Do Precipitation-Induced Shallow Landslides Occur under Unsaturated Conditions?

自然山体滑坡在世界范围内的社会经济影响是巨大的。在美国，自然山体滑坡每年造成生命损失、财产和环境损失超过20亿美元。浅层滑坡，通常是几米厚的未岩化土壤地幔或风化土层的平移性斜坡崩塌，可能主导着山坡环境中的质量运动过程。当它们发起或结合形成泥石流时，它们具有特别的破坏性。浅层滑坡和泥石流通常是由强降水或强地面震动引发的，在一次气象或地震事件期间可能会影响大片地区。根据最近的一项调查，在过去一个世纪里，全球40起最具破坏性的山体滑坡灾害中，约有一半是由长期或强烈的降雨造成的。近年来，对滑坡成因的科学认识取得了新的进展，特别是在世界各地的山坡环境中，在强降水或长时间降水作用下发生的滑坡，表明滑坡面可能位于地下水位以上，处于接近饱和的条件下。滑坡分析的经典方法假设土质材料要么是完全饱和的，要么是完全干燥的，忽略了土壤吸力对斜坡稳定性的贡献。因此，这种方法过于保守，不能准确地预测浅层滑坡。土力学的最新进展揭示了部分饱和土体中的应力状态。此外，地质力学和地貌学中的物理证据和科学认识都表明，渗透引起的滑坡的破坏面可能发生在地下水位以上和接近饱和的条件下。本项目寻求进一步的物理证据，并利用在部分饱和土壤中定义应力的新知识来确定在部分饱和土壤条件下滑坡发生的可能性。这项研究的总体目标是：检验以下假设：(1)滑坡，特别是在强降水条件下浅层和平移的斜坡破坏，可能发生在地下水位以上；以及(2)当材料接近饱和且吸力减小到几乎为零时，破坏发生。为了验证这些假设，我们将：(1)在实验室环境下测量非饱和水文和力学性质；(2)使用中型浅层滑坡模拟器进行滑坡启动试验；(3)实施特定场地的程序，对浅层滑坡的发生进行长期、实时的监测，从而在现场尺度上检验这两个假设的有效性。通过澄清浅层滑坡是在饱和条件下发生还是在非饱和条件下发生，这项工作为回答以下重要问题提供了物理基础：我们是否需要一个包括土壤吸力或有效应力或两者都包括的新范式来预测浅层滑坡。在这里获得的知识对促进对气候变化、地形和可持续土地利用之间的相互作用的理解具有变革性。强降水条件下浅层滑坡引发的大规模移动是长期地貌演变的一部分，对气候变化和土地利用演变的社会提出了巨大的挑战。检验地下水位以上有效应力变化概念用于滑坡成因分析，为今后的预测模型提供了良好的物理基础，并将提高我们预测和预报浅层滑坡灾害的能力。与美国地质勘探局国家山体滑坡信息中心的合作确保通过针对研究人员、K-12学生和普通公众的外联活动进行广泛的传播。通过研究生和本科生培训，将研究使命与教育和多样性使命相结合，积极招募代表性不足的群体，并将研究活动纳入现有课程，确保对不同的学生、研究人员和从业人员产生广泛影响。

The socioeconomic impacts of natural landslides are enormous worldwide. In the U.S., natural landslides result in the loss of life, as well as property and environmental damages exceeding US$2 billion annually. Shallow landslides, typically translational slope failures a few meters thick of unlithified soil mantle or regolith, may dominate mass-movement processes in hillslope environments. They are particularly destructive when they initiate or coalesce to form debris flows. Shallow landslides and debris flows are commonly triggered by intense precipitation or strong ground shaking and may affect extensive areas during a single meteorological or seismic event. According to a recent survey, about half of the 40 most destructive landslide disasters worldwide in the past century resulted from prolonged or intense rainfall. Recent advances in the scientific understanding of landslide initiation, particularly for those landslides that occur under intense or prolonged precipitation in hillslope environments around the world, indicate that the failure surface may be above the water table and under nearly saturated conditions. The classic methodology for landslide analysis assumes that earthen materials are either fully saturated or completely dry neglecting the contribution of soil suction to the stability of slopes. Thus this methodology is overly conservative and incapable of accurately forecasting shallow landsliding. Recent advances in soil mechanics have shed light on the state of stress in partially saturated soil masses. Furthermore, physical evidence and scientific understanding in both geomechanics and geomorphology all point to the likelihood that the failure surface of infiltration-induced landslides may occur above the water table and under nearly saturated conditions.This project seeks further physical evidence and uses the new knowledge in defining stress in partially saturated soil to identify the likelihood of landslide initiation under partially saturated soil conditions. The overall goals of this research are: to test the hypotheses that (1) landslides, particularly shallow and translational slope failures under heavy precipitation conditions, can occur above the water table; and, (2) that the failure occurs when materials are nearly saturated and suction stress is reduced to nearly zero. To test these hypotheses, we will: (1) measure the unsaturated hydrologic and mechanical properties under laboratory settings, (2) conduct landslide initiation tests using an intermediate-scale shallow landslide simulator, and (3) implement a site-specific program for long-term, real-time monitoring for shallow landslide occurrence, thus examining the validity of both hypotheses at the field scale. By clarifying whether shallow landslides occur under saturated or unsaturated conditions, this work provides a physical basis for answering the important question regarding whether we need a new paradigm that includes either soil suction or effective stress or both to predict shallow landslides. Knowledge gained here is transformative to advancing the understanding of the interactions among climate change, topography, and sustainable land use. Mass movement due to shallow landslide initiation under heavy precipitation is part of long-term landform evolution, and presents a great challenge for society subject to climate change and evolving land use. Testing the concept of effective stress variation above the water table for landslide initiation analysis provides a sound physical basis for future predictive models and will enhance our ability to predict and forecast shallow landslide hazard. Collaboration with the USGS National Landslide Information Center ensures broad dissemination via outreach activities targeted toward researcher, K-12 students, and the general public. Integrating the research mission with an educational and diversity mission via graduate and undergraduate student training, proactive recruitment of underrepresented groups and incorporating the research activities into existing courses ensure broad impact to a diverse audience of students, researchers, and practitioners.