open access

https://doi.org/10.14943/doctoral.k16538

https://hdl.handle.net/2115/95708

海成粘土における除荷挙動の実験研究とモデル化

英語

2025-06-30

Hokkaido University

XIII, 103p.

There is a rising demand for land reclamation in coastal cities with economic and demographic growth in East and Southeast Asia. Marine clays play a critical role in these projects, as their deformation characteristics significantly impact subsoil quality. The long-term loading behavior of marine clays has been studied by many researchers.However, there are relatively few existing studies focusing on the unloading behavior after preloading, and strain rate dependency on the unloading behavior of marine clays remains unclear. This study aims to accumulate experimental data on the unloading behavior of marine clays and develop a strain rate-based model for improving the prediction accuracy for marine clays’ unloading behavior. The authors conducted a series of constant rate of strain (CRS) consolidation tests from loading to unloading, and long-term unloading oedometer tests for marine clays to observe the unloading behavior. Controlling the preloading duration corresponding to different strain rates at the end of preloading to elucidate the stress history’s effect. Moreover, instead of a parameter σ'p (preconsolidation pressure) for normal consolidation visco-plastic behavior, the authors develop and propose a new visco-plastic model by introducing a concept of plastic rebound boundary and a new parameter R for swelling behavior in unloading. This parameter R represents the normalized distance from the current stress state to the plastic rebound boundary in logarithmic effective consolidation stress. Therefore, the visco-plastic model for behavior in the loading stage has been developed into the swelling visco-plastic behavior in the unloading stage of marine clay. Comparing the simulation and test results, the simplified visco-plastic swelling model shows great agreement with the test results. This thesis provides a novel perspective on the unloading behavior of marine clays and experimentally validates the efficacy of a strain rate-based model in describing their swelling behavior. The new model offers significant theoretical support for optimizing foundation design and prediction in future coastal land reclamation projects. The thesis is composed of 8 chapters. Chapter 1 introduces the study’s background, objectives, and the need for improving the accuracy of prediction for unloading behavior of marine clay. Chapter 2 provides a literature review of experimental research, such as incremental loading (IL) oedometer tests, constant rate of strain (CRS) consolidation tests and inter-connected consolidation tests, on the loading and unloading behavior of marine clays, as well as some recent constitutive models developed to simulate unloading behavior. Chapter 3 introduces the physical properties of Ariake clay, Louiseville clay and Onsøy clay, respectively. The consolidation characteristics of the three marine clays are compared based on their compression curves. Chapter 4 describes two types of CRS consolidation test methodologies: constant unloading strain rate consolidation test and multi-stepwise unloading strain rate consolidation test. The experimental results demonstrate the direct influence of unloading strain rate on the unloading behavior. In addition, the strain rate dependency is compared between loading and unloading stages.Chapter 5 presents two types of incremental loading oedometer test methodologies: fourstage unloading oedometer test and long-term unloading oedometer test. The influence of overconsolidation ratio (OCR) and preloading duration on the unloading behavior is investigated, and the strain rate dependency during unloading is quantitatively evaluated.Chapter 6 describes the proposed model's development process in detail and presents its simulation results based on the experimental data. First, the loading behavior of the new model adopts the formulation proposed by Watabe et al. (2008; 2012). Then, a new strain rate-based swelling model for unloading is developed by incorporating the concept of a plastic unloading boundary (Amerasinghe and Kraft, 1983; Tachibana et al., 2020) and reference overconsolidation ratio (OCRref). Finally, numerical simulations are conducted to evaluate the model's performance. The simulated results show good agreement with the observed unloading behavior of marine clays. Chapter 7 further discusses the new visco-plastic model, including a comparison between the swelling behavior during unloading and loading behavior. The advantages of introducing the reference overconsolidation ratio OCRref are elaborated, and the proposed model is systematically compared with some existing models. Finally, Chapter 8 summarizes the study's findings and provides some recommendations for further research.

2025-06-30

甲第16538号

博士(工学)

(主査) 教授 渡部 要一, 教授 石川 達也, 准教授 磯部 公一, 教授 宮森 保紀

工学院（環境フィールド工学専攻）

博士論文

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