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.
