Effect of root system of the Dicranopteris dichotoma on the soil unconfined compressive strength of collapsing walls in hilly granite area of South China

Benggang is a unique soil erosion landform in the subtropical hilly granite area of southern China. The stability of the collapsing walls is directly related to the severity of the Benggang erosion. In this study, four soil layers of the collapsing walls of Benggang of South China were investigated, and the influence of the moisture content and root weight density on the unconfined compressive strength of different soil layers was analyzed. The results indicated the following: With increasing moisture content, the unconfined compressive strength of the four soil layers first increased and then decreased. The unconfined compressive strength followed the sequence of the laterite layer, surface layer, transition layer and sandy layer, in descending order, which demonstrated that the Benggang was stable at the top and unstable at the bottom; furthermore, this type of structure could easily lead to the headward erosion of Benggang. The presence of Dicranopteris dichotoma root systems improved the soil unconfined compressive strength. The optimal root weight density for the four soil layers was 0.75 g/100 cm³. The unconfined compressive strength of the root-soil complex and rootless soil exhibited a positive linear correlation, and the regression equation was y = 1.29x − 8.02 (R² = 0.999). The moisture content and root weight density obviously interacted and affected the soil unconfined compressive strength. These two factors acted synergistically within a certain range and jointly increased the soil strength. Two-factor regression simulation and variance analysis revealed that the unconfined compressive strength of each soil layer significantly changed (p < 0.05). Models of an unconfined compressive strength attained a satisfactory adaptability, and the optimal moisture content and root weight density combination was obtained for each soil layer. Our conclusions indicated that the influence of the soil-fixing effect of plant roots was limited to shallow soil, other measures should be implemented in deep soil of Benggang.