Abstract: The suction caisson foundation can be used as the anchoring foundation of the tension leg platform(TLP), which is mainly affected by the uplifting load. The pullout failure mode is the key to analyze the uplift bearing capacity of the suction caisson foundation. But studies on the mechanism of the ultimate uplift bearing capacity are still few and not thorough enough. In this paper, the uplift bearing capacity of the suction caisson foundation are sutided based on by a series of laboratory model tests. Two uplift failure modes, including the top and bottom tension failure mode, are derived. The test results show that when the top tension failure mode occurs, the negative pressure at the top and bottom of soil plug is approximately equal. The bearing capacity increases with the increase in the negative pressure at the top of soil plug. In addition, the uplift bearing capacity of caisson is made up of its self-gravity, the frictional force of inside and outside wall and the tensile force at the top of caisson. When the bottom tension failure mode occurs, the bearing capacity increases with the increase in the negative pressure at the bottom of caisson. In this condition, the uplift bearing capacity of suction caisson foundation is made up of its self-gravity, soil plug gravity, the frictional force at outside wall and the tensile force at the bottom of caisson. When the load reaches the ultimate bearing capacity, the negative pressures both at the top and bottom of soil plug are less than the undrained shear strength Su and the reverse bearing capacity coefficients at the top and bottom of the caisson are both less than 1.0. Considering the top negative pressure in the caisson, the calculation method of the uplift bearing capacity of the suction caisson is proposed, and the uplift bearing mechanism of the suction caisson foundation in soft clay is revealed. These can provide a reference for the analysis of the uplift bearing capacity and engineering design of the suction caisson foundation.

Key words: suction caisson foundation, uplift bearing capacity, soft clay, failure mode, negative pressure