Influence of biochar addition to clayey sand on desiccation cracks formation and water infiltration under wetting and drying

Alternating rainfall and prolonged drought in the tropics promote desiccation cracking of soil surface and rainfall infiltration-induced instability on roadside slopes. Mixing the surface soil with biochar is effective in supporting vegetation that protects the slopes, but its direct effect on soil’s hydrological characteristics in short and long terms still requires investigation. This study evaluates whether amending compacted clayey sand with biochar can reduce cracks and alleviate infiltration and pore water pressure buildup under repeated wetting–drying (W-D) cycles. One-dimensional column tests were conducted in which compacted soil with different biochar types (rice husk and wood chip) and dosage (0%, 10% and 30% by bulk volume) was subjected to successive W-D cycles. Surface cracking was quantified as Crack Intensity Factor (CIF) from image analysis, while the pore water pressure monitored at different depths were used for inverse analyses to estimate cycle-specific macroscopic, effective saturated conductivity ( k s,eff ) and van Genuchten parameter α eff . Although biochar addition increased void ratio after compaction, many biochar-induced pores seem ineffective for flow, so the increase in intact (uncracked) saturated hydraulic conductivity ( k s ) was limited. After W-D cycles, biochar, especially wood chip, reduced surface cracking relative to the control soil, resulting in 6-h infiltration rates that were not markedly different from those of the unamended soil despite the higher void ratio. For all soil mixtures, W-D cycles increased k s,eff and α eff by generating cracks and connected porosity, accelerating PWP transmission to depth. However, lower dosage (10%) of biochar generally delayed and reduced PWP rise compared with 0% or 30%, indicating an optimum dosage for barrier performance. This optimum is explained by combination of less cracking and sensitivity of the macroscopic parameters k s,eff and α eff to cracks. This holistic effect is captured by soil diffusivity reflecting the macroscopic, effective parameters.