GR8 GEO participated in 2020 Liquefaction Experiments and Analysis Projects (LEAP-2020) research program by performing Type-B (blind) numerical simulations for seven (7) of the centrifuge tests using the Ta-Ger constitutive model implemented in FLAC. to predict the seismic response of a sheet-pile retaining structure supporting liquefiable soils. The present paper presents the Type-B predictions in comparison with the experimental results. Calibration of the Ta-Ger model based on available laboratory DSS data focused on capturing the liquefaction resistance for a large range of cycles, the post-liquefaction rate of shear-strain accumulation, the overburden effects and realistic shear stress-strain responses. A single numerical model was built in FLAC in prototype scale in accordance with the design geometry of the LEAP 2020 program Numerical analyses were performed varying only the input seismic motion applied to the model. Numerical analyses captured the primary mechanisms of the system response such as liquefaction in the free-field, negative pore pressure behind the wall accompanied by the outward rotation of the wall.

Our team had the closest predictions for the largest number of centrifuge tests compared to the other program participants (a total of 10 teams from around the world participated in the numerical modeling exercise using more than 6 different constitutive models). A paper in the LEAP 2020 Workshop Proceedings will soon be published. Stay tuned!


Panagiota Tasiopoulou, Yannis Chaloulos, Nikos Gerolymos, Amalia Giannakou and Jacob Chacko have co-authored a paper titled: “Cyclic Lateral Response of OWT Bucket Foundations in Sand: 3D Coupled Effective Stress Analysis with Ta-Ger Model” which has been published in Soils and Foundations. In this paper, the multiaxial sand constitutive model Ta-Ger (Tasiopoulou and Gerolymos, 2016a,2016b) implemented in the finite deference code FLAC3D is employed in the analysis of the lateral response of bucket (skirted) foundations subjected to wind/wave loading. The model has been adjusted to reproduce the cyclic response of sand for undrained, fully drained and the intermediate conditions, using a unique set of calibration parameters. Numerical validation is conducted against centrifuge test including a bucket foundation in dry sand, by building a 3D numerical model.

In order to gain qualitative insights into the effect of drainage conditions, the same problem used for validation was analyzed under saturated conditions and a range of soil permeabilities. It was shown that when flow is allowed the response up to a cycle threshold resembles that of fully drained conditions. Increasing the permeability delays the occurrence of liquefaction and the associated development of large deformations. You can find it online here!


Panagiota Tasiopoulou, Jacob Chacko, Yannis Chaloulos, Amalia Giannakou and Nikos Gerolymos have co-authored a paper titled: “Insight into the Cyclic Response of OWT Pile Foundations in Sand: Numerical Simulation of PISA Field Tests” which has been published in the Proceedings of the 4th International Symposium on Frontiers in Offshore Geotechnics (ISFOG 2020) in Austin, Texas. This paper presents numerical simulations of selected field tests of monopiles in Dunkirk sand conducted as part of the PISA program, in the finite difference code FLAC 3D using the Ta-Ger constitutive model for sand. The simulated tests included medium diameter piles which were subjected to both monotonic and cyclic lateral and overturning loading. Comparison between numerical results and test measurements show that the simulations can reproduce the basic mechanisms of the monopile/soil system response, under both monotonic and cyclic loading. Features reproduced in the simulations include the increase of the system stiffness during cyclic loading and the associated decreasing rate of accumulation of lateral displacement and rotation. Lateral soil support in terms of p-y curves as well as distributed moment due to vertical shear stresses along the pile perimeter were obtained from the numerical analysis. It is shown that the numerical methodology can be used to gain insight into soil-OWT foundation interaction mechanisms and derive the soil reactions acting on the foundations as a result of lateral and overturning loads.