Urban infrastructure and underground structures are currently suffering from progressive deterioration and aging. In urban areas, the frequency of localized torrential downpours and the amount of damage have increased. Examples of damage observed throughout the country are seepage failure, piping, and depressions. Onsite investigations of depressions and seepage failure due to piping have revealed that, in addition to muddy water seepage, loosening of soil and narrowing of particle distribution are major causes of local failure. For these phenomena, an important task is to elucidate the effects of internal erosion due to fine particle movement on local deformation and failure.