The Mechanism And Evaluation Of Liquefaction-induced Large Settlement Of Sandy Ground Under Long-duration Earthquakes

Recent years, strong earthquakes with long duration happened frequently all over the world. Large settlement induced by liquefaction during or following these earthquakes resulted in quantity of infrastructure and lifeline system crushed or broken. This research pays attention to the mechanism and evaluation of liquefaction-induced large settlement of sandy ground under long-duration earthquakes. To achieve the goal, a series of element compression tests and cyclic triaxial tests (under liquefied or non-liquefied condition) are studied to achieve the characteristics of sand deformation. Then, a model of volumetric stain after shaking which accounting for consolidation and cyclic strain history and a simplified method to calculate the settlement are proposed. Two sets of centrifuge model tests are researched to reveal how the ground settles under the long duration earthquake. As a result, both the model and the simplified method are validated to perform well. In case study, the thesis proposes a new way to figure out the critical accumated shear strain in the boundary value problem, and makes a case study. In detail, topics included in thestudy are shown as follows:(1) In the cyclic tests, the accumulated shear strain could be chosen as the index of plastic damage of sand unit, and the critical accumulated shear strain marks the initial liquefaction well.(2) For non-liquefaction situation, the mechanism of its volumetric strain only relies on the re-consolidation process, and this reconsolidation index is nearly equal to the swelling index in the same condition.(3) For liquefaction situation, the mechanism of the volumetric strain is composed of re-sedimentation and the re-consolidation processes. The re-sedimentation is closely related to the cyclic shear strain history, especially that after liquefaction, and the more the accumulated shear strain ratio is, the more the re-sedimentation volumetric will be. Besides, the re-consolidation behavior is significantly affected by the previous consolidation history, and the post-liquefaction reconsolidation will follow the trend of the previous normal consolidation curve, but the re-consolidation index is as1.3-1.5times large as the compression index in the same condition, thus the re-consolidation index can be estimated via the compression index.(4) A model of volumetric stain after shaking which accounting for consolidation and cyclic strain history and a simplified method to calculate are proposed. The model has accounted for both the consolidation and cyclic shear strain histories, and the simplified method to evaluate the earthquake induced settlement. Considering the concept of the model, they will not only be applicable to settlements induced by normal earthquakes, but also by those induced by long duration earthquakes.(5) Then, two sets of centrifuge model tests are researched to reveal how the ground settles under the long duration earthquake and the model and the simplified method are validated. And they have advantage over the former settlement evaluations when it comes to the settlement of the long duration earthquake.(6) The critical accumulated shear strain under boundary value condition was proposed, and a method to calculate the accumulated shear strain from the only accelerlation on the ground surface was also suggested. A case study from the East Janpan Earthquake was made by using the proposed methods at last.