Review on effect of biochar on soil strength: Towards exploring usage of biochar in geo-engineering infrastructure

Biochar is an environment friendly material that has been widely adopted in various fields, such as agricultural, environmental and energy. On the contrary, the use of biochar in geoengineering infrastructure is still rare. The review critically summarizes the influence of biochar on soil strength in the context of geoengineering infrastructure. For an ease of understanding, a new index, biochar strength factor (BSF), has been introduced to assess the strength of biochar amended soils with respect to bare soil (BSF more than unity reflects an increase in strength, whereas BSF less than one indicates a decrease in strength). Further, in the review, a discussion has been put forward about the various pyrolysis production methods of biochar and its influence on physicochemical properties (i.e., particle size, density, porosity, surface area, etc.). Feedstocks and pyrolysis conditions govern physicochemical properties of biochar and alter soil bulk density, porosity, hydrophobicity/ hydrophilicity, aggregate stability, and water retention/holding capacity. Due to high porosity, low density, high compressibility, and water retention capacity, biochar addition is likely to reduce the BSF (decrease in shear, compressive, and tensile strength) for most of soils (except clayey). On the other hand, the biochar strength factor is greater than unity (BSF > 1) for clayey and expansive soil. BSF was found to vary significantly from as low as 0.25 for silty sand to as high as 2.97 for lean clay. However, the inherent mechanism seems yet to be investigated thoroughly. Compared to other cementing and reinforcement materials, the production, cost-effectiveness, and economy are also a matter of research. The future scope for understanding the soil-biochar interaction in geoengineering has been briefly discussed.