Consolidation property of ultra soft soils before and after treatment of surface-layer improvement by vacuum preloading

During the design of foundation treatment of dredger fill, it is a crucial problem to determine the consolidation properties of ultra soft soils. A series of consolidation tests with loading in increment are carried out on the ultra soft soils before and after surface-layer improvement by vacuum preloading. The consolidation characteristics and the coefficient of consolidation of ultra soft soils are investigated considering different states and directions. The test results show that the coefficient of consolidation remaines at a low level under lower stress levels. With the increasing consolidation pressure, the coefficient of consolidation has an increase, but the increment ratio decreases gradually. The ultra soft soil is different from the normal sedimentary ones, resulting in that the ratio of vertical to radial structural yield pressures of the ultra soft soils is significantly greater than that of the normally sedimentary soils. When the stress exceeds the structural yield pressure of the ultra soft soils, the radial coefficient of consolidation has no significant difference from the vertical one, and this characteristic has a large difference from that of the normally sedimentary soils.