Behavior of granular rubber waste tire reinforced soil for application in geosynthetic reinforced soil wall Comportamento de solo reforçado com resíduo granular de borracha de pneu para aplicação em parede

Resumo Large quantities of waste tires are released to the environment in an undesirable way. The potential use of this waste material in geotechnical applications can contribute to reducing the tire disposal problem and to improve strength and deformation characteristics of soils. This paper presents a laboratory study on the effect of granular rubber waste tire on the physical properties of a clayey soil. Compaction tests using standard effort and consolidated-drained triaxial tests were run on soil and mixtures. The results conveyed an improvement in the cohesion and the angle of internal friction the clayey soil-granular rubber mixture, depending on the level of confining stress. These mixtures can be used like backfill material in soil retaining walls replacing the clayey soil due to its better strength and shear behavior and low unit weight. A numerical simulation was conducted for geosynthetic reinforced soil wall using the clayey soil and mixture like backfill material to analyzing the influence in this structure.

Behavior of granular rubber waste tire reinforced soil for application in geosynthetic reinforced soil wall

Introduction
Scrap tires are increasing every year and their disposal is a major environmental problem.Particles of rubber tires are been using in landfill engineering as subgrade reinforcement for construction roads over soft soil and others researches suggest their application in civil engineering because of their low density, high durability, high thermal insulation and low cost compared with other fill materials (Cetin, [1]; SZELIGA, [2], RAMIREZ, [3]).In Brazil is being using a specific granular rubber of scrap tires mixed with asphalt for the cap asphaltic road.This research aimed to understand the viability of this granular rubber as reinforcement material in geotechnical works (layers of landfills, embankments on soft soils and temporary landfills), obtaining a first knowledge of the behavior of reinforced clayey soil with this granular rubber from scrap tires.Finally, use of this alternative material would decrease the demand of natural resources, reducing the environmental impact and adding value to this waste.

Clayey soil
The clayey soil used in this study is a residual tropical soil (Figure 1) with a limit liquid of 53%, a limit plastic of 39% and specific gravity 2.72.It was collected in the Experimental Field II located in the PUC-Rio Campus.The grain size distribution curve is shown in Figure 3.This clayey soil is classified as MH according to SUCS.This soil has a micro-granular texture, constituted by quartz, altered garnet, clay minerals (mainly kaolinite) and iron and aluminum oxides.

Granular rubber
The granular rubber used for reinforced the clayey soil (Figure 2) is by-products of the tire retread process.The specific gravity of this material is 1,12.Its middle diameter is 1,0 mm, varying between 0,2 mm and 2,0 mm.The particle size distribution is shown in Fig- ure 3.This material is composed of 50% by weight of particular cars tire and 50% by weight of trucks tire.In Brazil this granular rubber is mainly used in road construction area.A mixture of asphalt and granular rubber is being used as an efficient technology to achieve Brazilian road requirements.

Mixtures
Clayey soil was mixed with 5%, 10% and 20% of granular rubber by dry weight of soil.Water was added according to the optimum moisture and the maximum dry density obtained from Standard Proctor Test (standard compactive effort) performed on each material (S100, S95/B5, S90/B10 and S80/B20).The abbreviations used to denote the soil and mixtures are shown in Table 1.

Experimental procedure
In order to obtain the optimum rubber content was performed triaxial tests on samples with 5%, 10% and 20% of granular rubber by dry weight of clayey soil (S100).Physical characterization and standard proctor tests were run on clayey soil and mixtures.

Physical characterization tests
Characterization tests were performed to determine the index properties of clayey soil samples, from the Experimental Field II.Plastic Limit.The specific gravity of soil solids, particle size analysis, liquid limit and plastic limit tests were performed using the material finer than size #40 (0,425 mm).An important step to obtain the specific gravity of soil solids is removing the entrapped air in the soil with a vacuum pump.Then was added water to the pycnometers until complete its capacity.Four pycnometers were used to calculate an average value of Gs.The particle size analysis for coarse soil was done according to the standards by sieving, while the distribution of particle size for fine soil was determined by sedimentation process using a hydrometer.

