Geotechnical and slope stability analysis in the landslide-prone area: A case study in Sawla – Laska road sector, Southern Ethiopia

Abstract

Examining the factors underlying landslides and assessing the stability of slopes is crucial for reducing the negative impact of landslides on the environment and the lifespan of infrastructure. This study assessed certain slope portions along the Sawla to Laska Road to determine the factors contributing to landslides and identify the mechanisms and conditions that lead to slope failure. Slope stability analysis was conducted using the limit equilibrium method, and a range of laboratory tests were conducted following ASTM standards. The analysis has shown that the three slope portions, specifically sections 1, 2, and 3, had distinct shear strength characteristics, as shown by variations in their cohesion and internal friction angle values. The average values for cohesion were 19.24, 10.41, and 12.9, respectively, while the internal friction angle values ranged from 11.32 to 17.8 for slope section three, 18.96 to 21.12 for slope section two, and 30.08 to 37.26 for slope section one. The limit equilibrium method was utilized to compute the safety factor of slopes. Five methods were employed to evaluate the safety factor: the ordinary method, Bishop method, Janbu method, Spencer method, and Morgenstern-Price method while considering three groundwater conditions (GWT at great depth, GWT at half of the slope, and GWT on the surface). Only slope section one exhibited a slightly stable safety factor that exceeded one under dry conditions. The slope-mass rating analysis showed that rock slopes one and two were deemed stable under class II, rock slopes three and four were partially stable under class III, and rock slope five was unstable under class IV. A slope mass range of 78 to 79 indicates excellent to very good rock mass quality and high stability potential for RSs1. In contrast, RSs5 shows a wide SMR range from 7 to 57, indicating a spectrum of very poor to fair to good quality rock mass and an associated range of stability from extremely low to moderate. RSs2 and RSs3 fall within the SMR range of 71.1–72 and 49.9–55.9, respectively, indicating good to fair rock mass quality with varying degrees of stability potential. Last, RSs4, with an SMR range of 39.6–45.1, shows poor rock mass quality and substantial stability concerns. RSs1 has a UCS of 28.68 MPa, indicating a "Medium strong" classification, and it belongs to the lithology type of "Trachyte." RSs2 showcases a higher UCS value of 32.25 MPa, similarly classified as "Medium strong," and is also identified as "Trachyte." RSs3, with a UCS of 28.83 MPa, falls under the "Medium strong" classification and is categorized as "Basalt." In contrast, RSs4 demonstrates a lower UCS of 18.41 MPa, leading to a "Weak" classification associated with the lithology type "Basalt." Lastly, RSs5 presents a UCS of 25.77 MPa, classified as "Medium strong," and shares the lithology type of "Basalt." The study identified that the landslide types in the study area were rockfall, earth flow, and soil/rock slides. The factors contributing to landslides in the study area were slope steepness, weathering, groundwater, and rainfall.