Abstract: Two trial test blocks were selected for the investigation of drawdown effect on the soil soft improvement by the dynamic compaction method in a highway project in Shangyu area. The groundwater was lowered to different depth by the vacuum dewater method. In order to compare the dynamic compaction effect on grounds with different water levels, the development and dissipation of the excessive pore water pressures as well as the ground settlement after the dynamic compaction were monitored. The results indicate that, under the higher groundwater condition, it is more difficult for the dissipation of the excessive pore pressures, and the smaller ground settlement under the unit compaction energy. With the deeper dewatering, a significant soil improvement effect can be found within a horizontal radial distance of six (6) meters and the depth of improvement was also increased.

Key words: Vacuum Dewatering Combined with Dynamic Compaction, Different Drawdown, Trial Tests, Excessive Pore Water Pressure Dissipation, Improvement Depth