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: 0244354
• 负责人: Jerry Yamamuro
• 金额: $ 29.97万
• 依托单位: University of Delaware
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-09-15 至 2004-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0244354&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.

该项目利用实验和分析技术对粘土在非常大应变下的行为进行了研究。岩土工程中的许多问题都与这一现象有关。有些问题涉及破坏后或不稳定问题，例如：斜坡稳定性破坏(砂土和粘土)；承载能力破坏(砂土和粘土)；液化和侧向扩展(砂土)；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测试样品(砂土和粘土)中剪切带内的土壤和CPT(砂土和粘土)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100%的剪切应变。与这些问题相关的第一个基本问题是：在如此高的应变水平下，土壤的行为是什么？目前，PI‘s知道在文献中没有可用的实验数据来测试土壤在良好控制的条件下产生的均匀应力和应变的大应变。对于剪切到大应变的三维应力路径测试来说，情况尤其如此。典型的单位土试件的实验室试验只能可靠地剪切到最大20%或30%的应变，而不会出现明显的试件不均匀，这会使实际的应力和应变大小以及孔隙率受到质疑。第二个基本问题是：现有的土性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，如修正的Cam-Clay模型和边界表面模型，是否与这个新的数据集一致？分析研究计划将仔细分析和利用试验数据，以评估传统的土的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，研究计划偏重于实验工作。基于PI目前未发表的粘土实验工作，还必须提出第三个基本问题。加载到不同三维应力路径上的所有试件和所有OCR是否都可以在没有持续剪切带的情况下剪切到非常大的应变？如果不是，可能不可能完整地执行下面概述的拟议的实验计划。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方案来解决这一特定问题。试验计划将包括以下任务：1)利用重复分段剪切技术对粘土试件进行非常大应变排水的轴对称三轴压缩试验，以获得远远超过100%的工程应变；2)使用前述重复分段剪切技术，对立方体粘土试件进行大应变排水真三轴试验。三维应力路径将从三轴压缩到三轴拉伸；3)分析剪切过程中不同点处试件中的空隙比分布，以确保应变和空隙比分布均匀；4)如果出现持续剪切带，则对实验任务进行补充，以研究这一现象。分析程序包括以下任务：1)在临界状态土力学框架内分析实验数据；2)根据实验结果，评估本构公式的后果并提出可能的修改建议，如修正的粘性土的CaM-Clay模型和边界面模型，以解释大应变和一般应力路径下的行为变化；3)如果发生持续剪切带，则用与剪切带相关的工作补充分析任务。它结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能够代表大规模现场应用的摩擦材料的数值分析。此外，拟议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。粘土中剪切带的力学在很大程度上是一个未被研究的课题。实验和分析计划极大地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影响。我们将进一步了解粘土破坏的一般情况。均匀应变破坏和应变局部化破坏是两种截然不同的破坏机制，对岩土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代码，以确保安全和具有成本效益的民用基础设施。将作出特别努力，招收代表性不足的少数族裔作为研究生和REU。