Subgrade Soil Properties and Their Impact on Asphalt Road Performance

The road system is an important part of any country's social and economic growth since, it is vital that towns, areas, and neighborhoods should properly linked. The Pavement Condition Index (PCI), as established by ASTM D6433, will be used to evaluate the pavement condition of Qurna-Chibayish road in Basra Governorate, Iraq, and its connection to natural subgrade soil qualities. The research includes visual (PCI) surveys as well as laboratory tests to evaluate subgrade soil characteristics, including grain size distribution, specific gravity, Atterberg limits, compaction, and California bearing ratio. The classification results of subgrade soil samples taken from (21) road sections reveal fine grained silty and / or clayey deposits with varied sand contents, according to the unified system, with AASHTO classes from (A-4) to (A-7). The soaked values of California Bearing Ratios (CBR) divided the soil sections into three, nearly equal groups: poor (CBR ≤ 5%); moderate (5 < CBR ≤ 10) and good strength (CBR > 10) conditions. The road evaluation process generally indicated satisfactory to good pavement conditions across most of the study area, where (53%) of the (PCI)-values fall within the range of (70 to 85) and (35%) within (85 to 100). Only (12%) received a fair rating with (PCI) lie between (55) and (70). It is evident from this study that, the Californian bearing ratio of the subgrade has a negligible effect on the pavement performance, as it is evaluated utilizing the pavement condition index. This is attributed to the addition of a fill layer that functions as a capping layer where the subgrade is greatly improved. Therefore, well designed and constructed pavement section would reduce the adverse effects of week subgrade soil and enhance high (PCI)-values.