Effects of Subgrade and Sub-Base Material Quality for the Deterioration of Flexible Pavement A Case Study on Jimma - Sekoru Road Segment

Abstract

A flexible pavement structure consists of several layers of which the most bottom layers are the sub base and pavement foundation or subgrade. A pavement structure is a layer structure which supports the vehicle load on its surface and transfers, supports the vehicle load on its surface and transfers and spreads the load to the sub base and subgrade. The effects of weak subgrade and poor sub base course materials on pavement design, construction, and performance prediction are very significant. This study examined the relationship between the subgrade geotechnical data, sub base material test data with the actual performance of pavement. The main objective of this study were evaluate the effect of subgrade and sub base material quality for the deterioration of flexible pavement. Investigating the quality of subgrade and sub base materials, studying the effect of subgrade and sub base for the flexible pavement deterioration, indicating the best remedial measures to solve deterioration problems of the road segment are the key objectives in this study. The effect of the subgrade soil types and the quality of the subbase materials were investigated to check and recommend solutions for the existed deterioration in study segment. The representative samples were taken in the selected subgrade and subbase course layers at which the road segment have been highly damaged. The road segment have built on a subgrade layer consisted granular, white clay and black cotton soil types. The distress spots have seen on areas which have black cotton and white clay subgrade soils as checked by field observation. Thus, natural granular soil, white clay and black cotton soil were taken to check the subgrade layer quality of Jimma-Sekoru road segment. And subbase course materials were also taken to check the quality of the layer on those spot areas. The finding were found using a series of laboratory tests such as gradation analysis, Atterberg limits, moisture density relationships and CBR tests for both subgrade samples and subbase layer materials. The liquid limit varies from 49.72 - 64.82% and plasticity index from 12.44 - 33.10%. The soaked CBR values of subgrade soil materials were between 3.65 - 15%. The minimum CBR value for subbase layer for moisture susceptible areas is 30% as per ERA but Jimma –Sekoru road segment subbase materials CBR is 15%. The results of the subgrade soils investigation showed that the road pavement structures are underlined by A-7-5 and A-7-6 category of soils according to AASHTO which showed that the soil is very poor subgrade materials. As the findings have shown, the quality of the subgrade materials are poorly graded, highly plastic, low moisture content, low dry density and below fair CBR strength according to ERA pavement design specification. The subbase materials quality is also the same as the subgrade materials based on ERA pavement design manual for Jimma-Sekoru road segment. Thus the quality of the subgrade and sub base materials bring the deterioration of JimmaSekoru road segment. As the study concludes, the subgrade soil needs improvement either mechanically or by chemical methods. The white clay and the black cotton soil have to be replaced by granular materials or stabilized by normal soil and/or chemicals. The subbase layer materials have to be changed and replaced by the other quality meet material based on ERA pavement design specification. Finally ERA, ORA and Jimma zone officers have to return their eyes to this deteriorated road segment to improve its subgrade and subbase layer for special and long live service.