Experimental Assessment of Cutback Bitumen Influence on Unsoaked CBR Performance of Sandy Soil

The performance of sandy subgrade soil is a critical factor in flexible pavement design, particularly in floodplain regions where seasonal moisture variation often weakens soil strength. This study investigates the influence of cutback bitumen on the compaction and bearing behavior of sandy soil collected from Bagerhat, Bangladesh, for potential pavement subgrade applications. The soil was classified as well-graded sand with low plastic fines (SW-SM), and laboratory tests were conducted following ASTM and AASHTO standards. Cutback bitumen was incorporated at ۴%, ۶%, ۸%, ۱۰%, and ۱۲% by dry weight of soil, while untreated soil compacted at optimum moisture content (OMC) served as the control specimen. Standard Proctor compaction testing established a maximum dry density of ۱.۵۸۵ g/cm³ at an OMC of ۱۶.۴۸%. Unsoaked California Bearing Ratio (CBR) tests revealed that the untreated soil produced the highest CBR value of ۱۴.۱۷%, whereas all treated samples showed significantly lower values. Among the stabilized specimens, the ۸% cutback mixture exhibited the highest treated CBR of ۳.۱۷%, yet remained substantially below the untreated condition. The reduction in strength is attributed to the bituminous coating around sand particles, which decreased direct grain-to-grain friction and weakened the granular load-transfer mechanism. The findings indicate that cutback bitumen is not effective for short-term strength enhancement of sandy subgrade soil, although it may still provide practical benefits in terms of moisture resistance, dust suppression, and erosion control for low-volume rural roads.

کلیدواژه ها:

Sandy soil stabilization ، Cutback bitumen ، California Bearing Ratio (CBR) ، Pavement Subgrade ، Flexible pavement