EAGER: Deformation Induced Soil Fracturing - Multi-Scale Multi-Physics Mechanism and Early Detection

这个早期概念探索性研究（EAGER）项目研究了由于地面沉降、滑坡或地震导致的土体沉降和变形过程中，土体中裂缝形成的多尺度和多物理过程的基础科学。它寻求一种新的土壤断裂理论模型的实验证据。这些发现将用于开发一种现场监测工具，为土壤破裂提供早期预警。该项目特别利用以下两个岩土工程问题作为案例场景研究的一部分。第一个问题与美国许多地区普遍存在的地下水开采引起的地面沉降有关。例如，在加利福尼亚州的圣华金河谷，超过一半的山谷下沉超过0.3米；据报道，在一些地区下沉10米，速度超过30米/年。当地下水抽采井附近存在差异压实和局部地质变化时，可能会在周围地面形成裂缝。这种由地面沉降引起的裂缝带宽度可达200 m，由多个平行、分支的裂缝和地堑块组成。据报道，这些特征对关键的建筑基础设施造成了数十亿美元的损失。然而，目前还没有能够有效预警土壤裂缝发展的监测技术。第二个问题与用于降低洪水风险的河堤的深层防渗墙可能引起的地震破坏有关。例如，加利福尼亚州的大萨克拉门托地区是美国最容易发生灾难性洪水的地区之一。该地区依赖于老化的堤坝系统，目前正在进行大规模的堤坝改造。一项典型的改善措施包括安装深切墙和额外填充物，以提高堤防，以减轻堤防的下渗破坏。在地震期间，随着堤防系统的变形，这些墙可能有破裂的危险。因此，在未来的洪水事件中，控制渗漏的能力可能会丧失。同样，需要一个早期预警监测系统来评估这种土壤裂缝在整个运行周期内的风险。当土壤发生断裂时，它很可能是局部的和尺度相关的，因此破坏的位置将难以预测。土壤压裂过程中的这种不确定性可能会导致重大的工程问题，因此需要任何能够提供早期检测的监测技术。为了进一步改变我们对土壤压裂过程的基本理解，需要实验证据来支持新的理论模型和现场测量工具。本项目旨在探索变形诱发土体裂缝萌生和扩展过程中多物理场、多尺度土孔水相互作用的实验证据，并论证高分辨率分布式光纤应变传感技术探测土体破裂早期特征的可行性。该项目的主要目标是：(1)测试在变形引起的土壤破裂开始和扩展过程中发生的多物理场和多尺度土壤-孔隙水相互作用的实验证据，以及(2)证明用于检测土壤破裂早期特征的高分辨率分布式光纤应变传感技术的可行性。在本项目中，将对土梁试件进行一系列的弯曲试验，其中将嵌入微型孔压传感器和光纤传感电缆。该项目将研究土体破裂萌生和扩展过程中超孔隙压力的产生和消散。假设采用高分辨率分布式光纤应变传感技术可以捕捉断裂过程区内不同的微尺度破坏模式。将对该技术用于现场土壤压裂预警系统的可行性进行研究。

This EArly-concept Grant for Exploratory Research (EAGER) project investigates the fundamental science of multi-scale and multi-physics processes of fractures that develop in soil while a soil mass is settling and deforming due to land subsidence, landslides or earthquakes. It seeks experimental evidence of a new theoretical model of soil fracturing. The findings will then be used to develop a field monitoring tool that provides early warning of soil fracturing. The project specifically utilizes the following two problems in geotechnical engineering as part of a case scenario-based study. The first problem is related to land subsidence induced by groundwater extraction that is prevalent in many areas of the United States. For example, in the San Joaquin Valley, California, more than half of the valley has subsided in excess of 0.3m; subsidence of 10 m and a rate of more than 0.3m/year has been reported in some areas. When differential compaction occurs near groundwater extraction wells and local variations in geology, cracks may develop in the surrounding ground. Such land subsidence induced fracture zones may be as much as 200 m wide and consist of multiple parallel, branching fissures and graben blocks. These features are reported to cause billions of dollars of damage to critical built infrastructure. However, monitoring technology that effectively gives early warning to soil fracture development is not available at present. The second problem is related to possible earthquake-induced damage of deep cut-off walls used in river levees that are constructed to reduce its flood risk. For example, the Greater Sacramento area in California is among the most at-risk regions in America for catastrophic flooding. The area relies on an aging system of levees, and massive levee improvements are currently underway. A typical improvement involves installation of deep cut off walls with additional fill to raise the levee in order to mitigate under-seepage failure of the levees. During an earthquake, the walls can be at risk of fracturing as the levee system deforms. As a consequence, the ability to control seepage in a future flooding event may be lost. Again, an early warning monitoring system is needed to assess the risk of such soil fractures during the lifetime of operation.When soil fracture occurs, it is likely to be localized and scale-dependent and therefore the locations of the failure will be difficult to predict. This uncertainty in the soil fracturing process can potentially lead to significant engineering issues and any monitoring technology that provides early detection is needed. To make a step change in our fundamental understanding of soil fracturing process, experimental evidence that supports new theoretical models and tools to measure it in the field are required. This project aims to explore for the experimental evidence of multi-physics and multi-scale soil-pore water interaction occurring during deformation induced soil fracture initiation and propagation and to demonstrate the feasibility of high resolution distributed fiber optic strain sensing technology for detecting an early signature of soil fracturing. The primary goals of the project are (i) test for experimental evidence of multi-physics and multi-scale soil-pore water interaction occurring during deformation induced soil fracture initiation and propagation, and (ii) to demonstrate the feasibility of high resolution distributed fiber optic strain sensing technology for detecting an early signature of soil fracturing. In this project, a series of flexural tests will be performed on soil beam specimens, in which miniature pore pressure transducers and fiber optic sensing cables will be embedded. The project will investigate the excess pore pressure generation and dissipation during the soil fracture initiation and propagation process. It is hypothesized that different micro-scale failure modes inside a fracture process zone would be captured by high resolution distributed fiber optic strain sensing technology. The feasibility of the technology for a field-based early warning system against soil fracturing will be examined.