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Benggangs are a special type of soil erosion in the hilly granite regions of the tropical and subtropical areas of Southern China. They cause severe soil and water loss, which can severely deteriorate soil quality and threat to the local ecological environment. Soils (red soil, sandy soil and detritus soil) were collected from collapsing wall of a typical Benggang in Changting County of Fujian Province, and their physicochemical and mineralogical properties were analyzed. Five different monovalent cations were used to saturate the soil samples to examine the specific ion effects on the shear strength and clay surface properties. Red soil had a higher clay content, plastic limit, liquid limit and shear strength than sandy soil and detritus soil. The studied soils mainly consisted of kaolinite, hydroxy-interlayer vermiculite, illite and gibbsite clay minerals. The soils saturated with K + , NH 4 + and Cs + had greater cohesion than the Li + - and Na + -saturated soils, e.g. , the cohesion of the red soil saturated with Li + , K + , NH 4 + -saturated soil, respectively. While the internal friction angle was slightly different, which indicated that different monovalent cations affected the shear strength differently. K + cations were 1.05, 1.23, 1.45 and 1.20 times larger than that of the Na + -saturated soil, respectively. While the internal friction angle was slightly different, which indicated that different monovalent cations affected the shear strength differently. K + -, NH 4 + - and Cs + -saturated clay particles had higher zeta potentials and thinner shear plane thicknesses than Li + - and Na + could increase the shear strength, and then increase the stability of the collapsing wall, thus might decrease the erosion intensity of Benggang. The results provide a scientific basis for the interpretation of and practical treatment of Benggang.+ , NH 4 + and Cs + could increase the shear strength, and then increase the stability of the collapsing wall, thus might decrease the erosion intensity of Benggang. The results provide a scientific basis for the interpretation of and practical treatment of Benggang., Competing Interests: The authors declare there are no competing interests., (©2024 Huang et al.)

'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 shear strength, mainly reflected in the cohesion.The roots enhancement effect in lateritic layer was better than that of sandy and detritus layers.New shear strength equations of root‐soil complexes were established based on the Wu‐Waldron model. [ABSTRACT FROM AUTHOR]

Failure of collapsing walls is an important process affecting the development of Benggang and is closely related to the soil shear strength. Plant roots can increase the soil shear strength. However, the effects and mechanisms of root reinforcement on the soil shear strength of collapsing walls remain unclear. To explore the shear strength characteristics of collapsing walls and their influencing factors under different vegetation conditions, Pennisetum sinese, Dicranopteris dichotoma, Odontosoria chinensis, and Neyraudia reynaudiana were adopted as experimental objects in the Benggang district of Anxi County, Southeast China. We measured the root characteristics and in situ shear strength of root–soil complexes by dividing soil with the four vegetation conditions into five soil layers: 0–5 cm, 5–10 cm, 10–15 cm, 15–20 cm, and 20–25 cm. The average shear strength of the root–soil complexes of the various plants ranked as follows: Pennisetum sinese (30.95 kPa) > Odontosoria chinensis (28.08 kPa) > Dicranopteris dichotoma (21.24 kPa) > Neyraudia reynaudiana (14.99 kPa) > bare soil (11.93 kPa). The enhancement effect of the root system on the soil shear strength was mainly manifested in the 0–5 cm soil surface layer. The soil shear strength attained an extremely significant positive correlation with the root length density, root surface area density, root volume density, root biomass density, for root diameters (L) less than or equal to 0.5 mm and between 0.5 and 1 mm, the soil shear strength could be simulated by using root volume density. The shear strength of undisturbed root–soil complexes measured with a 14.10 pocket vane tester was higher than the value obtained with the Wu–Waldron model (WWM). The correction coefficient k′ varied between 0.20 and 20.25, mostly exceeding 1, and the average correction coefficient k′ value was 4.94. The average correction coefficient determined in this test can be considered to modify the WWM model when conducting experiments under similar conditions. [ABSTRACT FROM AUTHOR]

• A typical and common herbaceous plant of Neyraudia reynaudiana roots was selected as the research objective. • The effect of both moisture content and root density to root-soil complex are discussed. • Roots could enhance the soil shear strength mainly reflected in the increase of cohesive force. • Moisture content was introduced into the shear strength model of root-soil complex. The collapse of collapsing walls is a key process during the Benggang development in southeast China and is closely related to the soil shear strength. Plant root reinforcement can effectively improve the soil shear strength. However, there are few studies on the effect of roots on the soil shear strength of collapsing walls. To explore the influence of roots on the soil shear strength of collapsing walls, the roots of Neyraudia reynaudiana was selected as the research object. This paper studies the tensile properties of the roots through root tensile tests and performs a direct shear test to study the influence of the roots on the soil shear strength by setting different mass moisture contents (15%∼30%) and root weight densities (0 ∼ 1.25 g 100 cm−3). Root tensile force is positively correlated with root diameter by a power function, while tensile strength is negatively correlated with diameter by a power function. Roots could increase the soil shear strength, and which was mainly reflected in cohesive force. The shear strength and cohesive force increase first then decrease with the increasing root density, while root density has no effect on the internal friction. The shear strength, cohesive force and internal friction of root-soil complex all decreased with increasing moisture content. The WWM (Wu and Waldron model) greatly overestimated the cohesion of herbaceous roots, and a new prediction model of the shear strength of root-soil composites was obtained by revising the WWM model by introducing the moisture content and root density (NSE = 0.94). In conclusion, Neyraudia reynaudiana roots could improve the soil shear strength, whereas the strengthening effect decreased under the condition of high moisture content. The results provide a basis for the selection and allocation of vegetation restoration measures in Benggang erosion in southern China. [ABSTRACT FROM AUTHOR]