Engineering properties of wildfire ashes

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While a large amount of research has been performed in recent years on characterizing wildfire ashes and determining the hydrological properties of slopes post-wildfire, fewer studies have concentrated on ash geotechnical behavior. This study addresses this need by examining the compaction, shear strength, and hydraulic behavior of wildfire ash and ash/soil layered specimens. Unique chemical and physical characteristics of wildfire ashes were shown to influence the ash engineering behavior. Ashes in the as-received condition have a silty sand grain size distribution with high surface areas (>1 m²/g). Chemically, ashes contained silica, aluminum, calcium (in the form of carbonates) and residual organic carbon from incomplete combustion. Maximum dry unit weights ranged from 13 16 kN/m³ at optimum moisture contents between 20 and 30%. Hydraulic conductivity of samples varied between 10^-4 and 10^-5 cm/s.  Shear strength of ash/sand layered specimens demonstrated that ash was fairly stiff, with an average friction angle of 28 degrees. Void ratios of specimens were consistently higher than expected for a silty sand fabric (usually above 1.0). The authors suspect that the unique chemistries present in ash (notably carbonates and organic char) contributed to the loose fabric structure and engineering properties that were atypical of a silty sand grain size distribution. This study is one of the first to present a comprehensive summary of wildfire ashes that concentrates specifically on the geotechnical behavior of the material and so expands the body of knowledge on wildfire ash.

9th International Congress on Environmental Geotechnics (ICEG2023)

Human-Induced and Natural Disaster Mitigation