Investigating unsaturated hydraulic conductivity and water retention characteristics of compacted biochar-amended soils for potential application in bioengineered structures

Abstract This study investigates the influence of biochar amendment on the water retention and unsaturated hydraulic conductivity (Kunsat) of compacted soils focusing on its potential application in bioengineered structures. Physical properties, water retention characteristics and Kunsat of two different soils, such as silty sand and pure sand and amended with 5% and 10% (w/w) biochar made from mesquite were measured with the help of instantaneous profile method (IPM). The water retention capacity of the biochar-amended soil was characterised by plotting soil water retention curve (SWRC) and estimating the saturated volumetric water content (θs), residual water content (θr), shape parameter ‘n’ and plant available water content (PAWC). The results revealed that the amendment of biochar in the soils increased the θs (by 20-30%), θr (60-370%) and PAWC (7-60%), and decreased the n (by 5-18%) and Kunsat (one-two order). The microstructural observation showed that the hydrophilic surface functional groups and intra-pores in biochar contributed to the improved water retention, while the altered pore size distribution and porosity of the soils due to biochar amendment increased the tortuosity of the water flow path and hence reduced the Kunsat. Based on the findings, biochar-amended soil is proposed to be used in bioengineered structures that could promote the growth of vegetation and improve the performance.