GOALI: Strength Loss in Clays During Earthquake and Other Cyclic Loading

地震引起的边坡破坏对强震地区边坡和挡土系统的设计产生了重大影响。例如，在1964年阿拉斯加地震期间，在天然粘土地层中发生了几次大规模滑坡，造成了安克雷奇的生命损失和基础设施和私人财产的重大经济损失。因此，震后开发的地震稳定性设计方法采取了过于保守的方法，假设了适合于完全扰动粘土的小强度值，导致设计系统地忽略了扰动和天然粘土的本质差异。这种过于保守的做法导致设计成本明显高于考虑天然粘土实际性能的设计。基于这项研究结果的设计将更经济，但会为公众带来足够的安全边际。因此，这项研究将有利于美国的经济和社会。拟议研究的社会和教育影响是广泛的，因为其结果将影响预测自然灾害发生和发展可持续岩土工程设计所需的基础科学。本课题涉及工程力学、岩土工程、地震工程、工程地质等多个方面。该研究的多学科方面将有助于扩大代表性不足的群体在研究中的参与，并对工程教育产生积极影响。这项与工业界学术联络的资助机会（GOALI）研究是工业界和学术界之间的合作，其好处是将研究成果快速应用于实践。本研究为量化天然黏土在循环荷载作用下的强度退化和破坏提供了一种新的方法。它将结合实验和理论发现来预测天然粘土矿床地震诱发破坏的开始。实验将在高质量的不同灵敏度的Bootlegger Cove地层样品上进行。这个地层中非常敏感的粘土引发了1964年地震期间在安克雷奇发生的灾难性滑坡。西北大学将与GeoEngineers公司合作进行这项研究，后者将协调BCF样本的钻探和取样。实验程序将包括对完整和重构样品的单调和循环试验。试验将阐明增量非线性、应力路径、固结历史和循环荷载的作用。建模活动将利用这些证据制定一个增量非线性本构模型，再现孔隙压力积聚和循环产生的破坏的影响。虽然已经提出了几种模型来处理天然粘土的复杂力学，但这些模型往往忽略了可能的破坏模式的丰富多样性。饱和流体土的工程力学理论提供了一些见解，但为了捕捉地震震动过程中发生的复杂过程，这些理论必须得到加强，以适应周期性衰减的刚度和强度过程。该方法将包括开发用于评估作为应力条件和循环次数的函数的循环强度退化的具体分岔标准。该模型将在计算机程序中数值实现，用于岩土工程问题的耦合动力分析。

Slope failures caused by earthquakes have had a significant impact on the design of slopes and earth retention systems in areas of strong seismicity. For example, several massive landslides occurred within natural clay soil formations during the 1964 Alaskan earthquake that caused loss of life and significant financial loss to infrastructure and private property in Anchorage. Consequently, design methods for seismic stability developed after this event take an overly conservative approach by assuming small values of strength that are appropriate for fully disturbed clay, resulting in designs that systematically ignore the substantial differences the behavior of the disturbed and the natural clay soil. This over conservatism results in designs that are significantly more expensive than one which accounts for actual behavior of the natural clays. Designs based on the results of this research will be more economical yet result in adequate margins of safety for the public. Therefore, this research will benefit the U.S. economy and society. The societal and educational impacts of the proposed research are broad, because its results will impact the basic science needed for predicting the occurrence of natural hazards and for developing a sustainable geotechnical design. This research involves aspects of engineering mechanics, geotechnical engineering, earthquake engineering and engineering geology. The multi-disciplinary facets of the research will help broaden participation of underrepresented groups in research and positively impact engineering education. This Grant Opportunity for Academic Liaison with Industry (GOALI) research is a collaboration between industry and academe, which has the benefit of rapid application of research results to practice. This research will develop a new approach for quantifying the strength degradation and destructuration of natural clays exposed to cyclic loading. It will combine experimental and theoretical findings to predict the onset of seismically-induced failure in natural clay deposits. The experiments will be conducted on high quality samples of Bootlegger Cove Formation of varying degrees of sensitivity. Very sensitive clays in this formation triggered the catastrophic slides that took place in Anchorage during the 1964 earthquake. Northwestern University will collaborate in this research with GeoEngineers, Inc., who will coordinate drilling and sampling of BCF specimens. The experimental program will include monotonic and cyclic tests on intact and reconstituted samples. The experiments will elucidate the role of incremental nonlinearity, stress-paths, consolidation history and cyclic loading. The modeling activities will use this evidence to formulate an incrementally non-linear constitutive model reproducing the effect of destructuration in pore pressure build-up and cyclically generated failure. Although several models have been proposed to cope with the complex mechanics of natural clays, such models often ignore the rich variety of possible failure modes. Engineering mechanics theories for fluid-saturated soils provide insights, but to capture the complex processes taking place during seismic shaking, these theories must be enhanced to accommodate cyclically-decaying stiffness and strength processes. The approach will include the development of specific bifurcation criteria for assessing the cyclic strength degradation as a function of stress conditions and number of cycles. The model will be numerically implemented in computer programs for coupled dynamic analysis of geotechnical problems.