Prediction of Unsaturated Hydraulic Conductivity in Bio-Treated Stabilized Lateritic Soil

Unsaturated hydraulic conductivity (USHC) is a key property that governs water flow in partially saturated soils. This study examines the USHC of lateritic soil treated with Bacillus megaterium using three soil–water characteristic curve (SWCC) models: van Genuchten (VG), Brooks–Corey (BC), and Fredlund–Xing (FX). Conductivity was estimated as the product of relative hydraulic conductivity, derived from model fitting parameters, and the saturated hydraulic conductivity (k$_{sat}$) measured through falling head tests. Specimens were compacted at different moulding water contents (MWC), microbial suspension densities, and energy levels to evaluate their combined influence on USHC. Results revealed that USHC generally decreased with increasing MWC. Higher values were obtained for specimens compacted on the dry side of optimum, due to the dominance of larger pores, whereas wet-side compaction resulted in lower conductivity because of finer pore structures. Variation in microbial suspension density produced only slight reductions in USHC, attributed to calcite precipitation from microbial urease activity, which partially obstructed pores. With respect to compaction, the VG model predicted rising USHC with matric suction, while BC and FX indicated decreasing patterns. These variations were linked to pore structure, calcite distribution, and model parameter sensitivity. The findings highlight the interactive effects of water content, microbial treatment, and compaction effort on USHC and confirm the usefulness of SWCC-based models in predicting hydraulic performance of bio-stabilized lateritic soils.