Finite element modelling of static liquefaction triggering mechanisms and evaluation of factor of safety

Static liquefaction risk is one of the major risks faced in Tailings Storage Facilities (TSFs), primarily due to the contractive and strain-softening nature of loosely deposited tailings. The safety margin against the triggering of static liquefaction is difficult to evaluate using conventional Limit Equilibrium Analysis (LEA), which considers either the peak or the residual strength (assuming liquefaction has triggered). In contrast, numerical methods such as Finite Element Method (FEM), offers deeper insight by capturing the full mobilisation process beyond failure, offering a more intuitive understanding of the available safety margin. This paper presents a case study of liquefaction triggering assessment conducted as part of the design process for a TSF.

Static liquefaction simulations in Plaxis commonly utilise static analyses with an automatic unloading approach to address strain-softening numerical issues. However, this unloading (strain-controlled) approach differs fundamentally from real-world load-controlled failure mechanisms. To more accurately capture load redistribution and failure propagation during liquefaction triggering, this study adopts dynamic analysis as a more realistic method for simulating progressive failure. This paper employs dynamic analyses to clearly visualise the disturbance thresholds associated with three triggering mechanisms: undrained surface surcharge, increase of phreatic level, and toe excavation. The stress-strain behaviour for each mechanism is examined in detail.

A new definition of Factor of Safety (FoS) is evaluated for each triggering mechanism, based on changes in both deviator stress (q) and mean effective stress (p’) required to trigger failure and their equivalency around the instability ratio. This offers a dimensionless alternative (therefore less dependent on the TSF size) to the conventional threshold disturbances for assessing the adequacy of safety against liquefaction.

The analyses are based on the NorSand constitutive model developed within Critical State Soil Mechanics (CSSM), and the limitations of such analyses are also discussed.