In August 2009, the Suruga Bay earthquake caused the collapse of an embankment on the Tomei Expressway, Japan. According to later investigation, the main cause was the embankment materials with slaking property became finer due to the seepage of rainwater and groundwater, and the embankment caused a decrease in strength. However, since it has been known that the fine granulation of the embankment materials by slaking will be completed in about 10 years after the construction, in actual, the collapsed embankment has been in service for 40 years. The authors do not evaluate that the fine granulation is the only cause of the collapsed embankment. Instead, we focused on the fact that the water content increased with the granulation, and confirmed the effect of the increase in water content on strength behavior by experiments. The results indicated that the reduction in strength of embankments made of slaking materials depends not only on the fine granulation of the embankment materials, but also on the increase in water content.