Following the rapid expansion of offshore wind farms in seismic areas, this study examines the hurdles encountered when applying conventional seismic evaluation methods, originally devised for onshore structures, to offshore installations. This includes the assessment of liquefaction offshore at large depths and its consequences on the response of offshore wind turbines supported by monopile foundations. With the aid of 3D dynamic finite element analysis of the entire SSI system (tower, monopile foundation and soil domain), it is shown that the resonant frequencies of the examined 5MW turbine were excited for the considered ground motion, inducing significant nonlinearity in the soil surrounding the monopile foundation. The vertical seismic motion, often overlooked in seismic design, is also discussed as it bears significance for the response of offshore wind turbines. Simple site response analysis for vertical ground motion emphasizes the need to consider the entire water column and soil profile depth to the bedrock for an accurate representation of the soil-water system's compression natural frequency in offshore environments.

Soil liquefaction is a subject of long-standing interest in earthquake geotechnical engineering. Although significant advances in liquefaction research have been achieved in the past decades, it remains an area of great difficulty and uncertainty, as evidenced by the extensive liquefaction-related damage in many recent earthquakes. Of particular concern is the widespread liquefaction observed in silty sand deposits, raising questions about the deficiencies of the current methods for liquefaction evaluation. This paper presents selected results of a long-term research aimed at developing a rational method for evaluating the liquefaction potential of both clean and silty sands. The method is based on a comprehensive experimental program comprising small-strain shear wave testing and large-strain undrained shear testing for a wide range of sand states and is built in the critical state framework. A remarkable feature of the method is the unified characterization of shear wave velocity for both clean and silty sands through a state parameter defined in a sound theoretical context. As shear wave velocity is a well-defined soil property and can be measured both in the field and in the laboratory, and since the state parameter has proven a useful state variable for characterizing soil behavior under both cyclic and monotonic loadings, the new method is attractive and promising in a wide range of geotechnical applications.

The existing engineering methodologies for liquefaction mitigation rely on free-field triggering in uniformly layered granular soil deposits. These methods routinely ignore cross-layer interactions in stratified deposits, consequences of softening and various mechanisms of mitigation on building performance, or interactions between and among structures in close proximity of each other. In this paper, through an experimental-numerical study, we show that these methods are unreliable, jeopardizing our ability to assess and mitigate liquefaction vulnerability from building to cluster, and to community scales. Fully-coupled, 3D, dynamic finite element analyses, validated with centrifuge experiments, show that combining ground reinforcement with drainage and densification (e.g., through installation of dense granular columns) can improve foundation’s settlement, but not necessarily to acceptable levels. To achieve desired levels of reduction in settlement, it is critical to minimize the likelihood of clogging in such drains, particularly in the presence of silt interlayers. These methods, however, may increase foundation’s tilt potential, which must be evaluated on a case-by-case basis. Unsatisfactory tilt is often uneconomical to repair, which may lead to the decision to demolish or relocate. And this engineering demand parameter (EDP) becomes particularly difficult to improve in urban settings and in stratified and non-uniform deposits. The combined influence if seismic coupling and stratigraphic variability on mitigation efficacy is shown to be significant in terms of foundation tilt, spectral accelerations, and flexural drifts experienced within the superstructure of both mitigated and unmitigated neighbors. These effects are notable for spacing-to-foundation width-ratios (S/W) as large as 1.0, which are common in cities. Additional measures and technologies may be needed to reduce tilt to acceptable levels in closely-spaced cluster configurations and realistically stratified deposits, while simultaneously strengthening both the ground and structures at an area-level and in a cost-effective and sustainable manner.