Leachability of 17-year-old stabilized/solidified contaminated site soils

Increased acceptance of in situ stabilization/solidification (S/S) depends on its long-term effectiveness and field performance at contaminated sites. Field trials of deep soil mixing application of S/S were performed at a contaminated site in West Drayton, near Heathrow Airport, in the United Kingdom back in 1996. Core samples were taken from this site at 2 months and 4.5 years and tested. This paper addresses the leaching properties of the contaminated site soils at 17 years in comparison to the properties at 0.24, 2.4, and 5 years. Seven different cement-based binders were used, including cement, PFA, lime, and bentonite to the soils, which were contaminated with a cocktail of heavy metals and organics. Two leaching methods, toxicity characteristic leaching procedure (TCLP) and the BS EN 12457, were applied to study the leaching behavior of the heavy metals in stablized/solidified soils. The leaching test results showed that leachate pH decreased with time in the first 5 years and then leveled off at 17 years. The BS EN 12457 leachate concentrations were lower than drinking water standards, suggesting that the performance of the S/S at 17 years' service time is satisfactory. The leaching behavior of the heavy metals in these S/S soils was mainly controlled by the alkalinity of the binders, with some of these heavy metals being absorbed into calcium silicate hydrates (C-S-H) gel phases. The binding mechanisms within the applied cementitious binders may be dominated by their leachate pH, which correlated with their corresponding metal hydroxide solubility. © 2014 American Society of Civil Engineers