This paper presents the unloading behaviors and properties of typical Shanghai soft soil based on two types of long-term unloading tests. The triaxial test results indicate that behaviors of Shanghai soft soil have a close correlation to stress history and stress history and are different than the conventional test result. The unloading shear stress is relative low when viscoplasticity deformation of soil occurs. The time was short for the last failure stage. When the deviational strain is about 0.01% the Young's modulus dropped by 70%. Difference of normalized Young's modulus of soils vanished when strain was less than 0.1%. The fitting line of effective stress to axial strain of these three specimens is a function of three order exponential decay. The oedometer test results showed that the consolidation coefficient varies with stress level and focuses at 0.02 to 0.06. The ratio of main consolidation coefficient to long-term consolidation coefficient varies with stress level. Time is a very important factor in the soil properties due to unloading.
Stress paths for a soil around a retaining structure in deep excavations are very complex. Soils will undergo durative and stepped unloading in excavations. The behavior and properties of soft soil in this process are different than the conventional test results. The behaviors of soft soil in unloading could not be described properly by conventional tests. However, this behavior affects the properties and accuracy in structures design and construction of excavation. So the unloading tests of soft soil were fulfilled to reveal some soil properties due to longterm unloading. These specimens are typical Shanghai soft soils. Deformation of soft soil is complex and related to the stress paths and stress history. The stress-strain characteristics of soft soils are timedependent also. Obtaining the soil properties by laboratory test is a necessary and useful.
