Geotechnical And Hydrogeological Challenges Relating to Outfall Repair Projects in Winnipeg, Manitoba

ABSTRACT: The City of Winnipeg is underlain by an extensive Paleozoic carbonate aquifer system that is confined under 9 to 60 meters of glacial drift. The upper zones of the carbonate bedrock are commonly well fractured, resulting in a fairly transmissive aquifer unit. Most of the City's older outfall chambers were built prior to 1970's, when groundwater levels in the carbonate aquifer were lower than current levels due to extensive consumptive use. As major wet industries were shut down and the consumptive use on the aquifer diminished, groundwater levels in the carbonate aquifer gradually recovered. Repairs and modifications on deep structures have been complicated in recent years due to the higher groundwater levels in the carbonate aquifer. Further, geotechnical investigations completed within the city have typical not involved hydraulic testing of the carbonate aquifer. The case study focuses on two outfall repair projects adjacent to the Assiniboine River within the City of Winnipeg: the Aubrey Outfall Chamber and the Ruby Outfall Chamber. The geotechnical assessments completed for both sites did not identify the degree of dewatering needed to depressurize the carbonate aquifer to safe levels for excavation. The groundwater issues were only realized during subsequent hydrogeological investigations completed for the contractor at the start of the construction project. This resulted in significant construction delays. As part of the dewatering efforts, open-standpipe and vibrating-wire piezometers were installed to confirm that dewatering and overburden pore pressure targets were achieved. The monitoring methodologies produced different results, which warranted a comparison of the relative advantages and disadvantages of the piezometer applications. The comparison highlighted the importance of site-specific hydrogeologic conditions in the design of a groundwater monitoring system. The study highlights the need for technical collaboration between geotechnical and hydrogeological professionals to fully characterize site conditions for construction purposes. The involvement of hydrogeological professionals with geotechnical investigations would be beneficial to avoid expensive project delays and unforeseen dewatering requirements.