Shear Strength of Soils Subjected to Fires

In recent years, a problematic cycle of drought, forest fires, precipitation, and rain-induced debris flows has affected California, but limited research has been performed on how the underlying soil is potentially weakened after additional soil-like material (ash) is produced. Additionally, while a large amount of research has been performed on characterizing forest fire ash, fewer studies have been performed on geotechnical characteristics of soil/ash mixtures. This study focuses specifically on the characterization of ash samples from a brushfire and the large displacement and residual shear strength of ash/soil mixtures and pure ash, respectively. Two types of samples are tested: (1) brushfire ash samples collected from fire exposed areas, and (2) ash/soil mixtures of pure brushfire ash and laboratory sand. Characterization of ash indicated a silty sand grain size distribution with chemistries that were expected for ash derived from biomass. However, it was determined that carbon content is not equivalent to loss-on-ignition (LOI) for biomass materials with high calcium content. Direct shear testing of ash/soil mixtures indicated that large displacement strength behavior was governed by frictional resistance between sand particles until ash quantity reaches 10% by weight, at which point the friction angle decreased by an average of 2°. Additionally, the residual strength of normally consolidated pure ash was high (above 30°), and the residual strength was reached at strains less than 1%.