Impacts of Wildfires on Geotechnical Properties of Soils

Recorded data indicate an increasing trend of wildfires in several regions of the United States and globally. The primary objective of this study is to examine the changes in the geotechnical properties of near-surface soils in fire-impacted areas. To achieve this, laboratory and field tests were conducted on soil samples collected from four wildfire-affected areas in California: Chino State Park Fire (Blue Ridge Fire), El Dorado Ranch Park Fire, SQF Complex Fire, and Creek Fire. In situ hydraulic conductivity tests were performed on soils from both burned and unburned areas. Additionally, soil samples from these areas were tested in the laboratory for index properties, shear strength, organic content, and pH. The results indicate that the mean angle of internal friction decreased by 10%–20% in burned soil samples compared to adjacent unburned soils across all study areas. Mean pH increased by up to 1.2 units in burned soils, while organic content rose by 25%–70%, likely due to ash deposition. Hydraulic conductivity in burned soils increased by up to 300% relative to unburned controls. Cohesion showed varying trends depending on the site. These findings definitively show that wildfires notably alter the geotechnical properties of soils. Such alterations can contribute to postwildfire geohazards, including increased erosion, debris flows, and landslides. Understanding these changes is crucial for developing effective postwildfire management and mitigation strategies to minimize the risk of these geohazards.