In general, under the repetitive dynamic load generated by rail cars running on the track, subgrade soil experiences changes of stress conditions such as deviatoric stress (σ d ) and bulk stress (θ). Due to the repetitive change of deviatoric stress (σ d ) with number of loadings, the resilient modulus (M R ) can be obtained by using the measured resilient strain (ε r ) after a sufficient number of loadings. At present, no plausible and unified test method has been proposed to obtain the resilient modulus of railway track subgrade soil. In this study, a possible test method for obtaining the resilient modulus (M R ) of railway track subgrade soil is proposed; this test, by utilizing repetitive triaxial compression testing, can consider all the important parameters, such as the confining stress, deviatoric stress, and number of loadings. By adapting and using the proposed test method to obtain M R , M R values for compacted track subgrade soil can be successfully determined using soil obtained in three field sites of railway track construction with changing water content range from OMC. In addition, shear modulus (G) ~ shear strain (γ) relation data were also obtained using a midsize RC test. A correlation analysis was performed using the obtained G and M R values while considering the strain levels and modes of strain direction.

The soils used as trackbed in Korea are selected using USCS utilizing basic soil properties such as Grain Size Distribution(GSD), % passing of #200 sieve (P 200 ), % passing of #4 sieve (P 4 ), Coefficient of uniformity (C u ), and Coefficient of curvature (C c ). Degree of compaction of the soils adapted in the code by KR should be evaluated by maximum dry density (γ d-max ) and deformation modulus E v2 . The most important influencing factor that is critical to stability and deformation of the compacted soils used as trackbed is stiffness. Thus, it is necessary to construct a correlation between the modulus and the basic soil properties of trackbed soil in order to redefine a new soil classification system adaptable only to railway construction. To construct the relationship, basic soil test data is collected as a database, including GSD, maximum dry unit weight (γ d-max ), OMC, P 200 , P 4 , C u , C c , etc.; deformation modulus E v2 and E vd are obtained independently by performing a Repeated Plated Bearing Test (RPBT) and Light Weight Deflectometer Test (LWDT) for ten different railway construction sites. A linear regression analysis is performed using SPSS to obtain the relationship between the basic soil properties and the deformation modulus E v2 and E v . Based on the constructed relationship and the various obtained mechanical test data, a new soil classification system will be proposed later as a guideline for the design and construction of trackbed foundation in Korea.