MECHANICAL BEHAVIOR OF VOLCANIC ASH SOILS MIXED WITH SHREDDED TIRES

Thanks to the efforts of different public and private entities, recent years have seen a growing interest in protecting the environment and in the use of non-renewable natural resources such as tires. One of the most widely accepted ways of using non-perishable materials has been their application in the construction of civil works. This paper focuses on the assessment of test tubes made up with mixtures of fi ne-grained Volcanic Ash Soil and Shredded Tires (VAS-TDA) to be applied as an alternative material for low cost lightweight fi lls, that allow the use of the greatest quantity possible of tires. The physical and mechanical properties of specimens made with mixtures of soil and shredded tires were examined, varying the location of the site of soil extraction and tire size (Gravel SizeSand and Gravel Size). It was determined that test tubes made up of 40% medium-sized shredded tires and 60% soil, reached low dry density values, and that increasing the content of shredded tires leads to greater plasticity and less compressibility. Similarly, resistance diminishes but only up to Californian Bearing Ratio (CBR) values appropriate for use in the nucleus and foundation of an embankment. For test tubes with a maximum content of 15% of type 2 shredded tires, there appeared to be little reduction in the resistance to the mixture, as long as a compaction energy of 2700 kN-m/m3 is applied.


INTRODUCTION
The problem of environmental pollution generated by tires is known worldwide, statistical data are found in the literature in which it is evident that these deposits represent billions of tons per year projected to 2025 [1][2]. They are often incorrectly stored and disposed of, these stocks represent a threat of uncontrolled fi res and other environmental risks [3]. Therefore, efforts are currently focused on investigating the reuse of these materials due to their non-biodegradable characteristic. The dumping of whole tires in landfi lls has been banned since 2003 as has their incineration without energy recovery. In January 2006, dumping shredded tires in landfi lls was also banned [4]. This has meant that the market for shredded tires for applications in civil engineering has increased slightly over the last decade. Some data obtained indicate that approximately 12 million shredded tires (0.13 million metric tons) in 1995 and 15 million (0.17 million metric tons) in 1996, were used for civil works that include slurries in collection systems, landfi ll covers, infi lls for road embankments, roadbeds and similar works [5]. One of the most commonly used recycling options in countries such as the United States and Canada is the use of shredded tires alone or mixed with some type of soil, as infi lls for embankments and in retaining walls, known as Tire-Derived Aggregate or TDA. This method has been used satisfactorily for embankments in North America since the beginning of the 1990s [6]. In Canada, it was fi rst used in 2000 to construct 300 longitudinal meters of embankment on soft ground [7]. Since then, it has been considered a low-cost solution, which is frequently applied in cases where infi ll weight has to be reduced in order to guarantee stability in the construction. One of the advantages of this material is that its unitary weights are 5.5 and 6.4 KN/m 3 [8], which is useful in the construction of civil works on roads built on low load-bearing capacity compressible soils where the stability and settlements are the more critical aspects. it is also non-biodegradable, of long-duration and resistant to biological agents. The cost of shredded tires in countries such as the US, varies between 5 to 50 dollars per m 3 depending on the quality; in Colombia, it varies between 195 and 231 dollars depending on its granulometry. A number of studies carried out around the world, have revealed the behavior of shredded tires. Hataf reported the elaboration of mixtures of sand and shredded tires of different sizes and in different volume percentages [9], concluding that the greater the percentage of shredded tires, the greater the load-bearing capacity(concluding that the load-bearing capacity is proportional to the percentage of shredded tires). Edinçliler and his group, on the other hand, compared the behavior of different sizes of tire shreds or pieces after recycling and mixing them with sand, confi rming that adding TDA to sand increases its cutting resistance, recording friction angle values of above 65° [10]. Cano and collaborators [11] conducted in-fi eld assessments of soil with shredded tires, concluding that these were more dense, less deformable, and more easily compactable. Li and collaborators measured the effect of the tire load-bearing zones in a smallscale sand embankment. In this project, we defi ne the reduction of stress transmission to lower layers due to the confi nement generated by reinforcement and a 50% reduction of the settlement rate when compared to an embankment without reinforcement [12]. Investigations confi rm the high compressibility of the remains of tires, a characteristic that increases the settlement rate and may lead to faults in the embankments [13]. Hidalgo and his group mixed shredded tires with sub-base granular material fi nding a reduction of the load-bearing capacity between the mixture and the subbase layer. These studies confi rm the importance of assessing this problem especially in loam-texture volcanic ash soils [14]. A number of authors have been interested in the infl uence of tire components in soils, examining whether there is a signifi cant infl uence on subsoil water and its structure due to the release of zinc, hydrocarbons, and aniline due to TDA. The results showed that the amount of these elements found in the soil did not exceed the environmental safety parameters [15]. The studies also referred to the possibility of combustion in exclusive shredded tire landfi lls not covered by a layer thick enough to protect the fi ll from ultraviolet rays and the incorporation of particles that could make the material ignite. However, as shown by Yoon, this problem can be solved by adding a percentage of soil or sand to the material. In addition to this, the sand mixture proved to be less compressible and more cutting resistant [16]. This study assesses the mechanical behavior of the TDA-volcanic ash soil mixture for application as a safe, low cost, lightweight fi lls, counteracting the results, compaction, and load-bearing capacity contained in the American Standard Test Methods (ASTM) guidelines for the use of shredded tires in civil engineering works.

