Effect of Soil Moisture Content on the Shear Strength of Dicranopteris Linearis-Rooted Soil in Different Soil Layers of Collapsing Wall

Soil erosion is a common form of land degradation in terrestrial ecosystems [1,2] and has a serious negative effect on human production and living activities [3]. Benggang (Figure 1b) is a unique type of soil erosion that collapses under the combined action of hydraulics and gravity, and occurs in seven provinces (autonomous regions) of southern China. “Beng” is collapse, and “Gang” is rolling hill [4,5,6]. According to a previous investigation, the annual average soil erosion modulus in the Benggang erosion area is 50 Gg/km², which is more than 90 times higher than the allowable soil loss in the south [5]. This is also called an “ecological ulcer” in tropical and subtropical China [7], which seriously endangers human production and life (Zhang et al., 2020). A collapsing wall is a steep cliff formed by undercutting or collapse of hillside soil [5], and its stability is the key to the occurrence and expansion of Benggang. Shear properties are an important index to quantify the stability of landslides. Under the action of external force, the shear resistance of soil changes greatly. Research on the mechanical properties of collapsing wall soil is of great significance for revealing the process of collapse and the development of Benggang.

Vegetation restoration has been widely used on stable slopes [8,9,10,11]. This is because plants can exert mechanical mechanisms of reinforcement and anchoring through shallow fine roots and deep thick roots [12,13,14]. Plant roots have strong tensile strength, whereas that of soil is weak. The composite material formed by roots and soil not only makes full use of the advantages of roots but also overcomes the weaknesses of soil [15]. Therefore, the shear strength and cohesion of soils with root materials are stronger than soils with no root materials [16,17,18,19,20]. In recent years, increasing attention has been given to the effect of root systems on Benggang soil reinforcement. Huang et al. [21] found that with the addition of Neyraudia reynaudiana root materials, a perennial herb of the Gramineae family, the shear strength of the lateritic layer improved significantly, but the internal friction angle changed only slightly. Shuai et al. [22] suggested that among the four herbs plants, the soil reinforcement effect of Pennisetum sinese and Odontosoria chinensis was higher than that of Dicranopteris linearis and Neyraudia reynaudiana. In the three layers of collapsing wall, the root system of Dicranopteris linearis significantly increased the shear resistance and compressive strength of the first layer but not in the other two [19,23]. In short, root density [19,24], species of plants [25] and soil properties and configurations [26] affect the effect of the root system on soil reinforcement.

Rainfall infiltration contributes to an increase in SMC, which changes not only the physicochemical properties of soil but also the ability of the root system to restrain soil [27]. The change in SMC becomes an important factor affecting rooted soil strength [28]. Lian et al. [29] explored the rooted soil shear strength of Robinia pseudoacacia L. and found that the cohesion of reinforced soil under saturated condition decreased by 50%~61%. Zhang et al. [15] indicated that the two shear strength indices of soil with a 12.7% SMC were significantly higher than those with a 20.0% SMC and that the effect of SMC on cohesion was higher than that of friction angle. In the saline loess root–soil complex, the cohesion and internal friction angle decreased linearly as the SMC increased [27]. The mechanical properties of the rooted soil are limited by SMC [30], and most related studies have concentrated on one soil. However, in the natural environment, many plants have a wide ecological range and are suitable for a variety of soils. The effect of SMC on the shear resistance of soil with different properties needs to be further studied. A collapsing wall is a heterogeneous soil profile that is divided into a lateritic layer (LL), sandy layer (SL) and detritus layer (DL) with totally different properties from top to bottom [26]. Field investigations have found that there are many similar plants growing in different soil layers, such as Dicranopteris linearis, Melastoma malabathricum, and Neyraudia reynaudiana. Among them, drought-tolerant, barren and acid-tolerant Dicranopteris linearis (Figure 1c) is the most widely distributed [22]. At present, many scholars have focused on the role of Dicranopteris linearis in the restoration of soil microbial function in degraded land [31] and the soil reinforcement ability of its roots [19]. The effect and mechanism of SMC alteration on the shear properties of the Dicranopteris linearis-rooted soil with different soil properties are ignored.

Wu and Waldron’s model (WWM) is a simple analytical model for cohesion depending on the tensile and distribution features of roots in shear zone and is frequently applied to quantify the reinforcement effect of roots on a given soil [32,33]. However, the WWM assumes that all roots are closely bound to the soil and break together during shear [34]. This assumption does not occur in most cases [21,22], nor does it account for the progressive failure mode of roots [14]. In addition, there is the problem of overestimation of cohesion. Therefore, many scholars [22,35] have revised it, with a correction coefficient that varies from 0.11 to 0.44. Meanwhile, the WWM does not consider that soil type and SMC affect root mechanical feature exertion [19,21]. In fact, SMC affects the degree of bonding between roots and soils [20,27], which restricts the mechanical properties of root materials and causes incorrect estimates of the cohesion increment. Modifying the k coefficient of the WWM based on the SMC is vital to improve its applicability.