Abstract: Spudcan foundations are preloaded to penetrate into the seabed to provide sufficient bearing capacity to bear the vertical loads from the upper platform and the horizontal loads from ocean environments. A cavity will be formed above the foundation, which affects the penetration resistance of the spudcan foundation. The theoretical solution to the resistance factor N_c for spudcan foundation penetrating into clay was deduced based on the characteristic methods by considering two extreme conditions with and without fully localized back-flow. Meanwhile, finite element limit analyses (FELA) were conducted to verify the proposed failure mechanism of the spudcan foundation, and centrifuge tests and numerical simulations were used to verify the presented theoretical results. It is found that two failure wedges are formed below the base of the spudcan foundation, and its volume decreases with the increasing shear stress at the base of the spudcan foundation. Resistance factor N_c increases almost linearly with the increasing of the roughness factor between spudcan and soil. Facter N_c under the condition of no fully localized back-flow is 20% lower than that under the condition of fully localized back-flow.