Standard proctor tests
Standard proctor tests were conducted on the clayey soil (S100) and mixtures (S95/B5, S90/B10 and S80/B20) to determinate the optimum moisture (wopt) and the maximum dry density (γdmax) of all materials.These tests were made according to NBR 7182, using the standard compactive effort.The compaction was made in a small cylindrical mold (internal diameter 10 cm and height 12,7 cm).It applied 26 blows in each layer (three layer in total) with a manual rammer, of 2,5 kg of weight, falling 30,5 cm of height.
Carefully was trimmer the top of the compacted specimen until to form a plane surface with the top of the mold.Then was determined the mass of the specimen inside the mold using a balance and subtracting the mold weight.Behavior of granular rubber waste tire reinforced soil for application in geosynthetic reinforced soil wall The material was removed from the mold to obtain a specimen for moisture.Then, with each moisture and dry density was plotted the points of the compaction curve.At least were used two molding water content points in each side of the curve (wet and dry side).

Triaxial tests
Standard triaxial testing procedures were followed.The specimens were saturated using backpressure and percolation of  water through the samples.The final saturation was estimated with Skempton parameter (B = 0,97).Consolidated-drained (CD) tests were conducted on clayey soil and mixtures.According to HEAD [5], the maximum rate of deformation was determined using the minimum time of failure 8,5t100.The rate of deformation used in the shear phase was 0,022 mm/min for all samples.After compact the clayey soil and the mixtures with their optimum moistures and maximum dry densities were  Behavior of granular rubber waste tire reinforced soil for application in geosynthetic reinforced soil wall molded samples with 3,8 cm of diameter and 7,2 cm of height.

Results and analysis
The results of specific gravity (Gs), liquid limit (LL), plastic limit (LP) and a resume of particle size are shown in Table 2.According with SUCS this soil is classified as a silt high plasticity (MH), but in this research it is named clayey soil due to it has more than 50% of clay.The soil particle size distribution curve is shown in Figure 3.
The compaction results showed the maximum dry unit weights and the optimum moistures of mixtures are lower than those for clayey soil (S100).This decrease is mainly due to lower specific gravity  Behavior of granular rubber waste tire reinforced soil for application in geosynthetic reinforced soil wall wall was 4.4 cm for clayey soil backfill.Moreover, S90/B10 backfill induced 2.3 cm of horizontal displacement on top wall.The horizontal displacement along the wall was less when S90/B10 material was used as backfill.The Figure 11 show the deformations generated after the last construction phase.
In addition, when it was used S90/B10 backfill the effective relative stresses were less than the effective relative stresses registered when S100 backfill was used.See Figures 13 and 14.

Conclusions
n Addition of granular rubber enhances the clayey soil behavior improving its shear strength.n For large deformation, the shear strength of mixture is higher than clayey soil and the development of strength has a better behavior than clayey soil.n The influence of the confinement in the mixture behavior is important.Exist a limiting confining pressure beyond which the presence of the granular rubber degrades the strength of the clayey soil.This could be explained because of the excessive confinement that restricts dilatation, in consequence the granular rubbers cannot mobilize tensile stress.Thus, this mixture will have a better performance than clayey soil under low confinement levels (Özkul and Baykal, [6]).n Results of the numerical simulation showed minors horizontal displacements in the precast concrete wall when the mixture S90/B10 was used as backfill.n Low effective stresses were generated when S90/B10 backfill was used in the retaining wall.n This mixture is an adequate material to be used in some geotechnical application as layers of landfills, backfill in retaining walls, small embankments on soft soils, temporary landfills, subgrade reinforcement for construction roads over soft soil.In these projects the presence of low confinement stresses enable the use of this material.n An important contribution to the environmental, low cost of the projects and major quality of this geotechnical structures would be possible with more researches in non-conventional material particularly with waste (Ramirez and Casagrande, [4]).

Table 1 -
Symbols used to denote the soil and mixtures

Table 2 -
Clayey soil physical characterization