Abstract: When a gravity structure, such as a caisson or a jackup spudcan, is pulled out from the saturated soft clay foundation, a negative pore pressure will be developing, which may greatly increase the pullout force. Therefore, it is very important to estimate the negative pore pressure developed in the soil during the unloading process. The development of excess pore pressure is studied based on the triaxial extension tests under the isotropic consolidation conditions. By combining the modified Cambridge model with the Henkel’s approach, the mechanism of negative pore pressure generation is revealed and a method is developed to predict the magnitude of the negative pore pressure. Meanwhile, the development rule of pore pressure under the isotropic consolidation conditions is obtained and explained why there will be a change of pore pressure from negative to positive. The research indicates that under the unloading conditions, the generalized shear stress induces positive pore pressure and the averaged principal stress generates negative pore pressure in the soil samples. The superposition of these two effects results in negative pore pressure in the initial unloading stage, and then becomes positive with the increase of strain. The research results can be used to predict the required force to pull out a structure from saturated clay foundation.

Key words: saturated soft clay, uplift force, negative excess pore pressure, modified Cambridge model, triaxial extension test