Freeze-Thaw Dewatering and Structural Enhancement of Fine Coal Tails

Abstract

An 18.2-ha coal tailings impoundment that reached full capacity in 1989 has since been undergoing reclamation activities. The impoundment is sectioned into coarse, intermediate, and fine tails regions. The fine tails region, which consists of saturated silt and clay-size soil and coal particles of high void ratio and negligible shear strength, offers unique problems in terms of reclamation to a dry-surface landscape condition. Freeze-thaw consolidation and dewatering is an effective process for transforming these fine coal tails to a condition of a weak soil with measurable shear strength. Consolidation of the fine tails is found to occur both within and below the frost front. Within the frozen zone, localized moisture migration results in the development of a three-dimensional consolidated soil ped and ice-lense structure. Upon thaw, the consolidated peds settle by gravity, with the water escaping to the surface through the fissured soil ped structure. Thermal-induced suction gradients developed at the advancing frost front result in moisture migration and dewatering of saturated fine tails below the frost front. These two freeze-thaw consolidation and dewatering processes are described, along with models for predicting potential volumes of freeze-thaw treated material for given climatic conditions. Field results showing the significance of these processes on the dewatering and strength enhancement of fine coal tails are also presented. This dewatering and strength development provides the opportunity to implement dry rather than wet landscape reclamation procedures during reclamation of these waste-storage facilities.