1. [1] Japanese National Railways, 1978, Design standards for structures and commentary, Soil structure (in Japanese).
2. [2] Railway Technical Research Institute, 2012, Design standards for railway structures and commentary, Soil structure (in Japanese).
3. [3] Itou, K., Momoya, Y., and Muramoto, K., "Strength characteristics of the railway roadbed improvement materials reusing deteriorated ballast," JGS, 47th Japan National Conference of Geotechnical Engineering, pp.1451-1452, 2012 (in Japanese).