Edinburgh Napier University

  The disposal of scrap tyres has become a major environmental problem around the world. For its recovery, a geomaterial has been used as part of civil constructions, commonly known as Rubber-Soil mixture (RSm). The use of RSm has the potential to be used as geotechnical seismic isolation system and hence provide protection against earthquakes. However, the static and dynamic behaviour of RSm is not well understood.
Various bulk (macro), particle (micro) properties, and the test conditions affect its characterisation. Whilst addressing this knowledge gap, the aim of this study was to understand the response of RSm under cyclic loading and evaluate its effectiveness in attenuating accelerations when used to retrofit a soil foundation.
An experimental programme was chosen for this research and is divided into three scales; particle, element and 1g model scales. Plain strain visualisations, oedometer, and x-ray tomographic tests were performed to elucidate RSm particulate behaviour.
To understand RSm dynamic behaviour, the evolution in stiffness and damping of the mixture was analysed from small-to-large deformations whilst altering rubber content and number of cycles. A sweep analysis was also performed via 1g shaking table tests to evaluate the cyclic performance of a scaled foundation-modified soil with RSm.
The findings in this thesis revealed that the macro behaviour of RSm is highly influenced by the particle properties, including rubber mass, size, shape, stiffness and its interaction with sand particles. Tests showed a greater change in void ratio of mixtures containing shredded rubber compared to crumb rubber. 3D x-ray tomographic images revealed an increase in contact and a decrease in rubber volume of RSm under loading.
This demonstrated that the high RSm compressibility is the result of both particle re-arrangement and rubber distortion. At an element scale, liquefaction resistance increased and mixtures did not liquefy by adding 20% rubber. The addition of rubber led to a reduction in soil stiffness whilst it increased the mixture resilience against cyclic loading, ameliorating the cyclic effect on stiffness and damping degradation. Material damping increased with rubber content at small-to-medium strains, whereas an upper value was revealed by adding 10% rubber at larger deformations. Test results supported the basis that energy dissipation in RSm is generated through particle sliding and rubber deformation, which takes over the dissipation mode after several cycles. Altering a host soil system by adding vertical discrete zones with RSm showed a lower amplification ratio and as a result mitigated part of the incident vibrations. The increase in damping capacity at a model scale was postulated to be the result of combining geometrical and material damping. The vertical disposition of the soft zone herein proposed could allow its application to both existing and new infrastructure.