Engineering practice of ex-situ stabilization for contaminated site soils

A systematic study on the ex-situ stabilization of multiple heavy metals-contaminated soils at a contaminated site in China is introduced. The key construction processes including excavation of contaminated soils, mixing of contaminated soils with a proposed novel stabilizer, evaluation of soil treatment effectiveness and backfilling of stabilized soils are presented. The stabilizer is composed of polymeric ferric sulfate, superphosphate, calcium hydroxide and chlorite powder at a dry weight ratio of 4:2:1:1. The dosage of the stabilizer is 2%~4% (dry weight basis) and the treated soils are cured for 3~7 days. It is found that the leaching concentrations of arsenic (As), antimony (Sb), copper (Cu), zinc (Zn), vanadium (V) and fluoride (F) are lower than the corresponding remediation target values. The Hitachi integrated soil remediation equipment is adopted in mixing the soils with the stabilizer. With the mixing equipment, the particle size of the soils is smaller than 20 mm, and the mixing uniformity of the soil-stabilizer system is found to be higher than 95%. The stabilized soils with heavy metal leaching concentrations meeting the remediation target values are backfilled to the in-situ site, lined and covered with composite barriers for the purpose of human health and environmental risk control. The structure of the cover barrier system, from top to bottom, is composed of compacted clay liner, non-woven geotextile, and HDPE geomembrane. The structure of the bottom barrier system, from bottom to top, is composed of compacted clay liner, non-woven geotextile, HDPE geomembrane and non-woven geotextile. After compaction with layered lifts, the thickness of the stabilized soils is controlled to be 3.0 m, and the degree of compaction of the stabilized soils is higher than 90%. The construction processes and key parameters obtained from the project are useful to facilitate the ex-situ stabilization of multiple heavy metals-contaminated soils and the risk control of treated soils being reused as geomaterials.