Do Precipitation-Induced Shallow Landslides Occur under Unsaturated Conditions?

在世界范围内，自然滑坡对社会经济的影响是巨大的。在美国，自然滑坡每年造成的生命、财产和环境损失超过20亿美元。浅层滑坡，典型的平移坡失稳，由几米厚的未岩化土壤地幔或风化层构成，可能主导着山坡环境中的体块运动过程。当它们开始或合并形成泥石流时，它们的破坏性特别大。浅层滑坡和泥石流通常是由强烈的降水或强烈的地面震动引发的，在一次气象或地震事件中可能会影响到广泛的地区。根据最近的一项调查，在过去的一个世纪里，全世界40次最具破坏性的滑坡灾害中，约有一半是由长时间或强降雨造成的。最近对滑坡发生的科学认识的进展，特别是对那些在世界各地的山坡环境中在强烈或长期降水下发生的滑坡，表明破坏面可能在地下水位以上，并且在接近饱和的条件下。经典的滑坡分析方法假设土材料要么完全饱和，要么完全干燥，而忽略了土吸力对边坡稳定性的贡献。因此，这种方法过于保守，无法准确预测浅层滑坡。土力学的最新进展揭示了部分饱和土体的应力状态。此外，地质力学和地貌学的物理证据和科学认识都表明，渗透诱发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。该项目寻求进一步的物理证据，并利用新知识来定义部分饱和土壤中的应力，以确定部分饱和土壤条件下滑坡发生的可能性。本研究的总体目标是：验证以下假设：(1)滑坡，特别是强降水条件下的浅层和平移边坡破坏，可能发生在地下水位以上；(2)当材料接近饱和且吸力应力降至接近零时发生破坏。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特性，(2)使用中等规模的浅层滑坡模拟器进行滑坡启动试验，(3)实施特定地点的项目，对浅层滑坡发生进行长期实时监测，从而在现场尺度上检验两种假设的有效性。通过澄清浅层滑坡是在饱和还是非饱和条件下发生，这项工作为回答关于我们是否需要一个包括土壤吸力或有效应力或两者的新范式来预测浅层滑坡的重要问题提供了物理基础。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相互作用的理解具有变革性意义。强降水条件下浅层滑坡引发的体块运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挑战。对地下水位以上有效应力变化概念进行验证，为今后建立预测模型提供了良好的物理基础，提高了浅层滑坡灾害的预测预报能力。与美国地质勘探局国家滑坡信息中心的合作确保通过针对研究人员、K-12学生和公众的外展活动进行广泛传播。通过研究生和本科生培训，积极招募代表性不足的群体，并将研究活动纳入现有课程，将研究任务与教育和多样性使命相结合，确保对学生，研究人员和从业人员的不同受众产生广泛影响。

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.