Research On Consolidation Characteristics Of Ultra Soft Soil

During the design of foundation treatment of dredger fill, it is a crucial problemto determine the consolidation properties of ultra soft soils. A series of consolidationtests with loading in increment are carried out on the ultra soft soils before and aftersurface-layer improvement by vacuum preloading. The consolidation characteristicsand the coefficient of consolidation of ultra soft soils are investigated consideringdifferent states and directions. The results indicate that, with the increase of preloads,the structure yield stress increases, compressive indices is almost the same whenpreload is less than structure yield stress, and decreases suddenly when preload islarger than structure yield stress, then it comes to a steady value. The secondconsolidation coefficient increases firstly, then decreases, finally gets to steady withthe increase of pressure, and the second consolidation coefficient would decreaselinearly when disturbance increases, and a computational model for secondconsolidation coefficient is put forward, and the model coincides well according to theseveral testing data, and does well in predicting, which is useful for the prediction ofpost-load settlement.The content of clay of ultra soft soil is larger than general remolded soil. Unliketo the normal sedimentary remolded soils, the coefficient of secondary consolidationof hydraulic fill ultra soft soil showed a peak value as the consolidation pressure wasapplied. Load ratio had significant influence on the coefficient of secondaryconsolidation of hydraulic fill ultra soft soil; the smaller incremental load couldreduce the coefficient of secondary consolidation. The coefficients of secondaryconsolidation of ultra soft soil and normal sedimentary remolded soils were increasedwith increasing moisture content.For the feature of hydraulic fill ultra soft soil showed a peak value as theconsolidation pressure was applied, we have carried out a series of experimental studyon the secondary consolidation of structural soft soils. Simultaneously, we haveanalyzed the difference of the cause of the peak of secondary consolidation betweenultra soft soil and structural soft soils.The conventional calculating methods of secondary consolidation coefficient ofsoils do not consider the influence of structure. In this paper, the secondaryconsolidation tests of undisturbed soil and artificial structured soil has been carried out to study the influence of the structural strength on the secondary consolidationcoefficient. It is shown that the proposed calculating method of preconsolidationpressure can effectively reduce the errors of casagrande. The artificial structured soilcan well simulate the influence of structural characteristic to the consolidation of theclay. The structural characteristic of the soil can make it accelerate into the secondaryconsolidation stage. The remnant structure of the soil may conducive to the secondaryconsolidation when the load close to structural yield pressure. At the same time, it hasshown that the secondary consolidation coefficient of the soil has a strong correlationwith its structural strength. A calculating method is developed, which is useful for theprediction of secondary consolidation deformation of structural soil.