The Mildred Lake Settling Basin (MLSB) is the largest disposal site for mature fine tailings (MFT) at the Syncrude Canada Ltd oil sands plant. Since 1996, MFT densification in the MLSB has significantly accelerated due to microbial activity. Methane-producing microorganisms, known as methanogens, have become very active. A field and laboratory research program has been performed to study the mechanisms leading to the rapid densification.

This research program consisted of historical monitoring data analyses, field investigations, small-scale column tests, and gas MFT densification tests. The field investigations have shown that the rapid densification of the MFT has occurred in the southern part of the pond ranging from 8 m to 15 m below the water surface. A connection existed between the rapid densification zone and the zone with intense microbial activity at the pond. The small-scale column tests demonstrated that, with increases of biogas generation, water drainage from the MFT was enhanced. Gas MFT densification tests showed that, stress histories and total pressure affected MFT densification property during microbial activity. Under high total pressure (6-7 m below pond surface) gas bubbles had difficulty to release. For MFT without pre-consolidation or under a preloading, during rapid gas generation, water was rapidly drained out. For over-consolidated MFT, water flowed back into MFT quickly during intense biogas generation. The concept of operative stress, the difference between the total stress and pore water pressure for the soil with large gas bubbles, was introduced to analyze the densification behavior of gassy MFT. Under high total pressure and under a preloading (1 kPa), excess pore pressure increased and operative stress decreased during rapid gas generation while water drainage from the MFT was accelerated. Total pressure and stress history also affected the structure and permeability of the MFT during microbial activity. Under low total pressure (1 m below pond surface) and without preconsolidation, the MFT permeability increased after intense microbial activity.