When applying a preload embankment to prevent residual primary consolidation settlement of a soft clay layer, it is necessary to decide, based on data from on-site measurements, if the preload has been left long enough to allow removal. One method of making this decision is to estimate the value of excess pore-water pressure. The authors observed from experience that this reduction of excess pore-water pressure exhibits a ‘half-life’ characteristic. From this characteristic it is possible to predict future reductions in excess pore-water pressure. The groundimprovement works at the liquid natural gas facility of the Kawagoe Thermal Power Station (Chubu Electric Power Co., Inc.) is an example of the application of this method for determining the time required to remove a preload under actual on-site conditions. As a result of applying this method a site was prepared that was free of residual consolidation settlement.