Initial Thermal Performance of Diavik’s A21 Dike

Following four years of construction, a new open-pit to access a diamondiferous kimberlite pipe (A21 Pipe), was officially opened at the Diavik Diamond Mine on August 20, 2018. The site is located approximately 300 km northeast of Yellowknife, NT, in a zone of continuous permafrost. The A21 Pipe is in a depression just offshore of South Island in Lac de Gras that had developed a through talik. A 2,200 m long ring dike was constructed to allow dewatering of the depression and to mine the kimberlite pipe by open-pit mining. Near the lake shore, the rockfill dike is founded on permafrost, whereas in deeper waters, the foundation is on unfrozen lakebed sediments and till over bedrock. A cut-off wall was installed to reduce seepage through the dike and its unfrozen foundation. Within the section of the dike founded on permafrost, the frozen soil and bedrock are used as a hydraulic barrier. It was therefore essential to understand the extent of the permafrost into Lac de Gras, especially on the South Abutment, where the water is shallow, and the lakebed topography forms a complex-shaped submerged peninsula with talik under the permafrost. Boreholes and geophysical surveys were used to estimate the extent of the talik and provide input for the design. During the initial phase of construction, extensive ground temperature monitoring and detailed observations were used to verify the extent of permafrost assumed in the design. These measures helped with the understanding of the complex ground thermal regime and provided information on the thermal behaviour of the foundation required to guide some of the construction decisions, such as the required initiation of active freezing by the use of thermosyphons or the timing of the placement of additional dike embankment construction material to accurately lock in winter frost and protect the permafrost from thawing.