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.