Effects of Strain Softening and Strain Rate on Penetration of Stiffened Suction Caisson into Nonhomogeneous Clay

Internal ring stiffeners improve the structural stability of suction caissons but alter soil flow during installation, complicating the assessment of penetration behavior in clay. This study investigates the effects of strain softening and strain-rate dependency on the mechanically jacked penetration of ring-stiffened suction caissons into nonhomogeneous clay. Three-dimensional large-deformation finite element analyses are conducted using the coupled Eulerian–Lagrangian method in ABAQUS (2020), incorporating a strain-softening and rate-dependent soil strength formulation. The numerical model is validated against published centrifuge test results, and parametric analyses examine the effects of soil sensitivity, relative ductility, caisson diameter, soil stiffness-to-strength ratio, and penetration rate. The results show that the strain-softening zone extends approximately twice the skirt thickness inward and 0.6 times the skirt thickness outward from the caisson wall. Increasing soil sensitivity and decreasing relative ductility intensify strength degradation and promote earlier soil backflow around the stiffeners. The strain-rate effect is concentrated mainly near the skirt tip and partially counteracts softening, yielding a combined soil strength factor of approximately 0.54 under representative conditions. Within the investigated ranges, caisson diameter and soil stiffness-to-strength ratio have limited effects on the strain-softening coefficient. These findings clarify the spatial interaction between soil strength evolution and stiffener-induced soil flow and support the interpretation of caisson penetration behavior under jacking conditions.