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This study investigates the effects of root distributions and stress paths on the shear strength of root-soil composites using a consolidated-undrained (CU) triaxial test. On the basis of the limit equilibrium, two root reinforcement coefficients (<i>n</i> and <i>m</i>) are proposed for characterizing the effects of shear strength parameters on the principal stress considering different root distribution angles and root diameters. Then, <i>n</i> and <i>m</i> are introduced into the conventional limit equilibrium equation to develop a new limit equilibrium equation for root-soil composites. The results demonstrate that the root distribution angles (<i>α</i>) and root diameters (<i>d</i>) affect the shear strength of the root-soil composites. Under a consolidated-undrained condition, the effective cohesion (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>c</mi><mrow><mi>r</mi><mi>s</mi></mrow><mo>′</mo></msubsup></mrow></semantics></math></inline-formula>) of the rooted soil is high and decreases in the order of 90°, 0°, 30° and 60°. For the same root distribution angle, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>c</mi><mrow><mi>r</mi><mi>s</mi></mrow><mo>′</mo></msubsup></mrow></semantics></math></inline-formula> increases with the increasing root diameter. Meanwhile, the effective internal friction angle (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>φ</mi><mrow><mi>r</mi><mi>s</mi></mrow><mo>′</mo></msubsup></mrow></semantics></math></inline-formula>) changes slightly. The failure principal stress of the root-soil composites is positively correlated with <i>n</i> and <i>m</i>. Furthermore, the deformation of the samples indicates that the run-through rate of <i>α</i> = 90° and <i>α</i> = 0° are both 0. Meanwhile, the lateral deformation rate declines from 17.0% for <i>α</i> = 60° to 10.9% for <i>α</i> = 90°.