Abstract:In order to explore the mechanism of plant roots on the soil shear strength of red soil layer in Benggang, an indoor direct-shear-test was taken to study the effect of Dicranopteris dichotoma roots on the shear strength of red soil layer under different water contents. The results showed that: (1) The shear strength of soil samples with different root weights generally decreased with the increasing water contents, and the shear strength of soil samples with roots was greater than that without roots. (2) Cohesion tended to increase first and then decrease with the increase of water contents, and increased with the increase of root weight density, but the increment gradually decreased. (3) There was a linear negative correlation between the internal friction angle and soil water content, but the internal friction angle had no obvious relationship with root weight density. (4) The influence of water content on shear strength was greater than that of root weight density. Water content and root weight density could be used to simulate the shear strength of the root-soil composite (NSE=0.84). In summary, plant roots could increase the soil shear strength of red soil layer on collapsing wall, but the strengthen effect of roots was reduced under high water content, and the stability of root-soil composite on collapsing wall could be increased by reducing the water injection.