EFFECT OF HETEROGENEITY ON FLOW CHARACTERISTICS IN POROUS MEDIA

Abstract

Heterogeneous and stratified porous media exhibit complex flow behaviour because variations in soil composition, pore structure, layer arrangement, and orientation directly influence seepage pathways and hydraulic conductivity. Reliable assessment of these effects is important for geotechnical, groundwater, waste-containment, drainage, and environmental engineering applications. The present study investigates the combined influence of chemical modification, microbiological treatment, and layer inclination on the horizontal hydraulic conductivity of heterogeneous porous media. Laboratory experiments were carried out using a specially designed horizontal permeameter with overall dimensions of 0.8 m × 0.5 m × 0.5 m and a 0.6 m long soil column. Two inorganic sandy-silt soils collected from the Yamuna alluvial region of Delhi were arranged in stratified form to represent heterogeneous porous conditions. Fly ash was selected as the chemical admixture and incorporated into the upper soil layer at 5%, 10%, 15%, 20%, and 25% by dry weight. Bacillus subtilis was used as the microbiological agent at bacterial cell densities of 10⁶, 10⁷, and 10⁸ cells/mL. Experimental testing was performed at layer inclination angles varying from 0° to 10° to quantify the independent and combined effects of chemical content, bacterial concentration, and specimen orientation on horizontal hydraulic conductivity. The experimental results demonstrated that the hydraulic response of the stratified media was strongly affected by all three governing variables. In untreated specimens, horizontal hydraulic conductivity increased with increasing layer inclination, indicating improved continuity of the preferential flow path along the oriented layers. The addition of fly ash consistently lowered hydraulic conductivity, and the degree of reduction increased with increasing fly ash dosage. At 0° inclination, reductions of approximately 10.83%, 27.23%, 40.07%, 56.04%, and 65.94% were obtained for 5%, 10%, 15%, 20%, and 25% fly ash, respectively. This behaviour is associated with pore filling, densification, and restriction of interconnected void spaces. Microbiological treatment also produced a marked decrease in hydraulic conductivity. Higher bacterial cell densities generated greater reductions because bacterial growth, extracellular polymeric substances, and pore occupation progressively restricted the available seepage channels. The combined use of fly ash and Bacillus subtilis produced the strongest hydraulic modification, demonstrating a synergistic effect between physical pore filling and biological clogging. vi The maximum reduction in horizontal hydraulic conductivity was approximately 91.33% at 0° inclination for the combined condition of 25% fly ash and 10⁸ cells/mL bacterial concentration. Although conductivity generally increased with inclination angle, treated specimens remained considerably less permeable than untreated stratified media throughout the tested range. In addition to the experimental investigation, Support Vector Regression (SVR) was used to model horizontal hydraulic conductivity as a function of soil characteristics, fly ash content, bacterial cell density, layer inclination, and number of layers. The model reproduced the nonlinear experimental behaviour with strong predictive performance and low error, demonstrating the usefulness of machine-learning techniques for representing complex heterogeneous flow systems. Overall, the study confirms that the hydraulic behaviour of stratified porous media can be substantially modified through controlled chemical and microbiological interventions. The findings provide a practical basis for reducing seepage and improving hydraulic performance in applications related to groundwater protection, engineered barriers, waste containment, soil improvement, and hydraulic infrastructure.