Characterization of Preferential Flow Paths in Compacted Sand-Clay Mixtures

Abstract

Laboratory soil-column experiments were conducted to study the distribution of preferential flow paths resulting from removal of colloidal-size clay particles. Flow paths were assumed to result when the fluid energy was able to overcome the cementitious bonding of particles along the pore walls. These experiments specifically studied the influence of clay (kaolinite) percentage in sand-clay mixtures and the effect of hydraulic gradients on pore evolution. Analysis of the effluent during the experiments indicated that clay particles were removed from the soil column, accompanied by an increase in porosity and hydraulic conductivity. Dye experiments were conducted on the same columns to stain the pathways where colloidal particle removal occurred. It was observed that pore distribution was fairly uniform in some cases, while other cases showed distinct preferential flow-path formation. Using concepts of image analysis, the dye pattern was converted to a binary file and analyzed as a spatial random process to characterize the spatial distribution of flow paths. The expected spatial density (λ) and the correlation length were chosen as the key parameters to quantify the random dye patterns and to describe the nature of flow-path evolution. A physically based model was used to identify a dimensionless parameter, G , which expresses the ratio of detachment and deposition forces at any space-time location. Soil detachability was identified as the other key parameter. This expresses the ease with which colloids can be removed from the pore walls. It was observed that λ exhibited an identifiable trend with G , while I was predominantly dependent on detachability of the mixture.