Charaterization of Sand-Rubber Mixture and Numerical Analysis for Vibration Isolation

Abstract

Scrap tyres provide numerous advantages from the viewpoint of civil engineering practices. Scrap tyres are light weight, have high vibration absorption, high elastic compressibility, high hydraulic conductivity, and temperature isolation potential. Scrap tyres have a thermal resistivity that is about seven times higher than soil; they produce low earth pressure and absorb vibrations. Many new techniques have emerged with time to utilize these advantageous characteristics for practical purposes in civil engineering. Though current reuse and recovery of scrap tyres has reduced the amount of landfills, but still there is a need for developing additional practices for the reuse of scrap tyres. Moreover, most of present practices do not use its vibration absorption capacity efficiently. To use the scrap tyres as individual material or mixed with soil in civil engineering applications, the systematic understanding of static and dynamic properties of sand-rubber mixtures (SRM) are of prime importance. In the present study an attempt has been made to characterize the SRM to use them as low-cost isolation material for low-to-medium rise buildings. Proposal of this isolation system using SRM is addressed in this study in four parts; in the first part, the estimation of shear strength and volumetric characteristics of the SRM were carried out. A total of seven different rubber sizes (six sizes of granulated rubber; 2 - 1 mm; 4.75 - 2 mm; 5.6 - 4.75 mm; 8 - 5.6 mm; 8 - 9.5 mm; 12.5 - 9.5 mm and one size of tyre chips; 20 - 12.5 mm) were considered for characterizing the SRM, and the rubber size which has higher shear strength characteristics is identified as optimum size for further studies. Second part deals with the effect of reinforcement on SRM with higher rubber content (50% and 75% rubber by volume). In the third part, dynamic properties of selected SRM combination with and without reinforcement were generated from experimental studies. In the last part, the numerical analysis was carried out using finite element program Strand7 to find out optimum dimension of proposed isolation scheme and reduction of spectral accelerations. In addition, the laboratory model tests were also carried out on square footing supported on unreinforced and reinforced SRM. The relative performances of reinforcement on settlement characteristics of SRM for 50% and 75% SRM have been compared with unreinforced SRM. Engineering behaviour of SRM has been studied by considering different rubber sizes and compositions by carrying out large scale direct shear test and Unconsolidated Undrained (UU) triaxial test. The shear strength characteristics such as peak shear stress, cohesion, friction angle, secant/elastic modulus, volumetric strain, failure and ultimate strength, ductility/brittleness index, and energy absorption capacity of sand and SRM were determined. The optimum percentage rubber content based on maximum shear strength and energy absorption capacity has been arrived. The granulated rubber size (12.5 - 9.5 mm) and percentage ratio, 30% by volume is found to be optimum size and content, which gives the maximum energy absorption capacity and lower brittleness index values compared to other rubber sizes. This chapter also describes the applicability of concept of Response Surface Methodology (RSM) to identify an approximate response surface model from experimental investigations on the engineering properties of sand and SRM. The experimental data were quantitatively analyzed by multiple regression models by correlating response variables with input variables in this study. To consume more tyres in SRM, rubber mix of 50 % and 75 % mixes are studied and these SRM results in lower shear strength and higher volume change when compared to 30 % SRM. To improve shear strength and reduce compressibility, geosynthetic reinforcement study has been carried out for 50% and 75% rubber by volume. Here geotextile, geogrid and geonets were used as reinforcement and number of layers and spacing between layers were varied. Finally type of reinforcement, number of layers and optimum spacing are arrived for the optimum rubber size of 12.5 - 9.5 mm for reinforced SRM. This study found that 4 layers with equal spacing of geotextile for 50 % SRM and geonet for 75 % SRM shows better strength when compared to other combinations. Further dynamic properties such as shear modulus and damping values at different strain level are estimated for red soil, sand, 30 % SRM and unreinforced and reinforced 50 % and 75 % SRM by carrying out resonant column tests and cyclic triaxial tests. The normalized shear modulus and damping ratio curves have been developed for these materials. The experimental results indicate that, shear modulus increases for 30% rubber by volume when compared to sand, thereafter the shear modulus values decreased with a further increase in rubber content in SRM. Whereas the damping ratio increases with increasing rubber content in SRM. For sand and SRM, with an increase in confining pressure shear modulus increases and damping ratio decreases. Based on the comprehensive set of experimental results, a modified hyperbolic model has been proposed. These properties are further used in the numerical analysis to find out the effectiveness of SRM as isolation material. Numerical dynamic analysis has been carried out on a 2-D finite element model of the soil-foundation-structure system. The building model has been generated considering the typical G+2 building resting on 20 m thick soil followed by rock depth and foundation is placed at 2.0 m below ground level. The beams and columns in the superstructure are modeled using 2-D frame elements. The soil column has been modeled using 4-noded 2-D plane strain plate elements. Considering the transmitting boundary condition, viscous dampers are implemented at the base of the computational soil domain in order to mitigate the reflective effects of waves. The Newmark family method (average acceleration method) has been used to calculate the displacement, velocity and acceleration vectors. Comprehensive numerical simulations have been carried out on the soil-foundation-structure system by varying rubber content in SRM (30%, 50% and 75% granulated rubber by volume), depth and thickness of SRM around footing and considering two input earthquake acceleration time history. It was found that earthquake vibrations are considerably reduced for SRM with higher rubber content. The optimum dimension of SRM giving maximum reduction in shaking level is found to be 3B below the footing and 0.75B (where B is the width of footing) on the side of the footing. Generally, the shaking levels at different floor can be reduced by 30-50%, with the use of 75% SRM. The results also indicated that the effectiveness of proposed system would depend on the characteristics of ground motion. To study the bearing capacity of square footing on SRM, laboratory model tests were carried out on square footing supported on unreinforced and reinforced SRM. The SRM combination which have been used for numerical studies are used in this model studies to know the bearing capacity and settlement characteristics. The optimum dimension of SRM around footing has been constructed. Model tests results show that, the bearing capacity decreases and settlement increases steadily with the increase in rubber content in SRM. Addition of reinforcement to SRM significantly improved the bearing capacity and reduced settlement characteristics. Reinforced SRM may be used as an effective low cost isolation scheme to reduce earthquake vibrations.