Effects of Gradation and Fine Contents at Different Moisture Contents on CBR of Subbase Material in Flexible Pavement

Nepal’s rapid road development demands significant construction materials, but meeting sub-base gradation requirements, as per Standard Specifications for Road and Bridge Works (SSRBW-2016), is challenging due to local material deviations, resulting in higher costs. This research studies the impact of gradation, fine content, and moisture content on subbase material properties and potential relaxation of gradation using soil mix samples from the Sunkoshi River, Nepal. Modified proctor tests showed that increasing fines reduced Maximum Dry Density (MDD) and raised Optimum Moisture Content (OMC). Lower Limit Grading (LLG) samples showed MDD declining from 2.251 g/cc to 2.128 g/cc, and OMC increasing from 7.54 to 9.82%, while Upper Limit Grading (ULG) samples exhibited similar trends. Strong linear correlations were observed between fines content and both MDD (R² = 0.89–0.97) and OMC (R² = 0.98). California Bearing Ratio (CBR) tests showed a reduction in strength with increasing fines and moisture. LLG samples recorded a maximum CBR of 41.2% at 0% fines (OMC), decreasing to 10.2% at 25% fines. ULG samples showed a more gradual decline from 35.0 to 9.0%. CBR values consistently peaked at OMC and were lowest above OMC. Linear regression showed strong correlations between CBR and fines (R² = 0.97) and CBR and moisture content (R² = 0.91–0.95). While SSRBW-2016 permits 10–15% fines passing the 0.425mm sieve and minimum CBR 30%, fines passing the sieve up to 20% met CBR requirements. The study recommends increasing the fines passing the 0.425mm limit to 20%, provided CBR requirements are satisfied, and to revise the SSRBW-2016 gradation envelope based on further studies.