Background The stress state and failure mode of plant roots in the shearing process will affect the root's ability of reinforcing soil, which is usually neglected in the classical models when quantifying the shear strength increment provided by roots and results in the overestimation of root reinforcement.

Methods Aiming to explore the resistance behaviors and failure model of roots in the unsaturated soil under shearing condition, the large scale direct shear test for Vetiver root-soil composite, root tensile and pullout test were conducted based on the various soil moisture content and root quantity gradient. And the fractured or pulled out root ratio of each treatment shearing sample were observed simultaneously. Based on which the shear strength increment by roots are calculated by a revised model considering effect of soil moisture and root failure.

Results  1) Under the condition of the same moisture content, the increased root shear strength was positively correlated with the root density. Under the condition of a certain root density, the increased root shear strength had a negative correlation with the moisture content. 2) The tensile force T_r and strength t with diameter 0.2-2.2 mm was 3-19 N and 5-29 MPa, and can be described as a negative power function and a positive power function with the increase of diameter D respectively (T_r=11.514D^-0.885, t=9.5763D^1.095). Root pullout force p also followed the positive power function with diameter (p=αD^β, α>0, β>0) and higher the moisture content, smaller the α value. 3) Among the four water moisture contents, Wu model values was 1.834, 1.864, 1.889, and 1.873 times of test values respectively, however the model value was 1.337, 1.028, 0.788, and 0.481 times of test values respectively when taking the root failure mode into account.

Conclusions 1) Under unsaturated conditions, the reinforcing soil capacity of the root system is mainly related to the ratio of the number of roots extracted during the shearing process. The higher the water content during the direct shear test, the lower the extraction strength of the root system, and the root extraction. The larger the ratio of number of the extract roots, the smaller the amount of roots that can fully contribute to the tensile strength, and thus the weaker the root system's ability to stabilize the soil. 2) The root tensile strength, pull-out force and diameter are all in a power-exponential function relationship, and as the moisture content increases, the fitting attenuation coefficient of the pull-out force increases. 3) After taking moisture content and root failure mode into account, the estimated value of the obtained root shear strength is closer to the experimental value than that by the Wu model.