Structural Monitoring of Rock Arches

岩拱是一种动态的自然结构，在各种环境力的作用下会发生弯曲、下垂、摇摆和震动。参观这些地貌的游客会面对它们脆弱的自然，然而很少有人欣赏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门的环境共振，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的变化（例如热应力，降雨），并最终能够辨别随着时间的推移内部强度的微妙变化。他们的方法的关键是测量拱门如何振动。岩拱是相对简单的机械结构，具有可测量的共振频率和模态。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，并将其输入到数值模拟中，以预测谐振模式形状和频率，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，岩石力学和地貌学领域的概念，目的是建立一种新的方法来定量评估岩石拱的动力学和结构性能的变化。这项研究将导致对自然拱门的生命周期，对不同外部载荷的反应以及导致其最终倒塌的破坏过程的新理解。我们的项目还为公众参与和科学教育提供了一个极好的机会。从长远来看，预期的结果可能会带来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知名。然而，目前还没有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成分和强度无法先验地知道。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拱的动态行为，估计基本的力学性能，生成数据的短期和长期的相对变化监测，并最终提供了一种手段来区分弹性和非弹性行为，并评估不断变化的结构健康。该项目的核心技术利用环境振动监测（即，在弱背景激励下测量拱的共振频率并检测随时间的变化，所述变化本质上可以是渐进的或间断的）。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，建立在结构健康监测领域的既定概念基础上。行为变化将通过测量环境振动的频谱特性和通过随时间变化的模态分析来评估。 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。共振频率偏移将与变形和环境测量相关，以评估驱动力和受监测拱对外部扰动的敏感性。测量是非侵入性和非破坏性的，并且所有仪器都被选择为具有小占地面积的移动的。其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.