Geotechnical Evaluation on Failed Section along Felegeselam-chida Road Project, Southwestern Ethiopia, Using PS-InSAR and Deterministic Approach
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Date
2020-09-26
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Addis Ababa University
Abstract
The current study of geotechnical evaluation on failed section were conducted in Southwestern Ethiopia, Felegeselam-Chida road project. This research aims to evaluate the status and features of the landslides through Persistent Scattering- Interferometry Synthetic Aperture Radar (PS-InSAR) and deterministic analyses along Felegeselam-Chida road project, south-western Ethiopia. Database for the 10 selected landslides along the road project is generated from project thirteen Single Look Complex (SLC) radar images that are downloaded from Alaska satellite. These time series images cover from Nov 3, 2014 to Jan 6, 2020 and processed to define the rate of deformation on the failed sections. The deterministic analysis further defined the safety factor of each failed section from the input data obtained from DEM analysis, geological factors and field-laboratory investigation (both primary and secondary) data using Plaxis software package both 2D and 3D. The PS-InSAR analysis showed that the mean velocity of displacement rate is ranging from 47.4mm/yr to -54.4mm/yr in the entire study area starting from Nov, 2020 to date. Additional analysis made considering the vertical displacement from satellite line of sight (LOS) to define the deformations for each selected failed sections. Accordingly, the mean velocity displacements for each are calculated and ranges from 20 to -100mm/yr at Km 57; 40 to -120mm/yr at Km 63; 60 to -60mm/yr at Km 65; 50 to -60mm/yr at Km 68; 50 to -150 mm/yr at Km 78-79; 50 to -100 mm/yr at Km83; 20 to 100mm/yr at Km86;10 to -100 mm/yr at Km87; 30 to -90 mm/yr at Km 93; and 20 to -120 mm/yr at Km 94. The results also confirm that the landslides are happening prior to the road construction and further accelerated and reactivated afterwards. In the other hand, the deterministic safety factors (FS) of the failed sections showed that they are almost instable, with FS ranging from 0.73-1.11, to be easily triggered through saturation and slope cuts. The end result of both the factor of safety and displaced material on each failed sections are, at Km 57 is 1.05 and 50m; at Km 63 is 0.97 and 65m; at Km 68 is 1.1 and 52m, at Km 78-79 is 1 and 60m; at Km83 is 1 and 50m: at Km 86 is1 and 60m; at Km 87 is 1.08 and 40m; and at Km 93 is 0.73 and 45m respectively. Finally, the result determined at Km 94 is FOS 0.95 and 50m displacement. Besides, the deterministic analysis gave insight to the stress and pore pressure conditions at failure; and mobilized shear strength at failure to assist the remediation effort. In this sense, the deviatory stress results gave clues for the prominent cause of failure. When the deviator stress is as high as 883.6 KN/m2, like at Km 78-79, indicating loading or saturation is causing the failures. Whereas the low results obtained 80 KN/m2 at of Km 57 may indicate cutting is the main cause of the failure.
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Keywords
Persistent Scattering, Ps-Insar, Line of Sight, Deterministic, Factor of Safety, Plaxis 2d and 3d