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Graded gravel is a typical filler for railroad subgrade and the procedures for designing and evaluating it are crucial. The current specifications only stipulate the physical indicators of graded gravel material groups and the mechanical indications following on-site compaction. The lack of indoor mechanical standards makes it impossible to predict the on-site construction. Furthermore, the specification specifies various varieties and vast ranges of gradation, making it difficult to construct a skeleton structure. The current specification uses the heavy compaction method (HCM) and static compaction method (SCM) for the test method of graded gravel, which are no longer compatible with the current vibration compaction molding process. In this work, the effects of grading types, compaction coefficients, and water content conditions on the mechanical properties of the subgrade surface grading gravel were studied. Then the correlation between the field and indoor mechanical indexes was investigated and the K30 predictive index of graded gravel was proposed and verified by combining it with the test section. The results show that compared with HCM and SCM, the correlation between vertical vibration compaction method (VVCM) and the field was greater than 90%. Compared with standard graded gravel, the California bearing ratio (CBR), Rc , and E0 of the strongly embedded skeleton compact gradation (VGM) graded gravel can be increased by 55%, 162%, and 171%, respectively. When the compaction coefficient increases by 1%, the CBR, Rc , and E0 can be increased by 12%, 28%, 31%, respectively. Compared with graded gravel with the optimal water content, the CBR, Rc , and E0 of dry graded gravel can be increased by 7%, 94%, and 14%. The proposed CBR1 design standard, Evd acceptance standard, and graded gravel design method can better predict the K30 index on-site. [ABSTRACT FROM AUTHOR]