Session: 10-04-01 Bucket, Gravity Foundations and Caissons

Paper Number: 80169

80169 - Potential and Consequences of Earthquake Induced Liquefaction Under a Gravity Based Structure 

The Adriatic liquefied natural gas (ALNG) receiving terminal is located 15 km off the northeast coast of Italy in the Adriatic Sea. The main part of the terminal consists of an 88 m by 180 m reinforced concrete gravity based structure (GBS) with two LNG storage tanks. The GBS is underlain by a thick sand unit, made up of loose to medium dense fine sand and silty sand layers. Due to an earthquake that occurred close to the site in 2018, a re-evaluation of the liquefaction potential and consequences of the sandy layers on the performance of the GBS was conducted.

The main focus related to liquefaction was differential settlement due to different thicknesses of the sandy unit. We first estimated settlements at 44 CPT locations across the GBS footprint using CPT based liquefaction triggering methods and simplified settlement calculations with adjustments to account for the added weight of the GBS. We then performed the same calculations but with the earthquake demand (cyclic stress ratio) estimated from 1D total stress nonlinear site response analyses. Finally, we performed 2D finite element analyses of the soil-structure system using the advanced constitutive model PM4Sand to model the liquefiable sand layers. An important element in these studies was the proper calibration of the constitutive model. We performed analyses using average values from all CPTs as well as spatially variable values. We conducted all analyses for 475 year and 5000 year return period earthquake scenarios based on a probabilistic seismic hazard analysis. In addition, we used lower bound soil parameters to force liquefaction to better understand the behavior of the GBS.

The results show that the total amount of displacement is small, and no significant differential settlement is expected at the GBS for the 475 year or 5000 year earthquake scenarios. This correlates well with the observation that the GBS registered no observed displacement or differential settlement above design limits during the 2018 earthquake. For the 475 year earthquake scenario, the simplified method shows that only a few layers in the sandy unit will liquefy, but for the 5000 year earthquake scenario, the entire unit will liquefy. A practical observation from the study is that as the complexity of the analyses increased, the predicted settlements decreased.

The case history presented in this paper provides example applications of simplified settlement calculations due to liquefaction that include the effect of an overlying structure. In addition, it provides a useful example of integrated liquefaction analyses using PM4Sand that can be used as a benchmark for other studies. Finally, it highlights the potential efficiency of performing more in-depth and robust analyses for infrastructure management and lifetime extension.

Presenting Author: Brian Carlton Norwegian Geotechnical Institute

Authors:

Brian Carlton Norwegian Geotechnical Institute
Patrick Lee ExxonMobil
Filippo Belloni Adriatic LNG
Amir Kaynia Norwegian University of Science and Technology