METHODOLOGY
Laboratory assays were carried out to determine the physical and mechanical properties of the soil with which the shredded tires was going to be mixed, and to classify the tires used into two categories: shredded tires with diameters of between 0.075 mm and 19 mm, and shredded tires with diameters of 25.4 mm. The methodology was divided into the following phases: Phase 1. Physical and mechanical characterization of the materials by applying the following tests: dry and wet sieve analysis (ASTM-D 421-85(2007) [17], ASTM D 2217-85 (1998) [18]), liquid limit and plastic limit tests and Plasticity Index (ASTM D4318-17) [19], specifi c gravity (ASTM D 854-10) [20], Scanning electron microscopy SEM-EDX,Standard Test Methods for Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter(ASTM D5084-16a) [21], constant head permeability for tires (ASTM D 2434-68) [22], resistance to degradation of coarse aggregate (ASTM C 535-09) [23], and soundness of aggregate by use of sodium sulfate or magnesium sulfate (ASTM C 88-05) [24]. Phase 2. Evaluation of the behavior of VAS/TDA mixtures involving the production of mixtures with different percentages of volcanic ash soil (VAS) and shredded tires TDA. Two types of treatment were designed for this procedure. Treatment 1: VAS1/TDA1, in which TDA1 corresponds to a PG texturematerial in proportions of 10%, 20%, 30%, and 40%. Treatment 2: VAS2/TDA2, in which TDA2 corresponds to a PG material in proportions of 5%, 10%, and 15%.

RESULTS
Characterization of materials: Volcanic ash soil deposits in Colombia cover around 11.6% of the territory [25], and are found in the superfi cial layer of the stratigraphic profi le with thicknesses of between 5.0 and 25.0 m. Sensitivity analyses with these materials have shown that they do not present this problem. Table 1 shows the physical analyses carried out with the volcanic ash soils used in this study. The shredded tires used in this study, were designated as Shredded Type 1 tire and Shredded Type 2 tire in sizes equal to 25.4 mm based on tires from heavy machinery, Table 1 and Figure 2 present the properties of the materials Scanning electron microscopy SEM EDX and atomic force microscopy AFM were conducted on air-dried soil samples (Figure 1), identifying the chemical composition of the particles of VAS1 soil. The chemical concentration of the soil particles in air-dried samples was identifi ed. The chemicals found in the highest concentrations were the following: carbon 16%, aluminum 13%, silica 12%, and iron 6%.

