Fully-Coupled cyclic time-history analyses of monopile foundations in sand

Sneak peek 📢 ! Check out our contribution to the Special Issue of Ocean Engineering: "State of Practice – Geotechnical Considerations for Offshore Wind" in the link below (limited free access until April 19th).

The paper presents the results of 3D coupled cyclic time history numerical analyses of a monopile supporting a 12 MW Offshore Wind Turbine, installed in dense cohesionless soils and subjected to a 600-s load history corresponding to the high phase of a 35-h design storm. The goal of the study is to investigate the governing mechanisms and gauge potential conservatisms or uncertainties in approaches for monopile analysis used in practice. The Ta-Ger constitutive model, implemented in FLAC3D and calibrated against site-specific cyclic tests, is used to model the complex soil response. Emphasis is placed on the effect of drainage conditions, an aspect typically overlooked in practice, although often stated as critical. Analyses show that the drainage of the system can substantially affect the response. In low-permeability soils (e.g., cohesionless soils with low-plasticity fines) widespread liquefaction may occur inducing high rotations above allowable limits. On the contrary, systems that can drain effectively within each cycle, develop moderate excess pore pressures which do not jeopardize performance. Current design procedures are often unable to accurately capture these effects possibly leading to either conservative or unconservative outcomes. Suitably validated advanced numerical analyses can be used as complementary tools to standard methods to assess these uncertainties.

Blind predictions of LEAP 2020 centrifuge tests of a sheet-pile wall with liquefiable backfill

Panagiota Tasiopoulou, Yannis Chaloulos, Amalia Giannakou and Jacob Chacko have co-authored a paper titled: “Seismic performance of sheet-pile wall supporting liquefiable backfill: Blind predictions of centrifuge model tests using Ta-Ger constitutive model” which has been published in Soil Dynamics and Earthquake Engineering. In this paper, blind numerical simulations of seven centrifuge tests with “identical” models of a sheet-pile wall supporting medium dense liquefiable backfill and subjected to seismic excitation were performed as part of the LEAP 2020 project. The analyses were conducted with FLAC using the Ta-Ger soil constitutive model. The Ta-Ger model parameters were calibrated against available laboratory DSS data on Ottawa sand with similar relative density with a focus on capturing at an element level: i) the liquefaction triggering resistance, ii) the post-liquefaction rate of shear-strain accumulation, iii) overburden effects on liquefaction triggering resistance and iv) realistic shear stress-strain responses. A single numerical model was built in prototype scale and analyses were performed by only varying the input seismic motion recorded at the base of each of seven centrifuge tests.

Comparisons of numerical predictions with measured centrifuge test responses indicate that the analyses successfully capture the primary mechanisms of the system response. These included liquefaction in the free-field and development of negative pore pressures behind the wall, with accurate predictions of outward wall displacement for the majority of the tests. Most centrifuge tests and numerical predictions consistently exhibit a systematic linear trend of increase in wall displacements with spectral acceleration at the predominant frequency of the system. The numerical analyses overpredicted wall displacements only for centrifuge tests not following this trend, indicating that the variations maybe due to experimental variations from their specifications that were not considered in the blind predictions.

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3D analyses of dynamic LNG tank performance on liquefiable soils improved with stone columns

Yannis Chaloulos, Panagiota Tasiopoulou, Takis Georgarakos, Amalia Giannakou, Jacob Chacko and StéphanUnterseh have co-authored a paper titled: “3D Effective stress analyses of dynamic LNG tank performance on liquefiable soils improved with stone columns” which has been published in Soil Dynamics and Earthquake Engineering. In this paper, the seismic performance of an LNG tank on liquefiable soil improved with stone columns is assessed through 3D effective stress time history analyses. The numerical methodology, which is validated against centrifuge tests, uses the Ta-Ger model to capture the complex sand response under earthquake loading while it simultaneously accounts for Soil-Foundation-Structure interaction by incorporating tank inertial response and both convective and impulsive hydrodynamic actions through a system of properly calibrated oscillators. The methodology introduces a novel technique for incorporating the "improvement" effect of the stone columns by means of "equivalent" soil properties calibrated through separate analyses of a representative remediated 3D soil "cell" explicitly simulating the stone columns within the soil. The analyses show that the presence of the

stone columns substantially reduces shear deformations in the improved soil. During shaking, tank settlements accumulate through a rocking, downwards ratcheting, mechanism, which is however constrained by the formation of a stable, nonliquefied soil zone below the tank. Post-seismic, reconsolidation settlements, primarily originating from unimproved zones below the stone columns, are also assessed. Once the complex soil behavior- and dynamic load- mechanisms are comprehensively addressed, differential settlements remain well below allowable limits, demonstrating adequate structure performance even under the high design shaking levels.

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