Investigation of Clays Undergoing Large Strains or Possible Shear Banding

该项目涉及的粘土在非常大的应变行为的调查，利用实验和分析技术。 岩土工程中的许多问题都与这一现象有关。 有些涉及到破坏后或不稳定性问题，如：边坡稳定性破坏（砂和粘土）;承载能力破坏（砂和粘土）;液化和横向扩展（砂）;以及所有类型的岩土工程破坏和破坏后事件。 涉及实验室实验的示例包括试样中剪切带内的土壤（砂和粘土）和CPT的校准室测试（砂和粘土）。在所有这些问题中，剪切带内的土壤可能承受超过100%的剪切应变，与这些问题相关的第一个基本问题是：在这些高应变水平下，土壤的特性是什么？ 目前，PI在文献中没有获得在良好控制条件下测试土壤大应变并产生均匀应力和应变的实验数据。 这对于剪切到大应变的三维应力路径测试尤其如此。对单位土样进行的典型实验室试验只能可靠地剪切到最大20%或30%的应变，而没有显著的试样不均匀性，这会对实际应力和应变大小以及孔隙比产生沿着。包括一个实验程序，以调查在非常大的应变粘土的行为。第二个基本问题是：如何现有的理论土壤行为与实验数据比较？现有的简单，但广泛使用的本构模型，如修改剑桥和边界面模型，与这个新的数据集一致？一个分析研究计划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，已扩展到有限应变。然而，研究计划是偏重于实验工作。根据PI目前未发表的粘土实验工作，第三个基本问题也必须提出。所有加载到不同三维应力路径上的试件和所有OCR是否都能被剪切到非常大的应变而没有持续的剪切带？否则，可能无法完整执行下文概述的拟议实验方案。 然而，在持续的剪切带形成的情况下，已经制定了补充实验和分析研究计划，以解决这个特殊的问题。 试验计划将包括以下任务：1）使用重复分级剪切技术对粘土试样进行非常大的应变排水轴对称三轴压缩试验，以实现远超过100%的工程应变; 2）使用上述重复分级剪切技术，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变化到三轴拉伸; 3）分析剪切过程中不同点处试样中的孔隙比分布，以确保均匀的应变和孔隙比分布; 4）如果出现持续剪切带，则用针对该现象研究的实验补充实验任务。分析程序将包括以下任务：1）在临界状态土力学的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，例如修正剑桥模型和粘性土的边界面模型，以解释大应变和一般应力路径下的行为变化; 3）如果出现持续的剪切带，用与剪切带相关的工作来补充分析任务。本研究包含了重要的原创知识概念。 它集成了粘土的基本实验工作，现有的弹塑性本构模型配方的应用，以及摩擦材料的数值分析，可以代表大规模的现场应用。此外，基本拟议的实验和分析工作将提供一个基准，用于模拟大变形问题的模型的验证。 粘土中剪切带的力学机制在很大程度上是一个未被研究的课题。 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条件。均匀应变和应变局部化破坏机理是两种截然不同的现象，它们对岩土工程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法规，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民族作为研究生和雷乌斯。

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.