The role of a track subgrade is to provide bearing capacity and distribute load transferred to lower foundation soils. Track subgrade soils are usually compacted by heavy mechanical machines in the field, such that sometimes they are attributed to progressive residual settlement during the service after construction completion of the railway track. The progressive residual settlement generated in the upper part of a track subgrade is mostly non-recoverable plastic deformation, which causes unstable conditions such as track irregularity. Nonetheless, up to now no design code for allowable residual settlement of subgrade in a railway trackbed has been proposed based on mechanical testing, such as repetitive triaxial testing. At this time, to check the DOC or stiffness of the soil, field test criteria for compacted track subgrade are composed of data from RPBT and field compaction testing. However, the field test criteria do not provide critical design values obtained from mechanical test results that can offer correct information about allowable permanent deformation. In this study, a test procedure is proposed for permanent deformation of compacted subgrade soil that is used usually in railway trackbed in the laboratory using repetitive triaxial testing. To develop the test procedure, an FEA was performed to obtain the shear stress ratio (τ/τ f ) and the confining stress (σ 3 ) on the top of the subgrade. Comprehensive repetitive triaxial tests were performed using the proposed test procedure on several field subgrade soils obtained in construction sites of railway trackbeds. A permanent deformation model was proposed using the test results for the railway track.

One of the major slope failures in Korea railways is surface failure. It is based on geological and geographical properties of Korea. There are various types of methods to protect surface slope failures. Performance for each method should be examined beforre construction and it is important to select optimum installation place. In this study, Net type Concrete Block (NTCB) is developed to ruduce surface failure of a slope and also the performance of NTCB is examined by various construction location under rainfall.

No abstract

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.

No abstract