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After a wildfire event, ash is a newly formed surficial soil layer with microscale properties such as roughness, morphology, and chemical composition that may impact how ashes form fabrics in situ and so affect the overall hydrological conditions of a burned area (infiltration capacity, permeability, etc.). To examine the effects of ash microscale properties on macroscale behavior, eight wildfire ash samples from California were characterized physically (specific gravity, specific surface area, particle size, etc.), chemically (elemental composition, organic and inorganic carbon content, etc.), and geotechnically (strength, compaction, saturated hydraulic conductivity, etc.). The tested ashes were found to contain predominantly organic unburned carbons and carbonates derived from the combustion of calcium-oxalate rich fuels in temperatures likely ranging from 300°C to 500°C. Ashes had high specific surface areas because morphologically, particles had highly texturized and porous surfaces. Additional water was necessary to coat the particle surfaces, which led to high liquid limits and compaction optimum moisture contents. Hydraulic conductivity values were within range for silty sands (10^−5–10^−3  cm/s), and specimens had friction angles near 30°. However, tested ashes consistently demonstrated high void ratios and low bulk densities during testing for strength, hydraulic conductivity, and compaction. These anomalies were attributed to unusual carbonate morphologies; the high interparticle friction of these phases allowed ashes to form looser fabrics than a typical silty sand and contributed to the measured high void ratios, low maximum dry unit weights, and high friction angles. Overall, we hypothesize that the relative amounts of inorganic versus organic constituents in our wildfire ash samples affected how the ashes formed fabrics and so affected their geotechnical properties. 

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 the overall engineering behavior of ash. 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 higher than expected 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. A wet deposited layer of fine-grained ash on top of compacted sand reduced the hydraulic conductivity of sand by 1 – 3 orders of magnitude. 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). Ash particles were irregular in shape with fibrous textures and had electrostatic attractive tendencies. 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