ABSTRACT
The paper presents the progress achieved from 1995 to 2001 in evaluating time-dependent consolidation of fine tailings. Results are being applied to the decommissioning of large uranium tailings ponds at WISMUT, in particular at Helmsdorf and Culmitzsch A tailings ponds both located in Southeastern Germany. Uranium tailings ponds of the former Soviet-German Wismut company in Eastern Germany cover nearly 6 km2 and contain about 150 × 106 m3 of tailings. Time-dependent consolidation behaviour of thick fine tailings is of critical importance for ongoing in-situ-decommissioning including the de watering by technical means and covering the tailings surfaces. Based on historic data and on first geotechnical data gained from drillings and soundings the actual consolidation state was recalculated for each tailings pond. For this the disposal period was backcalculated using a one-dimensional model applying the non-linear finite consolidation theory (Schiffman et al. 1984). To gain input parameter needed for such modeling a new advanced laboratory compression test was developed to determine the fine tailings consolidation behaviour characterized by the functions effective stress vs. void ratio and permeability vs. void ratio. A new cone penetration test including a piezocone was adopted to measure the hydraulic conditions in situ and the consolidation state of the fine tailings. Pore pressure gauges were installed in fine tailings in situ to measure the consolidation progress continuously. Data from the new laboratory tests and field measurements were used for a second run of the time-dependent consolidation modeling with respect to the covering of the fine tailings during remediation. Modeling was calibrated by newly collected geotechnical lab and field data. Results from both modeling and geotechnical investigations/measurements are in agreement with fine tailings consolidation behaviour predicted by the non-linear finite strain consolidation theory. In addition the paper presents the use of different one- and two-dimensional time-dependent consolidation models for cost-effective designing of the cover, for designing relevant construction elements of the final cover and for controlling and monitoring the construction progress of the cover on the tailings ponds.
