA), in 2001 (National Climatic Data Center, US) Jan. Feb. Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov. Dec. Total 80 84 77 93 87 146 132 67 154 79 110 74 1184 for drinking water (IDEM). Manganese, however, does not represent health concerns (EPA, 2002). The exposed steel belts, which contain 2–3% of manganese by weight, are thought to be the source of the manganese in the groundwater samples of other tire shred–soil embankments (Humphrey et al., 2000). Based on laboratory leaching tests, Edil and Bosscher (1992) noticed that the concentration of manganese increased with continued leaching. To evaluate the effect of tire shreds on the groundwater quality more accurately, organic materials should also be analyzed in future research. 7. Conclusions A tire shred–sand test embankment was constructed at State Rd. 31, in Lakeville, IN (USA). The fill material was composed of tire shreds and a sandy soil mixed in equal proportions by volume. The height, length and width of the test embankment were about 2.1, 20 and 17.7 m, respectively. Based on the investigation and monitoring of the tire shred–sand test embankment, the following observations and conclusions are proposed: 1. Settlement of the test embankment was monitored for 1 y after opening of the road for traffic using nine settlement plates located at three different sections of the embankment. After 200 days of road traffic, the settlement stabilized at small values. The maximum settlement observed was approximately 12 mm. 2. Vertical and horizontal inclinometer monitoring was conducted during the 1-y period following the opening of the road for traffic in order to check the lateral movement and the differential settlement of the test embankment. The maximum relative lateral movement with reference to the bottom of the embankment was only about 2 mm. No evidence of significant differential settlement was observed. 3. Samples of groundwater were analyzed periodically for metals. Except for manganese, which does not pose a health concern according to the US EPA (2002), all metal levels were well below the standard limits prescribed for secondary drinking water (EPA, 2002). They were also below the maximum contaminant level for drinking water according to the guidelines of the Indiana Department of Environmental Management (IDEM, 2000). 4. Temperature measurements were made during 1 y in order to check for the possible development of exothermic reactions that might lead to the initiation of fires in the embankment. No evidence of internal heat generation was detected. 5. Floating of the tire shreds in a soil matrix is desired to prevent self-ignition. The minimum mixing ratio that produces such an arrangement can be determined by laboratory tests. Other mixing ratios can be explored in order to increase the strength of the mixtures. 6. Evidence of slope stability problems, cracking of the road pavement or erosion was not observed. Based on the above findings and observations, it can be concluded that the use of mixtures of tire shreds and soil in embankment fill construction is very promising and should be promoted. Performance of the test embankment was quite satisfactory. Advantages of this material include the fact that it is lightweight, relatively cheap, easy to compact, free-draining and relatively incompressible. Additionally, this use is beneficial to the environment in that a waste material is recycled. The positive impact of the construction of the test embankment on the local community paves the way for continued use of tire shreds in civil engineering applications. 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