Abstract
Introduction and Objective: Extensive studies have been conducted in the field of the effects of vegetation on shallow landslides and landslides, but the role of different plant species and vegetation forms on the stability of forest landslides and landslide slopes is still not well known. Therefore, proper quantification of the effects of vegetation on soil shear strength is very important for assessing the stability of landslides and landslide slopes along forest roads and can help in better management of stands and effective restoration of landslide-prone areas. In this study, the effect of vegetation species and forms on some physical and mechanical characteristics of soil at different depths was investigated and the most efficient species and forms for stabilization were identified.
Materials and Methods: First, 12 100-meter transects were established parallel to the Shastakalate forest roads of Gorgan on stable, landslide and drift-prone slopes overlooking the road. After the transects were established, landslide and drift patches (first control), stable patches (second control) and pure patches consisting of different vegetation forms of herbaceous (Coryx), shrub (raspberry), shrub (hemlock) and tree (alder, angelica and maple) vegetation forms were identified in three replications each in a simple random manner. In each replication and at the central point of each patch, a soil sample was taken using a standard cylinder with a diameter of 3.5 and a height of 4.4 cm at depths of 0 to 20, 20 to 40 and more than 40 cm in order to determine the water and soil density. 2 kg of soil was also collected from each depth for other soil tests. The total number of soil samples collected was 72. Direct shear test was used to measure soil cohesion and soil internal friction angle. In the laboratory, the soil sample was placed inside a metal box with a square cross-section that was horizontally divided in half. First, a vertical force was applied to the sample through the loading plate, and then shear stress was gradually applied horizontally so that the two halves of the box containing the sample moved relative to each other. The amount of shear force and the corresponding shear displacement were measured. The test was performed several times and for different values of vertical stress, the amount of shear stress at the time of rupture was obtained. Finally, the stability of the thrust range was determined using the safety factor. Atterberg limit tests were performed according to ASTM D 4318. This research was conducted in the form of a completely randomized block design and under a factorial experiment with two factors, types of stains in 8 levels and soil depth in three levels, and a total of 24 treatments in SPSS software.
Results: In the stable slope, the slope covered with raspberry, hemlock, sedge, angelica, maple and alder increased significantly with increasing soil depth, while the soil adhesion value decreased. The highest soil adhesion value was recorded at a depth of 0.0-2 meters of soil and the highest internal friction angle value was recorded at a depth of 0.4-0.6 meters of soil. The amount of organic carbon was also higher in the surface layers of the soil than in the lower layers. The highest adhesion value of 16809.01 N/m2 was reported in the 0-0.2 meter layer of the slope covered with sedge, which was the closest value to the soil adhesion of the stable slope in that soil layer (19598.67 N/m2). The lowest soil adhesion value was recorded in the surface layers (0.0-2.0 meters) of the slopes covered with tree species. In the present study, the highest safety rate was observed in the 0.0-2 m layer of the stable slope and slopes covered with herbaceous species of Carax. In the 0.2-4 m layer of soil, there was no significant difference between the safety rate of the stable slope and the slope covered with evergreen, which indicated the ability of this species to stabilize drift slopes. In the 0.0-4.6 m layer of soil, tree species after the stable slope had a higher safety rate compared to other vegetation forms. The slope safety rate at a depth of 0.0-2 m was significantly higher than that at depths of 0.0-2.4 and 0.0-4.6 m. The lowest safety rate was observed at the greatest depth, i.e. 0.0-4.6 m. There was a positive and significant correlation between soil cohesion and organic carbon content. There was a negative and significant correlation between the slope safety rate and clay content, internal friction angle, and soil density. That is, with increasing clay content, internal friction angle and soil density, the safety rate decreased. In contrast, there was a positive and significant relationship between the safety rate of the slope and organic carbon and soil cohesion. With increasing organic carbon and soil cohesion, the safety rate of the slope increased. The soil type in terms of clay content was CH in the drift slopes covered with alder, angelica and maple, and CL in the stable slopes covered with raspberry, carrion and hemlock, which indicates a change in soil class in the presence of herbaceous, shrub and shrub species.
Conclusion: The results showed that soil cohesion and safety of the slope improved at a depth of 0.2-0.5 m in the soil and in the presence of herbaceous vegetation forms, and when the species related to this form were not present, the slope was significantly unstable at this depth. The lack of cover or removal of understory vegetation cover in drift slopes reduces the safety of the slope and may actually be detrimental to slope stability. The decomposition and function of roots through rhizosphere secretions and microbial byproducts play a role in increasing soil adhesion and the process of clod formation, and thus strengthens soil cohesion. Accordingly, it is suggested that on slopes prone to landslides, planting and strengthening the establishment of tree, shrub and herbaceous species should be done to stabilize and improve slope safety up to a depth of 0.6 meters.