Samples of the fill (coal mine waste) used in the construction of the Ozuki by-pass were immersed in beakers of water for 15 and 30 day periods. After air drying, the particle size of the samples was measured and test of their liquid and plastic limits were carried out. The results of these tests and of the particle size measurements are reported. Immersion in water resulted in a 1.5% increase in clay content and an increase in the liquid limit. Likewise, the plasticity index increased from 4.3 to 8.3.