Unloading Response of Decomposed Granite and Its Implications for Borehole Stability During Offshore Monopile Installation

Borehole instability in weathered granitic seabeds complicates the installation of rock-socketed offshore monopiles. Previous studies have examined construction-related instability and granite disintegration; however, the mechanisms governing borehole instability and its evolution following excavation in weathered granitic seabeds remain incompletely understood. This study combines laboratory testing, constitutive modelling, and coupled finite element analysis to investigate unloading-induced borehole instability. A unified elastoplastic model incorporating soil structure and overconsolidation was calibrated separately for completely and highly decomposed granite (CDG and HDG) using oedometer and drained triaxial tests. The calibrated parameters indicated slower structural degradation and faster loss of overconsolidation in CDG than in HDG. The model captured the principal drained unloading response trends of HDG, including pronounced axial strain accumulation as effective confinement approached zero. The coupled analysis predicted a maximum inward wall displacement of 12.43 mm seven days after unloading, concentrated within the unsupported HDG. Increasing the unsupported length from 4.17 to 8.37 m increased the predicted maximum displacement from 10.30 to 13.72 mm. These findings connect the measured sensitivity to confinement loss with borehole deformation and support limiting unsupported length during monopile installation.