Fly ash utilization in soil-bentonite slurry trench cutoff walls.

Abstract

The addition of fly ash, derived from coal-fired power plants, can significantly reduce the hydraulic conductivity of soil-bentonite slurry trench backfill mixtures. This was demonstrated with the use of falling-head permeability tests on trial backfill mixtures. The addition of up to approximately 40% fly ash by weight caused a reduction in the hydraulic conductivity of these mixtures, apparently by reducing porosity and by increasing the mixture specific surface. Little benefit was gained by adding more fly ash. The addition of small amounts of fly ash to medium s and -bentonite mixtures resulted in a reduction in hydraulic conductivity of over two orders of magnitude. The effect of fly ash upon fine s and -bentonite mixtures was much less pronounced. The observed effects of fly ash were adequately modeled by the Kozeny-Carman function, except for mixtures containing medium s and and very little ash. The model overpredicts conductivity for the latter mixtures. The benefits of fly ash-enhanced soil-bentonite backfill depend upon the achievement of a relatively homogeneous mixture. A model for the ideal, homogeneous mixture of particulate solids was developed, based upon the statistical characteristics of linear combinations of r and om variables. Three index tests were chosen for use with this model and for examining the mixing behavior of s and -fly ash-bentonite mixtures. The selected index tests were for percent fines, cation exchange capacity, and loss-on-ignition. After characterizing the individual mixture components separately with each of the three index tests, the results were used to validate the linear combination mixture model against index test results on numerous trial mixtures. A mixing index was defined as the ratio of the measured variance of index test results, determined on a set of mixture samples, to the theoretical target variance derived from the mixture model. Eight mixing tests were run and the relative homogeneity of each resulting mixture was determined six or seven times during the mixing process. Homogeneity with respect to the cohesionless fly ash particles was more difficult to achieve than with respect to the cohesive bentonite particles. A relatively high ash content (30% versus 10%) and a relatively low moisture content (23% versus 28%) appeared to aid in the mixing process. Adding prehydrated bentonite rather than air-dry powdered bentonite had no consistent effect upon the mixing process. For preliminary design purposes, an ash content of approximately 30% should be chosen for use in the backfill mixtures. If no native clay fines are present, up to 4% bentonite may be required to achieve a hydraulic conductivity of less than 1 $\imes$ 10$\\sp{-7}$ cm/sec. The backfill should be prepared relatively stiff (slump of 2 to 4 inches) and a tumbling-type mixer such as a portable pug mill should be used.