George Gazetas
Dr George Gazetas
Emeritus Professor of the National Technical University of Athens, Greece
Biography
George Gazetas has been for 30 years Professor of Geotechnical Engineering at the National Technical University of Athens, following an academic career in the US, where he taught at SUNY-Buffalo, Rensselaer (RPI), and Case Western Reserve University. His main research interests have focused on the dynamic response of footings, piles and caissons; the seismic response of earth dams and quay-walls; soil amplification of seismic waves; and soil–structure interaction problems. Much of his research has been inspired by observations after destructive earthquakes (including the Canterbury-Christchurch and Kaikoura earthquakes of N. Zealand). An active writer and teacher, he has been a consultant on a variety of (mainly dynamic) geotechnical problems. He has received a number of awards for his research contributions and teaching, including the Walter Huber Civil Engineering Research Prize from ASCE, and the Excellence in University Teaching Award from the Institute of Research &Technology in Greece. He has been the Coulomb (2009), Ishihara (2013), Mauggeri (2019) Lecturer. In March 20th, 2019, he delivered the 59th Rankine Lecture in London.
Abstract
The paper deals with rigid gravity walls of various types under strong seismic shaking. The objectives are: (a) to reveal some of the limitations of the pseudo-static methods of analysis, and (b) to investigate the main causes of the poor performance of this type of walls when they constitute quaywalls in harbours.
When retaining “dry” soil, gravity retaining walls have shown remarkable seismic resilience, and the pseudo-static Mononobe-Okabe (MO) type methods have been found to be adequate (even if somewhat conservative). The role played by the underneath “foundation” soil, not explicitly addressed in MO, is being investigated parametrically with an elaborate finite-element model subjected to the 0.6 g Takatori, 1995, and the 0.4 g Sakarya, 1999, accelerograms. Contrary to MO, it is shown that on very loose/soft foundation soil the wall suffers a substantial rotation but hardly any sliding at its base. The opposite is true when the supporting (foundation) ground is extremely stiff: the wall experiences an insignificant rotation but substantial sliding, and a well-defined MO-style failure surface develops in the backfill. The MO pressures are found to be acceptably conservative overall, even though the differences between wall response on loose and very-dense foundation soil cannot be distinguished pseudo-statically. The asymmetry of both the retaining wall and the near-fault ground motions leads to wall displacement sensitive to reversal of excitation polarity, a strictly dynamic effect.
The potentially detrimental role of water, when the system is submerged, has three principal components: the increase of the driving inertia of the backfill soil due to its saturation; the decrease of shear strength due to reduction of effective normal stresses; and the development of excess pore-water pressures which may increase the total pressures on the wall and weaken the foundation soil. To explain the mechanisms responsible for the large seaward displacement and rotation which the caisson-type rigid quaywalls have been suffering in earthquakes, a detailed numerical analysis is presented of the response of a quaywall in Rokko Island during the devastating 1995 Kobe earthquake. Utilising the basic published soil properties with the Pastor–Zienkiewicz elastoplastic constitutive model, an effective stress dynamic analysis is performed using as bottom excitation the accelerogram recorded at –32 m in the nearby Port Island. Residual lateral displacement of the wall, settlement of the ground behind it, and heave of the seabed in front of the wall toe, are well predicted by the model; rotation is substantially underpredicted. The evolution during shaking of excess pore water pressures reveals a complex interplay between several, simultaneously occurring, competing mechanisms: (i )the development of oscillatory inertia forces on the wall, in-phase or out-of-phase with the retained soil; (ii) the simple-shear deformation of the soil from the vertically incident shear waves and the ensuing development of positive excess pore water pressures in the soil; (iii) the extensional deformation developing in the active wedge of the retained soil, with the ensuing generation of negative excess pore water pressures; (iv) the weakening of the foundation soil leading to mobilisation of a bearing capacity failure mechanism; and (v) the dissipation and redistribution of water pressures during and immediately after shaking. The significance of the negative excess pore-water pressures in the retained soil and the subsequent small contribution of the soil thrust in the performance of the wall are emphasised. By contrast, the wall inertia plays a major role, to the point that enlarging the width of such gravity walls would not necessarily lead to better seismic performance.
