Abstract: To investigate the influence of plant roots on the mechanical properties of root-soil composite, triaxial tests were conducted on root-soil composite with varying root content. The mechanical properties dependent on root content and nonlinear failure criterion of the root-soil composite were analyzed. Results demonstrate that roots enhance the shear strength of the soil by providing tensile strength. Roots primarily impact the cohesion of the rooted soil, while slightly affect the internal friction angle of the root-soil composite. At a root content of 0.36%, the cohesion increased by 64.91%. The principal stress difference increases rapidly at the initial stage of strain, but such change slows down when the axial strain is greater than 2%. The specimen failure is characterized by dilative shear and the formation of longitudinal cracks. Increasing the confining pressure weakens the dilative shear effect of the root-soil composite. The initial tangent modulus increases with increasing confining pressure and root content. The maximum and minimum failure stress ratio of the root-soil composite is 0.99 and 0.63, respectively. The nonlinear strength failure criterion reflects the failure characteristics of the root-soil composite with varying root content. The failure envelope is nonlinear at low confining pressure and linear at high confining pressure, with the critical stress correlated to the confining pressure.

Key words: root-soil composite, root content, triaxial test, shear strength, failure criterion