Recently, ballastless tracks with cement grouted into ballast have been constructed on a full-scale construction operation. However, where roadbed constituents are of clay, roadbed soil outflow or even voids in the roadbed are likely to occur after heavy rainfall. In this study, the authors carried out tests with a full-scale ballastless track model laid on a saturated clayey roadbed. As a result, the defect mechanism of clayey roadbeds under ballastless tracks has been clarified. In addition, the authors have confirmed the improved effect of two countermeasures against roadbed failures by means of tests using the fullscale model.