Abstract: The undrained unloading tests of isotropic consolidation on soft dredger fill of Tianjin Binhai are performed using the stress-path triaxial apparatus to determine the effects of different unloading stress paths and unloading rates on stress-strain relationship, pore pressure variation and failure strength characteristics. Some findings are as follows. The stress-strain curves under each unloading path are approximately hyperbolic curves. Under the unloading stress path of UU0.0(radial unloading, axial not unloading), the deformation of samples exhibits axial compression, and the curves of pore pressure have sharp yield points. On the contrary, the deformation of samples under the path of UU2.0(both axial and radial unloading), UU∞(axial unloading, radial not unloading) and UL1.0(axial unloading, radial loading) show axial tension. In this way, the pore pressure increases with the increase of strain, and finally the growth rate of the pore pressure slows down and tends to increase steadily. Under the same stress paths, the larger the unloading rate, the slower the development of the pore pressure in the initial stage of unloading and the greater the peak pore pressure. The stress-strain curves at unloading rate of 0.1, 0.2 and 0.3 kPa/min are found that the initial tangent modulus is greatly affected by unloading rate under unloading compression paths, while it is not influenced under the tensile paths. The value of unloading failure strength is the maximum under the UL1.0 path, the minimum in the UU2.0 path, and centered in the UU∞ path. Under the same unloading path, the failure strength increases with the increase of the unloading rate. According to the normalization of stress-strain curves, the formulas, which considers the influence of unloading rates and unloading stress paths, are developed to estimate the initial tangent modulus and the unloading failure intensity.

Key words: soft dredger fill, unloading stress path, unloading rate, stress-strain, initial unloading modulus, unloading failure strength