Apparent Contact Angle and Water Entry Head Measurements for Organo-Silane Modified Sand and Coal Fly Ash

Organo-silane (OS) surface modification chemically alters the surface of silica-based materials such as soils and industrial byproducts, rendering them hydrophobic. Such modification offers the potential to transform porous media with relatively high hydraulic conductivity into a water-repellent barrier to infiltration. This paper reports on experimentally observed relationships between dry density, apparent contact angle, and water entry head for Ottawa sand and a Class-F coal fly ash (CFA). Modification with two different OS products resulted in maximum apparent contact angles of approximately 120 degrees and 140 degrees for sand and CFA, respectively. Both products and materials exhibited an asymptotic relationship such that increasing OS dosages beyond a threshold level yielded diminishing increases in contact angle. The water entry head was found to range from 0 to 14cm of water and 0 to 542cm of water for OS-modified sand and OS-modified CFA, respectively. Positive correlations were established between dry density, apparent contact angle, and water entry head. Soil hydrophobicity was found to have a negligible effect on saturated hydraulic conductivity; the governing parameter for infiltration control is water entry head. The Washburn equation was found to provide reasonable estimates of experimentally observed water entry head, especially at large values of apparent contact angle. These results suggest that designing with water repellency is a plausible concept for infiltration control. (C) 2018 American Society of Civil Engineers.