Abstract: As the engineering practice indicated that the rheological behavior is obviously under unloading of the soft clay. According to the typical pit unloading paths, a series of unloading rheological tests were done with stress-control triaxial devices under different unloading conditions. The unloading conditions include confining stress, unloading stress paths and drainage conditions. Through experiments, the unloading rheological deformation of soft clay can be divided into instantaneous rebound deformation and hysteretic rebound deformation. When confining pressure and axial pressure unloaded under the same time, the initial strain rate is relatively larger on a short time after unloading; at the same condition, the hysteretic rebound effect is not obvious because the soil have negative pore pressure. When the axial pressure unloaded, the hysteretic rebound effect is more obvious. At the same unloading path, the rheological properties are more obvious under drainage condition because of that the soil makes water swelling, and the negative pore pressure will eventually be dissipated. Based on the test results, the linear function between the Merchant rheological model parameters and unloading conditions is established; and then established the soft soil unloading rheological empirical model which can consider the effects of the confining pressure and the unloading path. These conclusions are very useful to research the soft clay rheological deformation model and the calculation of excavation unloading rheological deformation.

Key words: soft clay, foundation pit, unloading, rheological test, model parameters