Constitutive Modeling of Inherent Sand Fabric Anisotropy and Numerical Applications within Critical State Soil Mechanics

主要研究者：Yannis Defalias机构：加州大学戴维斯分校题目："固有砂组构各向异性的本构建模和临界状态土力学中的数值应用"众所周知，天然土壤沉积物由于其沉积及其对随后的组构的影响而具有固有的各向异性。 特别是对于桑迪，织物诱导的固有各向异性可能对土壤在塑性变形范围内的后续加载的力学响应产生显著影响。 其他相同的砂样（相同的砂、相同的孔隙比等）如果用不同的方法制备，它们可能会以完全不同的方式表现出来;或者，如果它们在不同的方向上加载相同，就好像它们是密度非常不同的样品。 本研究的主要目标是对这种固有的砂组构各向异性及其在地质结构数值分析中的影响进行本构建模。为了实现这一主要目标，需要一个砂本构建模平台。 该平台基于现有的砂土本构模型（Manzari和Dafalias，1997年; Li和Dafalias，2000年），尽管其简单性已被证明能够模拟密度、围压和单调或循环加载条件下的砂土响应，具有相同的模型常数，但是，只有一个例外 它不能解释由于组构形成的固有初始各向异性的影响。 在塑性力学理论中引入各向异性的概念，有一个很好的理论框架，即所谓的节理各向同性不变量，但如果在最大程度上使用它，它将产生极其复杂的本构模型，这对于实际应用和分析来说是不可行的工具。 为了避免这种复杂性，将通过一种非常简单的方法在所选择的本构平台中引入固有的各向异性效应。 的主要理论工具，由微观力学观察颗粒和空隙的取向分布的动机，将是引入一个标量值的各向异性参数A，这将影响一些，但不是全部，平台本构模型的基本功能，以实现各向异性行为的模拟。 将调查一些不同的途径， 初步结果表明，这是一个非常有前途的方法，尽管它的简单。 在这个过程中，CSSM的基本前提上的唯一性的临界州线在孔隙比围压空间的质疑，而不是，它的依赖于组构通过各向异性参数A提出。 一旦固有的各向异性已经成功地建模，第二个重要的步骤将是评估其效果的岩土结构的响应水平进行不同的负载和分析的数值实现的建议的本构模型。可以预见，固有各向异性对砂土的胀缩特性的影响，将对单调和循环荷载下的整体地质结构响应产生巨大影响，除了研究结果对特定区域的土本构建模和分析产生合理影响外，还将出现一些更广泛的影响特征。 提高我们在现代计算时代使用自然现象模拟的能力，以满足基础设施优化和安全设计的社会需求，这一点非常重要。 这种现象不仅是地质力学的，而且后者被纳入其他系统的更大网络中。 因此，诸如地质力学模拟的模拟组件的不良性能对于现象模拟的较大系统（例如，在地震设计中）的整体性能是有害的。 提供一个可靠的土壤建模方法，考虑到织物效应的重要问题，实际上是一个贡献，更好的整体性能，这个更广泛的系统模拟自然现象。 另一个更广泛的影响，这个奖项的成功完成将有，是大规模的教育。 尽管研究的先进性，关键州线的依赖于一个标量值的测量结构各向异性的中心思想有一个固有的简单性，可以很容易地传达给岩土工程的本科生，并构成一个标准的除了教科书的土力学无处不在。 最后，一旦主要研究结果发布和分发，将其纳入岩土工程专业使用的许多不同建模技术（而不仅仅是本研究中使用的建模平台）将是一件简单的事情。

PI: Yannis DefaliasInstitution: University of California at DavisTitle: "Constitutive Modeling of Inherent Sand Fabric Anisotropy and Numerical Applications Within Critical State Soil Mechanics"It is well known that natural soil deposits are inherently anisotropic due to their deposition and its effect on the ensuing fabric. In particular, for sandy soils the fabric induced inherent anisotropy may have dramatic effects on the mechanical response of the soil to subsequent loading in the range of plastic deformations. Otherwise identical sand samples (same sand, same void ratio, etc.) may behave in totally different ways if prepared by different methods; or, if they are identically loaded in different directions, as if they were samples of very different densities. The constitutive modeling of such inherent sand fabric anisotropy and its effect in the numerical analysis of geo-structures is the main objective of this research award.In order to address the main objective, a sand constitutive modeling platform will be required. This platform is based on an existing sand constitutive model (Manzari and Dafalias, 1997; Li and Dafalias, 2000) within the framework of Critical State Soil Mechanics (CSSM), which despite its simplicity has been proved capable of modeling the sand response under a large variation of densities, confining pressures and monotonic or cyclic loading conditions with the same set of model constants, but, with one exception. It cannot account for the effect of inherent initial anisotropy due to fabric formation. And such effect is often truly drastic, as shown experimentally.The incorporation of anisotropy in the theory of plasticity has a well understood theoretical framework in terms of the so-called joint isotropic invariants, but if used in its utmost generality, it will produce extremely complicated constitutive models which are not feasible tools for practical applications and analysis. In order to avoid such complications, the inherent anisotropy effect will be introduced in the chosen constitutive platform by a very simple method. The main theoretical tool, motivated by micromechanical observations on particles and voids orientation distribution, will be the introduction of a scalar-valued anisotropy parameter A, which will effect some, but not all, of the basic features of the platform constitutive model in order to achieve the simulation of anisotropic behavior. A number of different avenues will be investigated, and preliminary results show that this is a very promising approach, despite its simplicity. In the process the fundamental premise of CSSM on the uniqueness of the Critical State Line in the void ratio-confining pressure space is questioned, and instead, its dependence on fabric via the anisotropic parameter A is proposed. Once the inherent anisotropy has been successfully modeled, the second important step will be to evaluate its effect at the level of the response of a geotechnical structure subjected to different loadings and analyzed by numerical implementation of the proposed constitutive model. It is expected that the effect inherent anisotropy has on the dilative and contractive characteristics of sand, will have a dramatic effect on the overall geo-structural response under monotonic and cyclic loading.Besides the plausible impact of the research findings on the specific area of soil constitutive modeling and analysis, some broader impact characteristics will emerge. It is of great importance to increase our ability to use simulations of natural phenomena in the modern era of computations, in order to address societal needs for optimum and safe design of infrastructures. Such phenomena are not only geo-mechanical, but the latter are integrated in a larger network of other systems. Thus, the poor performance of a simulative component such as a geo-mechanical simulation, is detrimental to the overall performance of the larger system of phenomena simulations (e.g. in seismic design). The provision of a reliable soil modeling approach that accounts for the important issue of fabric effect, is in fact a contribution to better overall performance of this broader system of simulations of natural phenomena. Another broader impact the successful completion of this award will have, is educational at large scale. Despite the advanced character of the research, the central idea of the dependence of the critical state line on a scalar-valued measure of fabric anisotropy has an inherent simplicity that can easily be conveyed to undergraduate students of geotechnical engineering and constitute a standard addition to textbooks of soil mechanics everywhere. Finally, its incorporation in many different modeling techniques the geotechnical profession uses (and not only the modeling platform used in this research) will be a simple matter once the main findings are published and distributed.