TECHNOLOGY DEPARTMENT OF CIVIL ENGINEERING

Abstract

As ERA’s road design manual recommendations, cuts in soils up to a height of about 3 to5 m usually designed based on past experience with similar materials in the surrounding project area. That is due the fact that observations and experience in the performance of cut slopes in the region, provides knowledge of stable cut slope angles and common stability issues affecting large cuttings in soil. Similarly, cut slopes greater than 5 m in height recommend a further geotechnical analysis, depending upon the risk they pose. That include any cut slope where failure would result in large rehabilitation costs or threaten public safety should normally be designed using rigorous techniques. This study presents the i nvestigation, and t he r i gor ous analysis made for evaluating the stability of the presumptive Backslope slope ratios provided in the road manual, which is being used for the typical road project experience such as Ethiopian Roads Authority Daye-Chir-Nansebo Road Construction Project, which is found in southern nation and nationality regional state. In this study a soil material sampling and testing were carried out at different station along the road project alignment at respective station such as km22+775, km34+206, km43+760, km48+000 and km56+000. Accordingly, make use of the actual site and laboratory test finding of the soil properties, the backslope stability analysis for the soil material were carried out ,and checked for its factor of safety against the presumptive back slope ratios or design provision being used for the road project such as (3:1) for height of cut below one meter depth, (2:1) for height of two meters depth and (1.5 to 1) for height of over two meters depth ,all are ratios represented with (H:V). Accordingly, the slopes analysis was made as per the conventional limit equilibrium analysis method for two-dimensional (2D) slope stability analysis for which standard geotechnical software or GESLOPE/W was used. accordingly, different known method for slope analysis such as ordinary method of slices, bishop method, janub method, spencer method and Morgenstern and Price were used. The analyzed factor safety output for each of the methods was considered for the evaluation of the factor of safety for the backslopes ratios provided from the manual. however, the Morgenstern and Price method of analysis was used for the final judgment of the slope stability analysis, against the minimum required factor of safety of 1.3 Generally, it was observed that the presumptive backslope ratios, being used for the road project, for the soil material in cut of different height, found to be within acceptable factor of safety