In this paper

the undisturbed roots-soil composites of Iris tectorum Maxim.

the dominant species from the source area of Lanniqing landslide

Zhaotong

were taken as the research objects

and the shear characteristics of the composites were studied in depth by direct shear tests

and compared with the experimental results of remolded composites with the same water content in lab. The results showed that the samples with no roots as a whole had the characteristics of strain softening

while the composites had the characteristics of strain hardening. The shear strength of composites was higher than that of samples with no roots

which increased with the increase of roots contents

so that the roots strengthening effect was obvious; however

the undisturbed composites did not exist the optimal root content similar to that of the remolded composites. The c value of composites has no obvious change rule

while the φ value increased gradually

and both increase rates also have the same trend. The sections of failure composites showed that the ability of roots to consolidate soil was optimal when they were the taproots and they were vertical through shear plane

and the vertical pressure was high. The diameter of lateral roots without taproot was smaller

and the inclined distribution in the composites

especially along with shear direction

would affect roots reinforcing effect. The failure mode of samples with no roots was the overall shear failure with incomplete horizontal failure surface

while the failure mode of composites was that the roots were pulled out of sample instead of being sheared or pulled out

and obvious relative displacement occurred between roots and soil. The results could provide a theoretical basis for the selection of slope protection vegetation in order to enhance the shear strength of composites.