Impact of Dicranopteris linearis roots on the shear strength of different soil layers in collapsing wall of Benggang

‘Benggang’ is a local term for a widespread type of severgully erosion with steep collapsing walls in granitic, low, hilly areas of southern China, and its development and expansion are closely related to the shear strength of the collapsing wall. Plant roots play an important role in improving soil shear strength. However, the shear strength of root‐soil complexes in different layers of collapsing walls remains obscure. We selected Dicranopteris linearis fern roots and adopted the direct shear method to evaluate the effect of root weight density (RWD) (0–1.25 g 100 cm−3) on the shear properties of the lateritic, sandy and detritus layers. The results showed that roots could enhance soil shear strength, and the maximum increase in the lateritic layer was 11.53%, higher than that in the sandy (5.84%) and detritus layers (3.17%). As the root content increased, the cohesion of the sandy and detritus layers increased and then decreased, and their maximum increase in cohesion and the fitting optimal RWD were lower than those of the lateritic layer. The internal friction angle was not affected by roots. When the root content was constant, the shear strength and cohesion of the lateritic layer were significantly higher than those of the sandy and detritus layers, while their internal friction angle was significantly lower than that of the latter two layers. The average increment of soil cohesion calculated by the Wu‐Waldron model (WWM) was 10.52 kPa, which was 0.30, 3.75 and 19.38 times the measured average values of the lateritic, sandy and detritus layers, respectively. The correction coefficient k′ was 0.02–1.18, and the k'¯$$ \overline{k\hbox{'}} $$ in the lateritic layer was the highest (0.82), followed by that in the sandy and detritus layers. By combining the modified WWM with Coulomb's formula, new shear strength equations for root‐soil complexes of D. linearis were established. The predicted shear strength compared well with the measured shear strength (R2 > 0.90, NSE >0.90). Overall, the roots only had a significant reinforcement effect on the lateritic layer, and they could still not change the mechanical properties of the collapsing wall, which were more stable in the upper layers and weaker in the bottom. Therefore, other measures should be taken in the bottom layers to improve the stability of Benggangs. Highlights Effect of D. linearis roots on the shear strength of collapsing walls in Benggang was studied. Roots could improve collapsing‐wall soil