Management of solid waste is a major environmental concern worldwide. Annually, 1.5 billion of tires around the world get out of utilizing cycle. For example, 46 million tires are disposed of in the U.K. These wastes cause a major health hazards for both human beings and animals.[1] Therefore, disposal of the used tires in landfills has been banned. Thus, recycling of these waste tires needs to be considered. There is a potential of using scrap tires in many geotechnical applications such as building foundations, light-rail construction, road construction, slope stabilization, backfill for bridge abutment and retaining walls.[2, 3, 4, 5, 6, 7] Such admixing of scrap tire rubber (STR) has received great acceptance and application in the last two decades in several geostructures, as this approach offers economic benefits compared with traditional techniques. Some properties of such scrap tire rubber are high compressibility, lightweight, high thermal insulation, good long-term durability compared to other geomaterials.[3, 8, 9, 10]
Some studies found that adding rubber materials resulted in improving the shear strength of soil.[11, 12, 13] However, many other studies such as Kawata et al.,[14] Neaz Sheikh et al.[15] and Youwai and Bergado[16] have reported significant reduction in the strength of soil due to added rubber materials. Tafreshi et al.[8] found that by using tire shreds-sand mixtures over the pipe resulted in significant decrease in the pressure distributed over the pipe, pipe deflection and the soil surface settlement. Furthermore, it was found that shredded rubber mixed with well graded sand has a better performance on the pipe responses compared with chipped rubber. Mohamad et al.[3] investigated the shear strength parameters of sand mixed with different contents of tire chips (10–50% by weight). They found that 10% of the used tire chips can improve the internal friction angle and the shear strength of sand. Similar trend was observed by Saberian et al.[9] who found that by adding 20% shredded tire chips to sandy peat resulted in increasing the angle of internal friction and cohesion to 39.8° and 94.8 kPa respectively compared with 17.8° and 11.2 kPa for the untreated samples. Moreover, specimen with 10% tire chips provided the maximum unconfined compression strength. This trend is agreed with the findings of Cetin et al.,[17] Akbulut et al.[18] and Gotteland et al.[18] Moreover, most of the geo-material exhibited directional dependence of the mechanical properties.[20, 21, 22] Behavior of the mixture is significantly affected by rubber content and rubber-sand particle size
(a) Grain size distribution curves of the used sand and granulated tire rubber, (b) Scrap tire rubber and (c) Sand-tire rubber mixture.
ratio in a manner that increase in the former and decrease in the later, resulting in softer behavior, lowers stiffness and lower strength.[6]
Damping ratio of the sand-rubber mixtures increases and the shear modulus systematically decreases with an increase in the granulated rubber content.[10] The large strain stiffness of the sand-rubber mixtures decreases as the rubber content increases.[4] Conversely, the critical-state friction angles increases from 28.9° for untreated sand to 32.0° for sand that contained 20% of rubber and tested at similar states.[4]
It is apparent that soil-scrap tire can be used in many geostructures as alternative backfill material. Such soft rubber particles significantly change the mechanical properties of soil. However, very little attention was given to its effect on the deformation of sand. Therefore, the present study was conducted with the aim of investigating the deformation of rubber sand mixtures, and how long the granulated tire rubber could reduce the lateral strain.
The soil samples used in the present study is classified as poorly graded sand according to the Unified Soil Classification System. The specific gravity of solids is 2.61 Gs value, which is low here due to the presence of gypsum CaSO4 • 2H2O in the composition of the used sand.
The granulated rubber used in this study is the by-product of the shredding process of used tires. Rubber particles were sieved to obtain a uniform size. The rubber material was angular and irregularly shaped, and has specific gravity (determined in accordance with ASTM D 854), maximum and mean particle sizes of such rubber of 1.1, 4.75 mm and 0.43 mm respectively.
Tire rubber contents by mass,
The stress-strain characteristics of sand-tire rubber mixtures are illustrated in Figure 2. The stress ratio can be defined as the ratio of the major principal stress to the minor one. The highest level of stress ratio obtained from stress ratio-major principal strain curve is defined as the maximum stress ratio. Radial displacements were obtained by using radial chain device that was supplemented by a number of sensors and mounted around the sample.
Relationship between (a) stress ratio and major principal strain and (b) volumetric variation and major principal strain of specimens based on different contents of tire rubber.
Figure 3 shows the variation of the maximum stress ratio
Effect of rubber content on maximum stress ratio of mixtures.
It is well known that the shear strength of clean sand depends totally on the friction between particles and, thus, the presence of tire rubber material as inter-particle layers affects friction and thereby stability. The increase in stress ratio associated with
is likely to be the optimum tire rubber content. However, the decrease in the stress ratio that occurred in sand when
This variation in the maximum stress ratio
The variation in the maximum and minimum void ratio of sand-rubber mixtures, void ratio of mixtures (
Figures 5(a–f) show the accumulated principal strains with the stress ratio. It was found that maximum stress ratio
Regarding the effect of tire rubber on the induced lateral deformation, it was found that tire rubber material TRM has significant effects on such lateral strains,
Relationships between principal strains and stress ratio for sand mixed with different contents of tire rubber
The main objective of this study was to evaluate the effects of scrap tire rubber on the geomechanical characteristics of sand. In order to achieve such an object, a series of drained triaxial tests were conducted on sand mixed with 0, 10, 20, 30, 40 and 50% tire rubber. The results obtained in this study can be summarized as follows:
Minimum
The behavior of mixtures can be divided into three zones based on the content of the tire rubber: i. Sand skeleton zone (for
Results showed that maximum stress ratio increased by 13.6 and 18.0% when 10 and 20% of tire rubber was added to sand respectively. However, stress ratio then decreased by 12.4, 21.7 and 36.0% for
Adding tire rubber to sand resulted in an increase in the major principal strain, indicating that the behavior changed to being more ductile. However, minor and intermediate principal strains of sand-tire rubber mixtures decreased significantly compared with sand. This indicates that sand-rubber mixtures can be used effectively behind the retaining wall to eliminate the lateral earth pressure.