Geotechnical Characterizatiion And Slope Stability Analysis Of Landslide; The Case Of Werie-Maykinatal Road, Tigray Region, Northern Ethiopia

Abstract

Recently, landslide has occurred in Werie – Maykinatal road section of the Mekelle - Abi Adi - Adwa road project in Tigray Regional State, northern Ethiopia. This study aimed at investigating the geotechnical characteristics of type soil/rock, evaluating the stability condition of the slopes, and provision of recommendations on remedial measures in order to address the landslide problems. The approached/ methods used include: field work, laboratory tests and slope stability analysis. The field investigation involved description of soils/rock, inventory and detailed characterizations of the landslide affected sites (slope angle, dimensions of failed slopes, and any signs of instabilities) as well as sampling of soils for laboratory analysis. For the present study, based on the field manifestation of instability two most critical slope sections were identified for detailed slope stability analysis. ASTM (American Society Test Material standard) method was used to determine the properties of the soils in the laboratory. Tests carried out include: grain size, Atterberg limit, natural moisture content, shear strength and specific gravity. A total of six samples representing from two most critical slope sections in Werie – Maykinatal road were studied, with a view of determining their sliding potential. The results of geotechnical analysis revealed that the soils contain 1.62-1.97 % clay and silt, 43.56-65.37 % sand and 31.28-54.16 % gravel. Based on the unified soil classification system, the soil samples were classified as well-graded gravel and Poorly-graded gravel with clay and sand with group symbols of GW and GP-GC respectively. Results of the direct shear tests revealed that the cohesion and angle of internal friction varies between 25.3-73.65 kPa and 17.16-27.93 degrees respectively. The stability of the slopes were analyzed using Geo studio Slope/W software and result show that the factor of safety (FOS) values from site1 and site2 are 1.203 and1.372 respectively. This show the slopes are marginally stable and are being affected by slope instability. A modified slope angle (gentle slope) is recommended with a FOS value of 1.683 and 1.793 forsite1 and site2 respectively. The factor of safety (FOS) for modified slope angle was higher when compared to the FOS values from natural slope due to the effect of steep slope on the natural slope. In order to mitigate the problems provision of gabion retaining walls integrated with drainages are recommended. Moreover, surface water drainages (road surface and side ditches) are suggested. Benching of the upslope can also minimize the slope instability of the road sections.