Investigation and modeling of seepage-influenced caisson-soil interface monotonic shearing

Suction caissons with various aspect ratios are widely employed as foundations for fixed and floating offshore structures. During storm-induced rapid uplift, negative pressure may accumulate beneath the caisson lid, generating seepage effects that reduce the soil effective stress inside the caisson while increasing it outside. These variations further modify the normal stress acting along the skirt wall. To investigate the resulting caisson-soil interface response, monotonic sand-steel shear tests were conducted under varying normal stress that are equivalent to those induced by seepage during caisson uplift. The tests revealed distinct strain-softening behavior, with the peak and critical friction angles showing minimal dependence on the magnitude or history of normal stress. Guided by these findings, a monotonic t-z model was developed to represent seepage-influenced interface shearing by incorporating both varying normal stress and strain-softening behavior. With parameters calibrated from constant normal load tests, the model reproduced the shear response under different stress paths and captured the seepage-induced reduction in skirt wall friction during caisson uplift.