Effect of friction interfaces on friction characteristics
The main influencing factors on the direct shear friction of the root-soil interface include the vertical load, dry density, soil moisture and vegetation type. In the present study, the root-soil cohesions of goosegrass exhibited significant differences with crabgrass and barnyard grass (P < 0.05), and the root-soil cohesions of crabgrass and barnyard grass were much higher than those of goosegrass. It can be concluded that the root surfaces of crabgrass and barnyard grass absorb soil particles more strongly, resulting in a situation that does not differ significantly from that of the bare control soil (P > 0.05). In general, the low root-soil cohesion of these three typical grasses indicated that there was no correlation between root-soil cohesion and the improvement in soil shear resistance at different soil moisture levels. These results are like those obtained by Wei et al. (2018) in an experimental study and Tian et al. (2015) in a theoretical study. This similarity may be because there is incomplete contact between the root and soil in the unconsolidated shear specimens; hence, the root-soil cohesion exhibited “false cohesion” for the frictional strength of the root-soil interface under vertical loading.
Meanwhile, there were striking differences in the friction angle and friction coefficient between the soil-soil and root-soil interfaces (P < 0.05), and the soil-soil interfaces had the lowest friction angle and friction coefficient among the different friction interfaces. This is because the frictional force of the soil-soil interfaces depends on the blocking action of soil particles, while for the root-soil interface, the root surface roughness is the element that most influences the frictional force at the root-soil interface. The greater the root surface roughness and/or the larger the root surface area, the greater the contact area between the root and soil, and thus, the larger the root-soil frictional force. Hence, it can be concluded that the goosegrass root-soil interface had a larger frictional force with greater root surface roughness than the other two typical root-soil interfaces.
Furthermore, the correlation coefficients of the shear strength and vertical load in the root-soil interfaces of goosegrass were higher than those in the soil with the other two typical grasses and those in the bare control soil. Related studies have indicated that the response of the shear strength of a root-soil interface to a vertical load varied greatly for different root samples and root surface roughnesses for the same plant and diameter class, while the shear strengths of the bare control soil samples were tightly distributed. Meanwhile, there were considerable differences in shear strength among the samples with the three typical grasses and different soil moistures, but the shear strengths of these samples were greater than that of the bare control soil. Thus, the root system did not fully utilize its friction effects under a low vertical load; this finding is similar to that of Baets et al. (2008). In addition, the frictional resistance of the root-soil interface tended to increase nonlinearly with increasing depth of the root system. The rate of increase in the frictional resistance of the root-soil interface was larger when the depth of the root system was shallower, while the rate of increase in the frictional resistance of the root-soil interface was smaller when the depth of the root system was deeper. These results are slightly different from those of Xing et al. (2010), who found that the frictional resistance of the root-soil interface increased as the vertical load increased. This difference may be because in our study, the interval of the vertical loading is consistent, while in the Xing et al. (2010) study, the lower interval of the vertical loading is smaller than the higher interval of the vertical loading, which led the change rate of the frictional resistance of the root-soil interface under the higher vertical load to be much higher than that under the lower vertical load.
Effect of soil moisture on friction characteristics
Soil moisture is one of the most important factors affecting the friction characteristics. In the present study, the friction angle, cohesive stress and friction coefficient of the root-soil interface and soil-soil interface followed a binomial distribution with increasing soil moisture. It can be concluded that the increase in soil moisture content is beneficial for the root-soil anchorage effect, although when the soil moisture increased to a certain range (15%~20%), the root-soil anchorage effect was reduced. These results are like those of the experimental studies of Li et al. (2013b) and Zhao et al (2021). The reason for this phenomenon is that when the soil moisture increased, the water films more rapidly on the surfaces of the soil particles and roots; hence, the degree of blocking between the root and soil particles decreased. At the same time, the distance between the soil particles and root system increased, which decreased the connection strength of the root-soil interface. All these factors led to a first increasing and then decreasing trend in the internal friction angle of the root-soil interface under the condition of an unchanged extrinsic effective pressure (Wei et al., 2016). In addition, for the soil moisture levels of 5% and 25%, the cohesive stress of the root-soil interface was significantly lower than those under the other experimental conditions (P < 0.05). Therefore, the root system could significantly improve the soil shear strength in the dry and rainy seasons, and it can be further speculated that seasonal variation has a significant influence on soil reinforcement by plant roots.
In addition, the drawing friction properties of root systems are crucial determinants of soil conservation for plants. The maximum pull-out resistance and pull-out strength are two important characteristic indexes of the mechanical characteristics of roots. When a root system is pulled out by an external force, there are two main failure modes: tensile failure and friction failure. In the present study, the process curve of friction failure (displacement vs. pull-out force curve) can be divided into three stages: first sharp decline, second sharp decline and slow decline. At the initial stage of pull-out, the displacement of the root system in the soil tends to zero, which means that there is sattic friction between the root and soil. When the sliding displacement ranged from 0.5 mm to 20 mm, the root system started sliding when the root pulling force became greater than a certain value, and the friction of the root-soil interface changed from a static friction to sliding friction, leading to the root system being pulled out at the maximum simple root pulling capacity. As soon as the sliding displacement increased, the drawing force decreased gradually and finally stabilized. Murielle et al. (2014) indicated that the reasons for this phenomenon were as follows. At the beginning of the drawing shear, the uneven surface of the root system interlocked with soil particles, and soil particles at the contact surface lifted, separated and rotated as the drawing shear increased. At the same time, the volume of the soil changed. All of these factors require an external force. Therefore, the drawing force rapidly increased to a maximum value in an early stage of the test. Then, the shearing process continued, and the root-soil interface tended to become smooth due to the movement and rearrangement of the soil particles around the root system. Finally, the frictional resistance and drawing force gradually decreased and stabilized. The maximum pulling resistance was much higher at high soil moisture than that at low soil moisture. When the soil moisture was low, the contact between the soil particles and root systems was quite loose, and the cohesion between the soil and root system was low, leading to the low pull-out strength of the root system (Ghestem et al., 2014). With increasing soil moisture, a bound water membrane formed at the interface of the soil particles and water in the soil, and the cohesion of the root-soil interface increased under the influence of water cementation, so that the maximum static friction of the root-soil interface and root pull-out strength increased to some extent.
Overall, based on the redundancy analysis, the interface type had the greatest impact on the root-soil friction characteristics in the present study. Goosegrass, as one of the typical root-soil interfaces in purple soil bunds, has better drawing friction properties than the other two typical grasses. The root system of goosegrass improves the effectiveness of soil conservation and the degree of surface coverage and thus should be more widely used in the eco-environment construction of purple soil bunds for soil and water conservation. It can be concluded that goosegrass is the best choice among the three vegetation types tested in purple soil bunds for soil and slope protection. Moreover, for further research and discussion, the other soil physical-chemical properties that influence the friction characteristics of the root-soil interface should be studied to establish a scientific theoretical model of the root-soil interface in purple soil bunds.