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Soil in embankments is often amended with several additives for supporting the vegetation growth on the side slopes. Biochar, a stable, nutrient loaded and moisture retentive material, could be amended to the embankment soils. The positive impacts of biochar on soil properties, soil water and nutrient retention, microbial activity and growth of crops are well established. However, the load-bearing capacity of biochar-amended soil which is utmost important for a material to be used in embankment is rarely reported in literature. The present study aims to investigate the shear strength and load-bearing capacity of biochar-amended soil for application in embankments. For assessing the shear strength and load-bearing capacity, California bearing ratio (CBR) and direct shear tests were conducted on bare and 5%, 10% and 15% (w /w) biochar-amended silty sand and pure sand. The results revealed that the biochar amendment decreased the dry unit weight (6–19%), and increased the shear strength (cohesion (c) and angle of internal friction (ϕ)) and load-bearing capacity of both the soils by 22–228% and 17–89% respectively. Further, the biochar amendment improved the vegetation root (34–108%) and shoot (29–157%) mass, ensuring the enhanced stability and protection (erosion). Thus, the results suggest the application of biochar-amended soil in embankments, however future field-scale trials are needed before application. [ABSTRACT FROM AUTHOR]

• A detailed review of soil water retention characteristics (SWRC) of biochar-amended soil is presented. • Biochar feedstock type, pyrolysis temperature, particle size and aging has a strong influence on the SWRC. • The SWRC of biochar-amended soil also depends on soil type and corresponding compaction condition. • Correlations among SWRC parameters and biochar properties were developed and presented. • Such a study will be quite handy while applying biochar-amended soil in geotechnical and agricultural applications. Biochar is a stable carbon-rich product obtained through pyrolysis of plant and animal-based biomass. Soil is often amended with biochar due to the beneficial features, such as carbon sequestration, improvement of crop growth and yield by enhancing soil fertility, removal of an organic and inorganic pollutant from the soil, and improvement of water retention properties of the soil. Fundamentally, the amendment of soil with biochar alters the soil water retention characteristics (SWRC) of the soil. This is primarily due to the inherent properties of biochar that includes large specific surface area (SSA) by the intra-particle pores, and the functional groups present in biochar surface. SWRC is vital for both agricultural soil and soil in geotechnical or geo-environmental engineering structures due to the presence of vegetation. The effect of biochar on the SWRC of agricultural soil was extensively studied. However, it is rarely studied for soil in geotechnical or geo-environmental engineering structures, such as landfill cover, bioengineered slope, embankment, green roof, etc. Soil in these structures subjects to a distinct condition, in terms of compaction state and design life compared to agricultural soil. Therefore, a comprehensive gathering of information on the effect of biochar on the SWRC and the factors controlling the biochar influence is needed for potential investigation and application of biochar amendment for soil in geotechnical or geo-environmental engineering structures. In this article, the effect of biochar amendment on the SWRC of soil has been reviewed and summarized by considering various factors controlling the influence of biochar on the SWRC. The literature review revealed that the amendment of soil with biochar could improve the SWRC while few occasions not influence the SWRC. Factors, such as feedstock type and pyrolysis temperature, particle size of biochar, soil type, and compaction state have strong bearing on the influence of biochar on the SWRC. [ABSTRACT FROM AUTHOR]