Structural Monitoring of Rock Arches

岩拱是动态的自然结构，在各种环境力的作用下弯曲、凹陷、摇摆和震动。游客们面对的是它们脆弱的本性，然而很少有人欣赏这些通常被认为是坚忍的地质结构的动态生命。调查人员将监测一些突出的岩石拱的环境共振，以及选定地点的日常和季节性变形。他们的目标是了解拱门如何应对环境变化（例如热应力，降雨），并最终能够辨别内部强度随时间的细微变化。他们方法的关键是测量拱门的振动方式。岩拱是相对简单的机械结构，具有可测量的谐振频率和模态振型。该项目使用宽带地震仪测量共振频率。然后，研究人员根据分析的现场测量结果生成每个拱门的3D模型，并将这些模型输入到数值模拟中，以预测谐振模式的形状和频率，并进行校准以匹配现场数据。这个多学科项目结合了结构动力学、地震学、岩石力学和地貌学等领域的概念，旨在建立一种定量评估岩拱动力和结构性能变化的新方法。该研究将使人们对天然拱的生命周期、对不同外部荷载的响应以及导致其最终倒塌的破坏过程有新的认识。我们的项目也为公众参与和科学教育提供了一个很好的机会。从长远来看，设想的结果可能会导致评估自然灾害和游客安全的新方法。岩石拱门是壮观的自然地标，将游客与地球表面过程的动态和美丽联系起来。例如，精致拱门是美国西部的标志，与许多国家纪念碑一样可识别。然而，目前还没有合适的方法来评估这些自然地标的结构健康和完整性，因为它们随着时间的推移或在强烈风暴或地震等破坏性事件之后发生变化。与工程结构不同，天然岩石特征中关键元素的内部组成和强度不能先验地知道。评估结构损伤的唯一方法是了解结构的当前行为和对外部载荷的响应，并能够感知性能的相对变化。我们提出了新的方法来评估天然岩石拱桥的动力行为，估计基本力学特性，为短期和长期相对变化监测生成数据，并最终提供一种区分弹性和非弹性行为以及评估结构健康变化的方法。该项目的核心技术是利用环境振动监测（即在弱背景激励下测量拱门的谐振频率，并检测其随时间的变化，这些变化可能是渐进的，也可能是间断的）。所使用的概念融合了三个科学学科：结构动力学、地震学和岩石力学，以结构健康监测领域的既定概念为基础。通过测量环境振动的频谱特性和时变模态分析来评估行为的变化。通过感知拱的振动方式，我们可以解释其行为和对外部载荷的响应，并检测机械参数的可逆或不可逆变化。共振频移将与变形和环境测量有关，以评估被监测拱门对外部扰动的驱动力和敏感性。测量是非侵入性和非破坏性的，所有仪器都被选择为可移动的，占地面积小。目标是建立一种新的方法来评估天然岩拱的长期结构完整性，其中一些是国际公认的地标。

Rock arches are dynamic natural structures that bend, sag, sway and shake in response to a variety of environmental forcings. Visitors to these features are confronted with their fragile nature, however few appreciate the dynamic life of what are often perceived as stoic geological structures. The investigators will monitor the ambient resonance of a number of prominent rock arches, as well as daily and seasonal deformation at select sites. Their goal is to understand how arches respond to changes in their environment (e.g. thermal stresses, rainfall), and ultimately be able to discern subtle changes in internal strength over time. Key to their approach is measuring how arches vibrate. Rock arches are relatively simple mechanical structures with measurable resonant frequencies and mode shapes. This project measures resonant frequencies using broadband seismometers. The researchers then generate 3D models of each arch from the analyzed field measurements and input these into a numerical simulation to predict the resonant mode shapes and frequencies, and calibrate to match field data. This multidisciplinary project combines concepts from the fields of structural dynamics, seismology, rock mechanics, and geomorphology, with the goal of establishing a new method to quantitatively evaluate changes in the dynamics and structural performance of rock arches. The research will lead to new understanding of the life-cycle of natural arches, their response to different external loads, and failure processes leading to their eventual collapse. Our project also provides an excellent opportunity for public engagement and science education. In the long-term, the envisioned outcomes may lead to new methodology for assessment of natural hazards and visitor safety.Rock arches are spectacular natural landmarks that connect visitors with the dynamics and beauty of earth surface processes. Delicate Arch, for example, is an icon of the western United States and is as recognizable as many national monuments. However, there is currently no methodology in place to evaluate the structural health and integrity of these natural landmarks as they change through time or in the wake of a damaging event such as a strong storm or earthquake. Unlike engineered structures, the internal composition and strength of critical elements in natural rock features cannot be known a priori. The only means available to evaluate structural damage requires understanding the structure's current behavior and response to external loads, and being able to sense relative changes in performance. We propose new methodology to evaluate the dynamic behavior of natural rock arches, estimate fundamental mechanical properties, generate data for short- and long-term relative change monitoring, and ultimately provide a means to distinguish elastic and inelastic behavior and evaluate changing structural health. The project's core technique utilizes ambient vibration monitoring (i.e., measuring the resonant frequencies of an arch under weak background excitation and detecting changes through time that may be either progressive or punctuated in nature). The concept used merges three scientific disciplines: structural dynamics, seismology, and rock mechanics, building on established concepts from the field of structural health monitoring. Changes in behavior will be assessed by measuring the spectral characteristics of ambient vibrations and through time-dependent modal analysis. By sensing how an arch vibrates we can interpret its behavior and response to external loading, and detect reversible or irreversible changes in mechanical parameters. Resonant frequency shifts will be related to deformation and environmental measurements to assess driving forces and sensitivity of the monitored arches to external perturbations. The measurements are non-invasive and non-destructive, and all instruments are selected to be mobile with a small footprint. The goal is to establish a new method to evaluate the long-term structural integrity of natural rock arches, some of which are internationally recognized landmarks.