Mechanical, Chemical, Hydraulic, and Microstructural Properties of Buckshot Clay Amended with Gasification Biochar

Abstract

This study investigated the effect of gasification biochar amendment on mechanical, chemical, hydraulic, and microstructural properties of Buckshot clay, a high-plasticity clay with poor engineering properties. The gasification biochar used for this study was a waste biochar produced as a byproduct of gasification at a lumber mill. The feed stock for this material was the residual pine material from the plant. The biochar was separated into two sizes: coarse (passing sieve No. 8 and retained on sieve No. 20), and fine (passing sieve No. 20). Shear strength, swelling, compressibility, collapsibility, hydraulic conductivity, water retention, cation exchange capacity, pH, and microstructural characteristics were determined for the clay amended with 5%, 10%, and 20% gasification biochar. For the tested materials, the results indicated modest increases in the shear strength up to 10% biochar content for both the coarse and fine biochars. There was no significant reduction in the maximum swell pressure for the fine biochar until the 20% level, which corresponds to a greater than 50% reduction. The coarse biochar exhibited a more gradual reduction, with each increase in biochar content resulting in a decrease in the maximum swelling pressure with a maximum reduction of approximately 50%. Slightly increased compressibility and more brittle behavior were also noted with increased levels of biochar amendment. Higher levels of biochar addition noticeably increased the compression index with the greatest increase occurring for the coarse biochar. Minor increases in the soil’s pH and collapse potential were also observed. The cation exchange capacity of the material decreased with increases in biochar. Gasification biochars exhibit large variability in properties and characteristics depending upon the source and process used to produce the biochar. This study aimed to present a framework to evaluate the potential benefits of the reuse of gasification biochars for improving geotechnical proprieties of marginal soils for select applications such as nonstructural fills. For site-specific applications, depending on the local source of gasification biochar, running laboratory tests on site-specific materials are needed.