Dredge materials with the water content more than 80% have been used for land reclamation in Tianjin, China. The dredge is transferred by pipeline to the reclamation place and then strengthened with the vacuum preloading technique. It has been found that with the thickness of the filled dredge layer becomes larger and larger, the quality of soil improvement is not as good as expected. Field investigation shows that after vacuum preloading the vane test strength can reach 50kPa at the ground surface, but decreases along with depth dramatically. The model test is carried out to detect the factors that may influence the efficiency of using vacuum preloading to improve dredge soil foundation. The water content, vacuum pressure and undrained strength are measured during the test. Test results indicate that the undrained strength decreases and the water content increases along with the depth below the ground surface and the distance away from the PVD even at the time when the consolidation degree should have reached 99% according to the conventional consolidation theories. The excavated soil profile shows that the PVDs undergo drastic distortion because of the large deformation of the dredge soil surcharged by vacuum pressure, which will decrease the drainage efficiency of the PVDs.