A study of arching effect in soils incorporating receycled tyres

Abstract

The term ‘arching effect’ refers to the phenomenon of stress redistribution in granular materials because of induced differential displacements and/or significant stiffness difference between the substituted and original materials. Arching is frequently observed in different geotechnical practice such as piled and geogrid-reinforced embankments, foundation piles, retaining walls, backfills, tunnelling, buried pipes, and excavation. Knowledge of arching mechanism serves to assist engineers in optimising their soil-structure design. Investigation of mechanical properties and potential applications of recycled-tyre-based aggregates has been carried out extensively over the last 25 years firstly to exploit the beneficial engineering properties of waste tyres such as their light weight as backfilling material and secondly to mitigate their environmental impact. However, to the author’s knowledge, no previous study of the arching effect in rubber-soil mixtures have been reported in literature at the time of this research. As rubberised soil is gradually establishing its place as a suitable and cost-effective alternative to natural aggregate in geotechnical work, it is worthwhile to have a thorough knowledge of the arching effect in rubber-soil mixtures. This thesis aims at experimentally investigating the arching effect in a coarse sand and rubber-soil mixtures using the Digital Image Correlation (DIC) technique and stress measurements. Active and passive arching modes in soil and rubberised soil were studied and compared. To do so, a replicate of Terzaghi’s original trapdoor apparatus was manufactured and filled with dyed sand and 10% and 30% (by weight) rubberised sand. Imaging and DIC analysis of the arching effect induced deformation in the trapdoor apparatus for the aforementioned granular material were conducted using VicSnap and VIC-2D computer programs. The distribution regimes of vertical and horizontal displacements and strains in the specimen cross-sectional area were obtained and interpreted. Additionally, shear strains, major and minor principal strains, and volumetric strains were computed and analysed. The effect of a central 5 kPa surcharge on these variables was also studied. It was observed that the DIC technique provides small-scale information and insights into how and where the arching effect creates zones of different local deformations within soil. The patterns and values of these deformation fields were found to be dependent on the test conditions and material type. As part of the experimental plan, the evolution of stresses over the trapdoor element and its adjacent, static bases, was recorded during the experiments using a number of stress sensors.