Stress-dilatancy relationship of rooted soil: shear mode effect

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4. Stress-dilatancy relationship of rooted soil: shear mode effect

Direct shear test (DST) and simple shear test (SST) have been commonly used to measure the shear strength of soils. DST, favoured for its simplicity, has been almost exclusively used for testing rooted soils but this method imposes a predefined shear plane, incapable of capturing the stress conditions of soil element before slope failure occurs. In contrast, SST better simulates shear behaviour but has never been used before to assess the root effects on soil shear strength and dilatancy. This study conducted DSTs and SSTs on sandy soil reinforced with vetiver grass roots (Chrysopogon zizanioides L.) to measure the stress-dilatancy relationships. Results revealed that DST consistently overestimated shear strength and dilatancy, with effective cohesion of rooted soil often more than 100% of that obtained from SST. The stressdilatancy analysis revealed that DST required higher mobilised stress to reach the maximum dilatancy than SST, whilst in rooted soils, the stress ratio continued to rise beyond the maximum dilatancy due to additional energy dissipation from the mobilisation of root tensile properties. These findings emphasise the need for caution when using DST for evaluating the shear behaviour of rooted soils and highlight the importance of considering both root cohesion and dilatancy effects in stability assessments and engineering applications.

3rd International Workshop on Soil-Vegetation-Atmosphere Interaction (RootS2025)

2a. Experimental laboratory characterisation of the multiscale bio-hydro-chemo-mechanical behaviour of rooted soils