Investigation of Clays Undergoing Large Strains or Possible Shear Banding

研究遭受大应变或可能出现剪切带的粘土

• 批准号: 0455321
• 负责人: Jerry Yamamuro
• 金额: $ 1.8万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-09-01 至 2007-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0455321&HistoricalAwards=false
• 关键词: Investigation; Clays; Undergoing; Large; Strains

This project concerns the investigation of the behavior of clays at very large strains, utilizing both experimental and analytical techniques. Many problems in geotechnical engineering relate to this phenomenon. Some relate to post-failure or instability issues such as: slope stability failures (sands and clays); bearing capacity failures (sands and clays); liquefaction and lateral spreading (sands); and all types of geotechnical failure and post-failure events. Examples involving laboratory experiments include soil inside a shear band in a test specimen (sands and clays) and calibration chamber testing of the CPT (sands and clays). In all of these problems the soil inside the shear zone may be subjected to shear strains in excess of one hundred percent.The first fundamental question related to such problems is: What is the soil behavior at these high strain levels? Presently, the PI's are aware of no available experimental data in the literature that has tested soils to large strains under well-controlled conditions with resulting uniform stresses and strains. This is especially true with respect to three-dimensional stress path tests sheared to large strains. Typical laboratory tests on unit soil specimens can only be reliably sheared to a maximum of 20 or 30 percent strain without significant specimen nonuniformities that call into question the actual stress and strain magnitudes, along with the void ratio. An experimental program is included to investigate the behavior of clays at very large strains.A second fundamental question is: How do existing theories of soil behavior compare with the experimental data? Are existing simple, but widely used constitutive models, such as the Modified Cam-Clay and bounding surface models, consistent with this new data set? An analytical research program will carefully analyze and utilize the experimental data to evaluate traditional soil behavior frameworks and simple constitutive models that have been extended to finite strains. However, the research program is weighted towards experimental work.Based upon the PIs current unpublished experimental work on clays, a third fundamental question must also be raised. Can all specimens loaded onto different 3-d stress paths and all OCRs be sheared to very large strains without persistent shear banding? If not, it may not be possible to execute the proposed experimental program outlined below in its entirety. However, in the event that persistent shear bands form, a supplementary experimental and analytical research program has been formulated to address that particular issue. The Experimental Program will consist of the following tasks: 1) Perform very large strain drained axisymmetric triaxial compression tests on clay specimens using repetitive staged shearing techniques to achieve engineering strains well beyond 100 percent; 2) Using the aforementioned repetitive staged shearing techniques, perform large strain drained true triaxial tests on cuboidal clay specimens. Three-dimensional stress paths will be varied from triaxial compression to triaxial extension; 3) Analyze void ratio distributions in specimens at different points during shearing to ensure uniform strains and void ratio distribution; 4) If persistent shear banding occurs, supplement experimental tasks with experiments directed toward a study of this phenomenon.The Analytic Program will consist of the following tasks: 1) Analyze experimental data within the framework of Critical State soil mechanics; 2) Based on the experimental results, evaluate the consequences to and propose possible changes to constitutive formulations, such as the Modified Cam-Clay and the bounding surface models for cohesive soils to account for variations in the behavior at large strains and for general stress paths; 3) If persistent shear banding occurs, supplement analytic tasks with work related to shear banding.This study incorporates significant original intellectual concepts. It integrates fundamental experimental work on clays, application of existing elasto-plastic constitutive model formulations, and numerical analysis of frictional materials that can represent large-scale field applications. Moreover, the basic proposed experimental and analytical work will provide a benchmark for the verification of models used to simulate large deformation problems. The mechanics of shear banding in clays is a largely uninvestigated topic. The experimental and analytical plans significantly increase the fundamental knowledge in this area.The proposed activity will have a broad impact on society. Further knowledge about the general condition of failure in clays will be developed. Uniform strain and strain localization failure mechanisms are two widely different phenomena and both have a great impact on the design of geo-structures. Understanding them will help develop better codes to ensure a safe and cost efficient civil infrastructure. Special efforts will be made to recruit underrepresented minorities as graduate students and REUs.

这个项目涉及到粘土的行为在非常大的应变，利用实验和分析技术的调查。岩土工程中的许多问题都与这一现象有关。一些涉及破坏后或不稳定问题，如：边坡稳定性破坏（砂和粘土）；承载力破坏（砂土和粘土）；液化和横向扩张（砂）；以及所有类型的岩土破坏和破坏后事件。涉及实验室实验的例子包括试样（砂和粘土）剪切带内的土壤和CPT（砂和粘土）的校准室测试。在所有这些问题中，剪切带内的土壤可能受到超过百分之百的剪切应变。与这些问题相关的第一个基本问题是：在这些高应变水平下土壤的行为是什么？目前，PI意识到在文献中没有可用的实验数据，这些数据已经在良好控制的条件下测试了土壤的大应变，从而产生均匀的应力和应变。这对于大应变剪切的三维应力路径试验尤其如此。典型的实验室测试单位土样只能可靠地剪切到最大20%或30%的应变，而没有显着的试样不均匀性，这使实际应力和应变大小以及空隙比受到质疑。包括一个实验程序，以研究粘土的行为在非常大的应变。第二个基本问题是：现有的土壤行为理论与实验数据相比如何？现有的简单但广泛使用的本构模型，如Modified Cam-Clay和边界面模型，是否与这个新的数据集一致？一个分析研究项目将仔细分析和利用实验数据来评估传统的土壤行为框架和简单的本构模型，这些模型已经扩展到有限应变。然而，研究计划侧重于实验工作。基于PIs目前未发表的关于粘土的实验工作，第三个基本问题也必须提出。所有加载在不同三维应力路径上的试样和所有ocr都能被剪切成非常大的应变而不出现持续的剪切带吗？如果没有，则可能无法完整地执行下面概述的拟议实验程序。然而，在持续剪切带形成的情况下，已经制定了一个补充的实验和分析研究计划来解决这个特殊问题。实验计划将包括以下任务：1)使用重复阶段剪切技术对粘土试样进行非常大应变排水轴对称三轴压缩试验，以获得远远超过100%的工程应变；2)采用上述重复阶段剪切技术，对立方体粘土试样进行大应变排水真三轴试验。三维应力路径将从三轴压缩转变为三轴拉伸；3)分析剪切过程中试件在不同时刻的孔隙比分布，确保应变和孔隙比分布均匀；4)如果发生了持续的剪切带，用针对这种现象的研究的实验来补充实验任务。分析项目将包括以下任务：1)在临界状态土力学框架内分析实验数据；2)在实验结果的基础上，评估本构公式的影响，并提出可能的改变，如修正的Cam-Clay模型和黏性土的边界面模型，以解释大应变和一般应力路径下的行为变化；3)如果持续出现剪切带，将分析任务补充与剪切带相关的工作。这项研究包含了重要的原创知识概念。它整合了粘土的基础实验工作，现有弹塑性本构模型公式的应用，以及可以代表大规模现场应用的摩擦材料的数值分析。此外，所提出的基本实验和分析工作将为用于模拟大变形问题的模型的验证提供基准。粘土剪切带的力学是一个很大程度上未被研究的课题。实验和分析计划大大增加了这一领域的基础知识。拟议的活动将对社会产生广泛的影响。进一步了解粘土破坏的一般情况将得到发展。均匀应变破坏机制和应变局部化破坏机制是两种截然不同的现象，它们对土工结构的设计都有很大的影响。了解它们将有助于制定更好的规范，以确保民用基础设施的安全和成本效益。将作出特别努力，招收代表性不足的少数民族研究生和再就业人员。