Unloading Pore Pressure Response of the Weak Clearwater Derived Clays Under a Tailings Dyke at the Suncor Millennium Mine

ABSTRACT: Suncor is in the process of removing Pond 8B which provides buttressing support for the downstream slope of Dyke 11B. To maintain stability, modifications are required to Dyke 11B before and during Pond 8B drawdown. The dyke modifications also involve partial excavation of the crest and downstream slope. Dyke 11B is an ex-pit dyke constructed on Clearwater Formation clay shales and Clearwater derived till. The clays are high plastic and highly pre-sheared. Understanding the pore pressure response of the glacially-rafted and ice-pushed weak Clearwater Formation clays to the unloading was critical for the design of the project. To confirm the design parameters and assess the potential risk of removing Pond 8B, a field program was designed and executed to determine the pore pressure response of the clays due to the unloading. It was recognized that a successful trial required relatively continuous data retrieval coupled with a high level of cooperation with Suncor Operations and their contractor. The unloading trial consisted of an approximately 650 m x 125 m instrumented area on the dyke. The piezometers installed in the excavation area were connected to a telemetry system, and the data was monitored from remote locations in real-time. The piezometers were protected during the excavation and remained operational until the day the excavation proceeded to the instrument location. The piezometer leads were disconnected during the excavation in an area of the instrumented section immediately before the excavation and re-connected to the instrument soon after the excavation to obtain as close to a continuous pore pressure response as practical. The project was considered a success from the perspective of obtaining data throughout the excavation process. This paper presents the details of the instrument monitoring program, the effort undertaken to maintain the instrumentation during the excavation, and the unloading pore pressure response during the excavation.