Swelling Suppression Mechanism of Compacted Expansive Soil Amended with Animal and Plant Based Biochar

Abstract

Biochar has been recently explored in regulating swelling of expansive soil. However, those studies were conducted mainly on loose soil (from agricultural perspective), that have much lower density as compared to that in engineered landfill cover/slope cover. Further, the mechanism of swelling suppression in biochars, of different origin (i.e., plant and animal based) is rarely explored. The study aims to investigate swelling suppression mechanism of a compacted expansive soil under influence of plant and animal based biochars. Atterberg limits, free swelling rate as well as one dimensional swell tests on expansive soil were carried out at four different biochar contents (i.e., 0%, 5%, 10% and 15%) and two biochar types (woodchip and pig manure). The results show that: (1) When the biochar type is kept the same, the swelling characteristics of the modified samples decrease significantly with an increase in biochar content. Based on XRD tests, water sensitivity of the expansive soil after modification is obviously reduced due to the contraction of the hydrophilic mineral lattice. However, beyond biochar content of 10%, the effect on swelling characteristics of expansive soil is minimal; (2) Woodchip biochar has a better suppression capacity than pig manure biochar at higher biochar contents (> 5%). SEM tests observed through comparison that the pore development of woodchip biochar greatly outperforms that of pig manure biochar. Hydrophilic minerals of the expansive soil with 10% woodchip biochar had the smallest crystal spacing and the smallest peak area. Adsorption, flocculation and cation exchange between biochar and clay particles limited the water entering the soil and reduced the thickness of the diffused double layer. Hence, it reduced swelling ability of expansive soil.