Enhanced Shear Strength of Wildfire Impacted Soils Using Biochar and Chitosan

The rise in global temperatures driven by the changing climate has led to more fuel and favorable wildfire conditions, resulting in more frequent fires and larger burned areas. Root strength, from vegetation, improves slope stability; however, loss of vegetation due to wildfires negatively influences the slope stability. Furthermore, these wildfires degrade soil structure, reducing shear strength characteristics, and inducing hydrophobic behavior in soils, thereby reducing water infiltration and increasing runoff, increasing the susceptibility to slope failures. This study proposes a novel solution in this regard, where sustainable materials derived from organic waste are used as amendments to enhance short-term slope stability and promote long-term vegetation regrowth. Specifically, this study investigates the improvement in shear strength parameters using two materials: biochar, derived from thermal decomposition of organic matter, and chitosan, a biopolymer derived from seafood waste. This study evaluates the shear strength of three wildfire-affected soils collected from the Four Corners wildfire site, Idaho, USA, with amendments of biochar (10% by weight) and chitosan (2% by weight). Chitosan notably enhanced both cohesion and internal friction angle in all three soils. In contrast, biochar significantly improved the internal friction angle in two soils, while in the third, it substantially increased cohesion but slightly reduced the friction angle. Biochar amendment increased friction angle by up to 27%, while chitosan increased cohesion significantly (from 0.80–2.46 kPa to 8.85–12.35 kPa) and friction angle by up to 23%. Further, slope stability quantified using infinite slope analysis revealed that amendments can notably improve the slope stability of wildfire-impacted soils. All three soils with chitosan amendment had a factor of safety (FS) over 1.5, even at a slope angle of 60°, and biochar amendment significantly improved the maximum slope angle at which the FS ≥ 1.5. Overall, the findings from the study demonstrate the potential of biochar and chitosan as sustainable solutions for mitigating slope failures in wildfire-affected areas.