ABSTRACT: Thaw settlement of degrading permafrost: a geohazard affecting the performance of man-made infrastructures at Umiujaq in Nunavik.

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In the context of global warming anticipated at high latitudes, the degradation of permafrost can induce geohazard such as ground subsidence affecting the performance of man-made infrastructures. A case study on the performance of a road built in 1991 at Umiujaq on the east coast of Baie d’Hudson in Nunavik (Québec) is presented. Some village infrastructures lying in part on warm frozen ice-rich marine sediments are already affected by the thawing of permafrost. For example, along a 200-meter segment of the road leading to the airport, three zones of ground subsidence were studied in summer 2006; 15 years after the road completion. The subsidence can be as high as 0.63 m and the total volume of subsidence is close to 530 cubic meters for a road embankment 7.4 m wide. In addition to the elevation measurement, ground penetrating radar and electrical resistivity profiling surveys were carried out for assessing the ground stratigraphy and permafrost conditions. The marine sediments are thawed over a thickness of at least 6 m under the subsidence zones leading to a void ratio decrease of about 10% following the thaw consolidation. The ground subsidence is not only due to the recent climate warming trend observed in Nunavik but also to the snow accumulation along the road embankment insulating the ground surface in winter and preventing further ground freezing. While the road is still suitable for vehicles, the only economically viable mitigation of permafrost instability is to give free the subsidence and reload the road embankment when needed as long as the thawing is not achieved.

Bibliography of Canadian Geomorphology