Thermal Conductivity of Biochar-Enhanced Sandy and Silty Soils

Near-surface soils increasingly endure vigorous thermal cycles, sharpening the need for backfills that hinder heat flow without inflating carbon footprints. This study probes biochar as a thermal modifier while deliberately varying both botanical source and pyrolysis pathway—variables seldom explored together. Seven locally abundant Iraqi woods, spanning soft to hard classes, were charred under a systematic matrix of temperatures and residence times in an oxygen-free furnace. The resulting chars were blended at graded dosages with well-graded sand and low-plasticity silt compacted to loosen, medium, and dense states, and thermal conductivity (K) was measured using a calibrated transient-needle probe. Results reveal that an intermediate-temperature pine char delivers the lowest intrinsic K and achieves pronounced insulation gains across all density states; benefits rise sharply up to about 10 wt. % dosage and level off thereafter, indicating an economical threshold for field application. By linking wood origin, , dosage, and relative density to the thermal response of sands and silts, the study furnishes a design-ready framework for climate-resilient geotechnical backfills while avoiding additional carbon burdens