Lateral constraint release-induced degradation of vertical bearing capacity for cutting edge in clay: A semi-analytical model for symmetric geometry boundary alteration

Large open caissons are widely used in long-span bridges. During open caisson sinking, staged excavation on both sides of the cutting edge progressively relieves lateral constraint, yet the quantitative relationship between this constraint release and the degradation of vertical ultimate bearing capacity (UBC) remains poorly understood. Based on classical bearing capacity theory, this study employs the Coupled Eulerian-Lagrangian (CEL) large-deformation method to investigate the excavation-induced bearing capacity degradation, and a semi-analytical model for symmetric geometry boundary alteration is proposed. The results show that the UBC decays linearly with excavation depth: each 1 m depth increase reduces UBC by 10–25%. Reducing the retained width from 5 m to 1 m accelerates the degradation by 60–85%. For the cutting-edge geometry, the UBC decreases linearly with the cosine of the inclination angle, and the decay rate increases with edge height. The proposed model is validated against field measurements from a large open caisson: the predicted sinking factor of 1.20 corresponds to a steady sinking state, which is consistent with the observed steady sinking on that day (field settlement = 50 cm). This study quantifies the effect of lateral constraint release on vertical UBC degradation in clay.