MATERIAL MIXTURE TESTS
By increasing the percentages of shredded tires in a mixture in treatment 1, the LL and PI values(either) diminish or are kept constant. For 40% shredded tires, a liquid limit of 28.54% was obtained, 24.61% (with standard deviation 0.48%) plastic limit, and a Plasticity Index of 3.93%. These values describe the same soil type but with low plasticity, refl ecting a reduction of the compressibility, because the material is considered appropriate and better for landfi lls. This behavior favors the material's use as subgrade for a road, which requires a subsoil of PI < 4. Shredded tires constitute a light-weight material with a density of 2.7 a 7.0 kN/m 3 [26] according to the other investigations. The maximum dry density for the TDA used in this study corresponds to a value of 7.16 kN/m 3 , the maximum dry density for Soil 1 is 11.67 kN/m 3 and for Soil 2, of 12.26 kN/m 3 . When the soil was mixed with TDA in both treatments, the dry density reduced as the percentage of shredded tire increased. With 40% TDA/ VAS, the maximum dry density reduces by 13% with respect to the density value for the soil in its initial state. The soils used in this project are sandy clay, which are characterized by their low permeability. The average permeability coeffi cient (K) obtained was 7.02 x 10-6cm/s with standard deviation 1.76 x 10-6 cm/s, which is a value that characterizes loam clay materials [27]. Additionally, fi ne-grained soils retain more moisture than coarse-grained soils and hence, the importance of guaranteeing adequate drainage in the construction of landfi lls with excavation materials. In contrast, shredded tires presented K values in the interval of 1.6 x 10-2 a 7.6 x 10-2 cm/s. By adding TDA to the soil a smaller contact area between particles is created where less water is retained and the infi ltration velocity increases, improving the soil's properties of hydraulic conductivity. The results showed that the larger the percentage of TDA, the greater the permeability coeffi cient. As shown in Figure 3, a permeability coeffi cient of 4.90 x 10-3 cm/s (with standard deviation 6.56 x 10-4 cm/s) was obtained for the mixture containing 40% TDA, this treatment has sig-

Compaction and resistance test
To test the mechanical properties,in this test a standard deviation between the values of 0,08 kN/m 3 -1,29 kN/m 3 for test type 1, and standard deviation between the values of 0,03 kN/m 3 -0,45 kN/m 3 for test type 2,the soil's    These values are appropriate for these materials to be used in the construction of all parts of embankments (foundation, nucleus and crown), whereas for 40% TDA1 and 60% VAS, the CBR value was 4% (w: 22% ), which is considered adequate for the construction of embank-ments although only for the foundation layers and nucleus, according to specifi cations of construction [28]. The behavior of the resistance of the mixtures of Soil type 2/ Tire type 2, was similar, although soil resistance in this case was below a CBR of 8.5% (w: 29%), and for the 15% TDA, 85% VAS mixture, a CBR of 5.9% (w:29%) was found. Figure 6 shows the variations in the resistance of the soil-tire mixtures.It is observed that the addition of the crushed tire decreases the optimum moisture content value. The VAS1 soil has a consolidation coeffi cient (Cv) of 65 x 10-4 (cm/s 2 ), which presents a relative increase when TDA is added to the mixture as shown in Figure 7.
With 40% TDA, a value of 66.8 x 10-4 (cm/s 2 ) is reached indicating that the consolidation time in mixtures with TDA is reduced when compared to pure soil. This is considered favorable for construction given that when consolidation requires unacceptable timeframes, methods are required that can accelerate the process [29]. The compression coeffi cient was also studied indicating that soil compressibility (Cc), which, for the material without TDA, is of 0.49, which corresponds to clay loam, and it reduces relatively when adding different percentages of shredded tires as shown in Figure 8, for 40% TDA, whereby a Cc of 0.44 was obtained, indicating that the settlement rate of the embankments built with this material, would be of lesser magnitude than those of embankments built using VAS from excavations. This is favorable for constructions given that, as the settlement rate is decreased, stability is ensured. The behavior observed in the consolidation process is similar to that of coarse material, whereby the material suffers an initial deformation for a short time followed by a relatively stable secondary consolidation.

CONCLUSIONS
It is possible and favorable to use mixtures of shredded tires and soil derived from loam texture volcanic ash MH -as long as these materials are not sensitive and do not collapse-to build embankments or landfi lls following the specifi cations of global norms related to the minimum CBR value of the materials and of norm ASTM D6270-98 (2004), for the use of tires. The behavior studied of the different percentages of mixtures presents benefi ts insofar as greater permeability and speed of consolidation, decreased rate of settlement, lower unitary weight, and a great contribution to the protection of the environment when compared to the use of pure VAS. According to the results obtained, the most suitable ratio appears to be 40% TDA and 60%VAS. The load-bearing capacity CBR of the loam soil reduces as TDA is added, limiting the percentage of tire that can be added to the mixture of these materials. However, the greater the size of the shredded tires, the lower the resistance of the mixture, according to this, tire sizes smaller than 1cm are recommended.