Moisture Retention and Hydraulic Conductivity of Coarse-Textured Soils Amended with Coal Combustion Fly Ash

Abstract

Previous studies have suggested that an increase in water holding capacity or matric potential (ψ) may result from the addition of coal combustion fly ash (FA) to coarse-textured soils, but common laboratory techniques for evaluating such characteristics can be time-consuming and difficult to replicate. Therefore, centrifuge-based methods were used to assess the matric potential (ψ) and hydraulic conductivity (K) as a function of the degree of saturation for a coarse-textured surface soil from the Southeastern US that was amended with acidic FA at application rates ranging from 0­15% (wt/wt). In repacked columns, a low ionic strength rainwater surrogate was used as the leaching solution. For comparison, similar amounts of standard clays and sand (i.e., kaolinite (KA); montmorillonite (MONT); and ottawa sand (OS)) were added to the test soil to demonstrate the sensitivity of the centrifuge-based methods. The water dispersible clay (WDC) content, an indicator of the susceptibility of the soil clay to dispersion, was also evaluated for the amended soils. A minor increase in matric potential was observed only at the highest FA application rates, while saturated K (Ksat) actually increased and then leveled off with increasing FA addition. In contrast, the matric potential and K for the other tested amendments was altered in the expected manner. KA and MONT decreased HC and increased matric potential at a given moisture content, while OS addition increased the soil HC and decreased the water holding capacity. Consistent respective trends were also evident in the particle size analyses of the amended soil. The seemingly inconsistent behavior observed for the FA amended columns may reflect changes in pore-water composition resulting from soluble FA components that increased the background ionic strength of the soil solution for the readily dispersive surface soil, as column effluents were generally less turbid with increasing FA addition. Changes observed in WDC for the various amendments support such a mechanism as the dispersible clay decreased and the ionic strength increased significantly for the FA amended soils.