The vibrofloation method without gravel backfill was only thought to be proper for treatment of medium and coarse sand with less than 10 % fine particles. Almost no project case available is concerning vibrofloation without backfill treatment of silty and fine sand. A laboratory test was then carried out to achieve the relating characteristics of the hydraulic silty and fine sand .In this test a reduced scale prototype model was set up

in which pressure gauges were embedded. From the data acquired during and after vibroflotation

the pore water pressure development and dissipation functions are established. The spatial distribution of pore pressure among soil was also studied. The cone penetration test(CPT) was made after each vibroflotation to discover the relationships among CPT

relative density and water pressure. Furthermore

the conclusions obtained from test were then used in guiding a ground improvement project in Shanghai.