Recently, ballast-less tracks with cement grouted into ballast have been constructed on a full-scale operation. Where roadbed constituents however are of clay, outflow of roadbed soils or even voids in the roadbed are likely to occur after heavy rainfalls. In this study, authors carried out tests with a substantial scale model of ballst-less track set on a saturated clayey roadbed. As a result, the mechanism of the clayey roadbed defects under ballast-less tracks has been clarified. In addition, authors have confirmed the improved effect of two countermeasures against roadbed failures by the tests with substantial scale models.

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