Spatial distribution of plant root forces in root-permeated soils subject to shear

• In-situ shear tests are used to investigate root forces in root-permeated soils.
• Location and orientation of roots affect root forces developed in the soil.
• Roots oriented opposite to the shear direction tend to develop tensile stresses.
• Lateral roots oriented in the shear direction tend to develop compressive stresses.
• Most of the mobilized percentages of root forces during shear are less than 45%.

This paper aims to investigate the spatial distribution of root forces in root-permeated soils that are subject to shear. In-situ large shear tests, 1.0 m × 1.0 m × 0.5 m, on plant root systems are carried out. The root system of Leucaena leucocephala (Lam.) (white popinac) and Acacia confusa (Formosa acacia) are used in this study. The axial forces developed in plant roots in soil that is subject to shear rely on the root location and orientation with respect to the shear direction. The majority of the roots in front of the tree stem, with respect to the shear direction, develop compressive forces during the shearing process, as the root orientation is low. Roots on the side of the tree stem tend to develop tensile forces during the shearing process. Most of the measured root forces at the back of the tree stem with respect to the shear direction are in tensile modes. Post-peak root forces continue to develop with increasing shear displacements. The percentage of the maximum mobilized root force in tensile modes during shear in this study reaches to approximately 68% of its ultimate tensile resistance at a shear displacement of 10–25 cm, whereas most of the instrumented data are less than approximately 45%. The maximum mobilized stress level for roots in front of the tree stem during the shearing process is low with respect to the ultimate tensile resistance of the root, and the maximum mobilized stress level for roots on the side and at the back of the tree stem is moderate. This study provides an in-depth look at the root forces developed in a plant root system subjected to shear.