Insight into seismic earth and water pressures against caisson quay walls

Motivated by the need to explain the large displacement and rotation that numerous caisson-type quay walls suffered in the port of Kobe during the devastating 1995 earthquake, a detailed numerical analysis is presented for the response of such a wall from Rokko Island. Utilising the Pastor–Zienkiewicz elastoplastic constitutive model, an effective stress dynamic analysis is performed using as input the accelerogram recorded 32 m below the ground surface in the nearby Port Island. The evolution during shaking of lateral displacements, plastic strains and pore water pressures sheds some light on the complex interplay of several simultaneously occurring phenomena: the development of oscillatory inertia forces on the wall, in phase or out of phase with the backfill soil and water pressures; the simple-shear seismic deformation of the soil and the ensuing initial development of positive excess pore water pressures in the backfill and the foundation soil; the extensional deformation developing in the ‘active wedge’ behind the wall, with the ensuing generation of negative excess pore water pressures; and the continuous dissipation and redistribution of water pressures. The conventional generalised Mononobe–Okabe theory is also reviewed, and extensive comparisons are made with the numerically computed effective and water pressures against the wall. Finally, a surprising role of the relative density of rubble behind the wall is highlighted.