Special attention must be paid to the fill construction where coal ashes are used because they have particular geotechnical properties.
Coal ashes are sandy coarse silt and have no plasticity. They have such small specific gravity as 2.1. Compacted coal ashes have strong shearing resistance at the optimum moisture content, but loose coal ashes have the risk of liquefaction when they are saturated.
In Hisasue valley in Kawasaki city, an extensive flow slide of fill construction where coal ashes were used occurred in June, 1965. The fill consisted of loose dumping ashes and it collapsed just when it had been saturated by the seeping groundwater to the critical state.
In Hodogaya in Yokohama city, a large fill construction has been built with coal ashes with special attention paid to the drainage of groundwater and to the compaction of fill material. The work has proved a success.