Shear Strength Characteristics of Bottom Ash–Rubber Mixture Reinforced with Geogrids

This technical paper outlines the shear strength characteristics of a new lightweight backfill material comprising bottom ash and tire granules of scrap tires through a series of large-scale direct shear tests. The proportions of rubber granules in the bottom ash–tire granule mixture varied between 0 and 40% with normal stresses ranging from 125 to 225 kPa. Furthermore, the interface shear strength characteristics of bottom ash–tire granule mix with uniaxial polyester geogrid of different aperture sizes are analysed. Test results on the unreinforced composite mix have shown that adding tire granules decreased the peak mobilized shear strength of the mix compared to pure bottom ash at higher normal stresses. The excessive volumetric strain of the composite mixtures with higher rubber content could be limited by the inclusion of geogrids. The geogrid reinforcement improved the interface shear strength of the composite mix with 40% tire granules up to 11% due to the frictional interlock and passive resistance offered by the geogrid when the mean grain size of the composite material is higher than the thickness of the transverse rib. The interaction coefficient of the reinforced bottom ash and bottom ash–tire granules mix varied between 0.90 and 1.1 at different normal stresses. Meanwhile, the effect of aperture size of geogrids was significant at lower normal stresses except for the composite mixture with 40% tire granules. Thus, the response of the unreinforced and geogrid reinforced bottom ash–tire granule mixture indicates the suitability of this mixture as a backfill material in reinforced retaining structures.