Seismic liquefaction analysis of a critical facility with PM4Sand in Plaxis

Abstract

In seismically active areas, liquefaction hazards have always been a complicated aspect to evaluate as part of the seismic design of a project. In the case of the design of critical facilities, this becomes crucial, as beyond design basis conditions may elevate the seismic loads significantly and create a considerable liquefaction risk in areas of deep alluvial deposits. Furthermore, traditional semi-empirical methods lose their applicability at depths larger than 15 m, which becomes problematic if one wishes to analyse the liquefaction hazard of deep Holocene deposits. Given this shortcoming and the rapid growth of numerical tools available for geotechnical earthquake engineering, the use of liquefaction-predicting constitutive models, like PM4Sand, provides the opportunity to obtain more accurate and physically-consistent results. For this purpose, this research is divided in three main parts. The first part covers the study of the onset of liquefaction with the use of two cyclic undrained direct simple shear test databases and the identification of liquefaction-triggering criteria, in terms of pore pressures and shear strains which can consistently define a liquefied state in sands. The second part includes the thorough analysis of the capabilities of the PM4Sand model, in Plaxis, through a benchmark calibration study using one of the previously mentioned laboratory test databases, concluding in the proposal of a modified calibration methodology based on pore pressure ratio (ru) and shear strain (γ) liquefaction-triggering criteria. The third and last part covered a practical case study oriented towards the design of a critical facility, where a beyond design liquefaction hazard analysis of a hypothetical site was evaluated incorporating the findings from the previous parts. A one-dimensional liquefaction hazard analysis was performed using a single earthquake signal and soil profile, where the consistency of the PM4Sand model in terms of liquefaction-triggering was evaluated and the numerically-obtained results were compared to those calculated through one semi-empirical method. Additionally, the one-dimensional model was extended and a two-dimensional liquefaction hazard analysis, including the presence of a simplified structure, was performed with the aim of evaluating the effects of soil-structure interaction and structural load variation on the liquefaction hazard of the soil profile over distance.