This paper presents data from ring-shear and one-dimensional compression tests, which have been used to cause breakage under large shear strains and large compressive stresses respectively. A comparison of the behaviour of carbonate sand in monotonic shear and compression is reported, focusing on void collapse, grading properties and particle shape evolution. Trends in breakage behaviour under compression and under shear were similar; however, ultimate steady-state properties differed significantly. The higher effect of shear breakage was evident, in that the steady-state gradings for sheared material had higher fractal dimensions, indicating that the gradings were finer, and morphological changes in particle shape were more significant. Steady-state gradings tended to be multi-fractal in nature rather than mono-fractal, and showed that a unique monotonic steady-state grading does not exist. Two types of ring-shear device were compared, and the differences between the results highlighted a dependence of steady-state properties on test apparatus configuration.

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Landslides due to catastrophic weather events, especially heavy rainfall, have risen significantly over the last several decades, causing significant damage and affecting the health and livelihoods of millions of people. Using tree roots to bio-engineer shallow slopes has been proven to be a cost-effective, sustainable measure and thus has gained increasing popularity. As slope failure often occurs under heavy precipitation, it is important to understand the mechanical interactions in the soil matrix surrounding a root to better estimate the reinforcement capacity of a root system, especially as the soil undergoes wetting from drier conditions. However, very few studies of root reinforcements have considered the effects of degree of saturation on behaviour. In this study, steel wires are used as a root analogue to explore the impact of root geometry, soil dilation and soil saturation on the pull-out behaviour of a root and three commonly used unsaturated soil strength models have been used to interpret the pull-out results. It was found that roots with larger diameter did not contribute to additional resistance. Also, a linear relationship between degree of saturation and pull-out strength was identified over a large range of suctions and one of the unsaturated soil strength models seemed to provide a more reasonable interpretation. The results will help future bioengineering slope design by improving the understanding of soil-root interface behaviour, including the effect of root diameter in slippage failure and greater emphasis on the importance of taking degree of saturation into account in unsaturated soil strength models.