Influence Of Subsurface Courses Materials On Pavement Performance: A Case Study in Yebu Agaro Road
The properties of subsurface materials would vary from place to place while it is true the performance of pavement along the route also differs. The research study had been undertaken at Yebu Agaro road section with the main objective to determine the Influence of Subgrade Soil on pavement performance. In order to realize the desired objective, a systematic methodology was adopted which includes field investigation, field test and laboratory tests. While the purposive sampling method was used to extract soil samples from the road section that are severely damaged showing different failures types and non distress surface. Those severe types of failures observed along the road section are reflective crack, pothole, rutting, alligator crack and block crack. From these failure types, five representative samples were extracted at different locations along the failures section and one non distress pavement of road section using a manual hand auger. The analyses of soil samples were carried out based on the Geotechnical properties such as wash gradation, Atterberg limit, and Modified proctor test, California Bearing Ratio (CBR) and field test, as well as Axle load analysis. It was found out that the liquid limit of subgrade soil varies from 33% 60.5% and the plasticity index from 20.3% 30.1% while there was a significant increase in moisture content at field and observed beyond the OMC in the laboratory result due to the distress of pavement affected by water infiltration through cracks, that tend to reduce the dry density. The recorded soaked CBR values of subgrade soil materials range between 8% 13% which was below the 15% minimum value specified by ERA manual. However, the existing CBR values of the subgrade soils indicate a poor material used for pavement structures. According to ASSHTO, the soil is classified as A 2 6 and A 7 6 category which means the existing soils are fair to poor as a sub grade material while USCS classification shows that the soil falls into SC group. In addition, the failures that are frequently observed on the road surface were significantly influenced by subgrade soil. For the sub base layer materials, it was noted to have satisfied the minimum requirements as compared with standard specification, except deficiency of its thickness. For the base course materials, inferior qualities of material were used at some section. The overall results showed that, heavy vehicles or traffic loads are one of the major causes of road failure along the study road section. Hence, the influence of other factors such as poor drainage courses, level of ground water table, variety of geologic materials along the road route and poor construction materials should be thoroughly addressed before the start of the rehabilitation of the road section in the future. Finally a possible remedial measure is recommended for every observed failure or distress on the pavement condition of the study area in order to sustain the design life of the pavement.