Experimental study on the road performance of reduced density fly ash–clayey sand subgrade mixtures from the Yellow River floodplain

To address challenges in subgrade construction using Yellow River floodplain soils, this study developed a reduced density filler by mixing aged fly ash with locally sourced clayey sand. The research aimed to establish a robust framework for bridging the gap between pavement design specifications (resilient modulus, E 0 ) and construction quality control (compaction degree, K ), enabling performance-based quality assurance. Laboratory experiments evaluated compaction characteristics, California Bearing Ratio (CBR), and Laboratory Static Resilient Modulus ( E 0 lab ) of the mixtures. Results showed optimal performance at 30%–36% fly ash content, achieving maximum CBR of 23.6% and E 0 lab of 54.56 MPa. A novel global regression model was established, directly linking CBR to both K and fly ash content ( FA% ), offering a powerful tool for construction quality assurance. Furthermore, a practical pathway was developed to convert E 0 lab to field design E 0 (via CBR and existing correlations), facilitating the translation of laboratory findings into engineering design. This research culminates in a unified framework for construction quality control, providing recommended K and FA% acceptance windows to guarantee target E 0 and CBR values. The developed mixtures are highly suitable for Class II (lower layers) and Class III/IV (top layers) highway subgrades. This study offers robust technical support for sustainable fly ash utilization and performance-driven subgrade construction.