ERI: Engineering Behaviors of Slopes Exposed to Wildfires

Csu Fullerton Auxiliary Services Corporation, Fullerton CA

Investigators

Abstract

As the global temperature of the planet increases, shifts in regional climate have resulted in hotter, drier summers in the state of California, which increases the risk for droughts and wildfires. In Summer 2020, a record-setting 4.4 million acres were consumed, affecting many lives and causing millions of dollars of property damage. However, the risks associated with wildfires do not stop once the fire has been extinguished. The ravaged slopes are still at risk for potentially catastrophic mudslides. As frequency and severity of wildfires increases, so does the frequency and severity of mudslides. Unfortunately, researchers still do not fully understand how burned soils and ash behave after a wildfire. How strong is wildfire ash? Does a burned slope become stronger or weaker once ash is deposited after a fire? Once a wildfire is extinguished, where does the ash go and how does it age? This Engineering Research Initiation (ERI) award strives to answer these questions by examining the engineering behaviors of slopes exposed to wildfires. Specific objectives include characterizing wildfires ashes and burned soils and measuring their strength, compressibility, and hydraulic properties. This research will assist those who assess the risks associated with post-fire mudslides. It is important that we understand the geotechnical engineering behavior of ash and burned soil, so that we can provide the most accurate and most effective tools to protect communities from post-wildfire hazards. This research will also be providing educational benefits to historically underrepresented groups in engineering, as the research will support the hiring of undergraduate and graduate student researchers from underrepresented communities. This research program focuses on examining wildfire-exposed areas from a geotechnical perspective, to determine if the addition of ash and dry ravel to soil slopes and basins weakens these areas and could potentially cause slope failures. Although there is a significant quantity of research in the literature concerning the factors that trigger debris flows, limited data is available on shear strength characteristics of wildfire ash and ash/soil mixtures. This is a large gap is our understanding of debris flow behaviors, as the soft, unconsolidated ash layer plays a crucial role in the stability of the post-wildfire slope. To supplement the available research in this area, this work will determine the strength and hydraulic characteristics of wildfire ash and ash/soil mixtures from central and southern California. Specific objectives include measuring the shear strength of wildfire ash and ash/soil mixtures using a combination of shear strength devices, measuring the saturated and unsaturated hydraulic properties of wildfire ash/soil mixtures, performing standard geotechnical index property testing of wildfire ash and burned soils, and performing geochemical testing on wildfire ashes. This work should provide a deeper understanding of the engineering properties of residual materials post-wildfire. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.