Numerical modeling of wave-induced liquefaction around a gravity-based structure

Abstract

Liquefaction occurs when pore water pressure exceeds the forces between soil grains, causing the soil to behave like a fluid. The recognition of the significance of liquefaction emerged with the construction of large-scale structures. Following the Calaveras Dam failure, Allen Hazen introduced the term "liquefaction" in his study on the same incident\cite{Hazen1919}. Subsequently, numerous research studies have been conducted to investigate the liquefaction phenomenon. This thesis focuses on the study of residual liquefaction, one of the two types of liquefaction, the other being momentary liquefaction. Residual liquefaction has phases: initiation of liquefaction, liquefaction phase, and compaction phase. However, the model does not involve the compaction phase. Therefore, in the scope of the thesis, the compaction phase is not investigated. In marine environments, liquefaction is triggered by large waves with periods ranging from 5 to 15 seconds and heights up to 2 meters or even 10-20 meters in offshore areas. Soil type, grain sizes, and relative density strongly influence the likelihood of liquefaction. The objective of the thesis is to construct a model in order to study the liquefaction under standing waves or the rocking motion of a caisson, or both. Rocking motion is the back-and-forth movement of a caisson that may occur when the structure is exposed to a storm. Rocking motion causes a significant generation of pore-water pressure and can potentially trigger liquefaction, leading to catastrophic consequences. The model uses the finite element method (FEM) in two steps. First, it derives soil stresses by solving Biot consolidation equations, considering quasi-static behavior, and using Darcy's law to determine pore water's movement and pressure. Then, it solves the excess pore pressure equation to obtain the period-averaged pore pressure. The source term generates the excess pore pressure in the equation. In this model, a slight modification has been implemented to differentiate between liquefied and non-liquefied areas. Essentially, the model identifies areas susceptible to liquefaction and indicates the boundaries where the liquefied region ends. The outcomes generated by the model show a favorable agreement with the previous experimental studies. Comparisons have been made with previous studies to validate the findings. In this thesis, various cases are simulated using the model, including progressive waves, standing waves in front of a caisson, and the rocking motion of a caisson. The calculations for each case are demonstrated, with a detailed explanation of their integration into the model. The impact of a buried pipe on liquefaction susceptibility is investigated under progressive waves. The significance of a bedding layer is also analyzed under standing waves and rocking motion.