Investigation of Thermal and Strength Characteristics of a Natural Backfill Composite Inspired by Synergistic Biochar-Biopolymer Amendment of Clay Loam

Thermally active structures (TAS) such as crude oil pipelines and high-power voltage cables are usually embedded within local soils for their sub-structural stability. Based on site requirements, soils are amended with synthetic cementitious materials to improve their strength. However, synthetic binders such as cementation, geo-polymerization, and bio-cementation undermine thermal insulation while improving strength. Moreover, commonly used synthetic binders such as cement and lime are extremely harmful to the geoenvironment due to their hyper-alkalinity and high carbon footprint. Therefore, this study proposes a novel backfill composite consisting of clay loam, biochar, and biopolymer for superior thermal insulation and strength characteristics. Local soil containing high clay content was chosen due to its low thermal conductivity and potential to form stable hydrogen bonding with the biopolymer. The clay loam is amended with 2.5% to 7.5% (w/w) biochar and 0.5% to 1.5% (w/w) of biopolymer and their various combinations. Out of nine soil-biochar-biopolymer composites devised in the current study, six composites exhibited superior strength and lowered thermal conductivity than the plain soil in both conditions, i.e., optimum moisture state and upon drying. The findings of this study establish the synergistic attributes of biopolymer and biochar amendment for developing a high-strength thermal-insulating soil composite.