Abstract: The application of vibroflotation without backfill in the improvement of liquefiable silty and fine sand at a power plant ground treatment is presented.On the basis of vibroflotation strengthening principle and the rising mode of vibration pore water pressure,the analytic solution of pore water pressure is deduced from the theoretical formula,and the sand critical liquefaction condition is presented.In view of the monotonic fluctuation function,the analytic solution is detached as monotonic function and fluctuation function and solved by the graphical method to get the exact solutions of sand critical liquefaction time.The paper takes the critical liquefaction time as the time to get the best density of sand and then optimization design for the vibroflotation load time can be made on the basis of the critical liquefaction time.The underlying mechanism of pre-vibration effect is also discussed,which is thought to be useful to improve the microstructure of sand.Finally,the suitability and effectiveness of vibroflotation without backfill on silty and fine sand are dicussed.