Numerical investigation of steel-casing opening design for penetration through riprap in a macro-tidal estuarine mudflat

The installation of steel casings in riprap-covered macro-tidal estuarine mudflats is governed by a complex interplay of soil, tidal and pore-water pressures, frequently causing the casing to refuse to penetrate or to deviate from its design alignment. Perforating steel casings can effectively mitigate the adverse pressure build-up described above, yet it simultaneously compromises casing strength and stability. Taking the steel casings of a typical Chinese seawall project as the reference, this study employs finite-element modelling to simulate the entire installation process and quantify how varying perforation patterns alter the internal forces and stress-concentration factors at the openings when the casing is driven through riprap-armoured macro-tidal estuarine mudflats. The results indicate that perforations lower the casing's global shear and bending capacity, which in turn loosens the soil plug at the tip and markedly reduces penetration resistance. This reduction in resistance, however, amplifies both the horizontal and vertical displacements generated when the casing strikes riprap blocks. Under dynamic impact and riprap-reaction forces, the stress-concentration curves for openings situated closer to the load becomes flatter and the corresponding factor diminishes, signifying a more uniform stress distribution over the cross-section. Moving the perforation away from the casing head progressively raises the stress-concentration factor and intensifies the local peak. Star-shaped perforations yield markedly smoother stress-concentration curves than either single- or twin-directional patterns. These findings furnish a quantitative basis for selecting perforation geometry and placement when driving steel casings through riprap armour in macro-tidal estuarine mudflats.